1//===- DeadStoreElimination.cpp - MemorySSA Backed Dead Store Elimination -===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// The code below implements dead store elimination using MemorySSA. It uses
10// the following general approach: given a MemoryDef, walk upwards to find
11// clobbering MemoryDefs that may be killed by the starting def. Then check
12// that there are no uses that may read the location of the original MemoryDef
13// in between both MemoryDefs. A bit more concretely:
14//
15// For all MemoryDefs StartDef:
16// 1. Get the next dominating clobbering MemoryDef (MaybeDeadAccess) by walking
17// upwards.
18// 2. Check that there are no reads between MaybeDeadAccess and the StartDef by
19// checking all uses starting at MaybeDeadAccess and walking until we see
20// StartDef.
21// 3. For each found CurrentDef, check that:
22// 1. There are no barrier instructions between CurrentDef and StartDef (like
23// throws or stores with ordering constraints).
24// 2. StartDef is executed whenever CurrentDef is executed.
25// 3. StartDef completely overwrites CurrentDef.
26// 4. Erase CurrentDef from the function and MemorySSA.
27//
28//===----------------------------------------------------------------------===//
29
30#include "llvm/Transforms/Scalar/DeadStoreElimination.h"
31#include "llvm/ADT/APInt.h"
32#include "llvm/ADT/DenseMap.h"
33#include "llvm/ADT/MapVector.h"
34#include "llvm/ADT/PostOrderIterator.h"
35#include "llvm/ADT/ScopedHashTable.h"
36#include "llvm/ADT/SetVector.h"
37#include "llvm/ADT/SmallPtrSet.h"
38#include "llvm/ADT/SmallVector.h"
39#include "llvm/ADT/Statistic.h"
40#include "llvm/ADT/StringRef.h"
41#include "llvm/Analysis/AliasAnalysis.h"
42#include "llvm/Analysis/AssumptionCache.h"
43#include "llvm/Analysis/CaptureTracking.h"
44#include "llvm/Analysis/CycleAnalysis.h"
45#include "llvm/Analysis/GlobalsModRef.h"
46#include "llvm/Analysis/Loads.h"
47#include "llvm/Analysis/MemoryBuiltins.h"
48#include "llvm/Analysis/MemoryLocation.h"
49#include "llvm/Analysis/MemorySSA.h"
50#include "llvm/Analysis/MemorySSAUpdater.h"
51#include "llvm/Analysis/MustExecute.h"
52#include "llvm/Analysis/PostDominators.h"
53#include "llvm/Analysis/TargetLibraryInfo.h"
54#include "llvm/Analysis/ValueTracking.h"
55#include "llvm/IR/Argument.h"
56#include "llvm/IR/AttributeMask.h"
57#include "llvm/IR/BasicBlock.h"
58#include "llvm/IR/Constant.h"
59#include "llvm/IR/ConstantRangeList.h"
60#include "llvm/IR/Constants.h"
61#include "llvm/IR/DataLayout.h"
62#include "llvm/IR/DebugInfo.h"
63#include "llvm/IR/Dominators.h"
64#include "llvm/IR/Function.h"
65#include "llvm/IR/IRBuilder.h"
66#include "llvm/IR/InstIterator.h"
67#include "llvm/IR/InstrTypes.h"
68#include "llvm/IR/Instruction.h"
69#include "llvm/IR/Instructions.h"
70#include "llvm/IR/IntrinsicInst.h"
71#include "llvm/IR/Module.h"
72#include "llvm/IR/PassManager.h"
73#include "llvm/IR/PatternMatch.h"
74#include "llvm/IR/Value.h"
75#include "llvm/InitializePasses.h"
76#include "llvm/Support/Casting.h"
77#include "llvm/Support/CommandLine.h"
78#include "llvm/Support/Debug.h"
79#include "llvm/Support/DebugCounter.h"
80#include "llvm/Support/ErrorHandling.h"
81#include "llvm/Support/raw_ostream.h"
82#include "llvm/Transforms/Scalar.h"
83#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
84#include "llvm/Transforms/Utils/BuildLibCalls.h"
85#include "llvm/Transforms/Utils/Local.h"
86#include <algorithm>
87#include <cassert>
88#include <cstdint>
89#include <map>
90#include <optional>
91#include <utility>
92
93using namespace llvm;
94using namespace PatternMatch;
95
96#define DEBUG_TYPE "dse"
97
98STATISTIC(NumRemainingStores, "Number of stores remaining after DSE");
99STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
100STATISTIC(NumFastStores, "Number of stores deleted");
101STATISTIC(NumFastOther, "Number of other instrs removed");
102STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
103STATISTIC(NumModifiedStores, "Number of stores modified");
104STATISTIC(NumCFGChecks, "Number of stores modified");
105STATISTIC(NumCFGTries, "Number of stores modified");
106STATISTIC(NumCFGSuccess, "Number of stores modified");
107STATISTIC(NumGetDomMemoryDefPassed,
108 "Number of times a valid candidate is returned from getDomMemoryDef");
109STATISTIC(NumDomMemDefChecks,
110 "Number iterations check for reads in getDomMemoryDef");
111
112DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa",
113 "Controls which MemoryDefs are eliminated.");
114
115static cl::opt<bool>
116EnablePartialOverwriteTracking("enable-dse-partial-overwrite-tracking",
117 cl::init(Val: true), cl::Hidden,
118 cl::desc("Enable partial-overwrite tracking in DSE"));
119
120static cl::opt<bool>
121EnablePartialStoreMerging("enable-dse-partial-store-merging",
122 cl::init(Val: true), cl::Hidden,
123 cl::desc("Enable partial store merging in DSE"));
124
125static cl::opt<unsigned>
126 MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(Val: 150), cl::Hidden,
127 cl::desc("The number of memory instructions to scan for "
128 "dead store elimination (default = 150)"));
129static cl::opt<unsigned> MemorySSAUpwardsStepLimit(
130 "dse-memoryssa-walklimit", cl::init(Val: 90), cl::Hidden,
131 cl::desc("The maximum number of steps while walking upwards to find "
132 "MemoryDefs that may be killed (default = 90)"));
133
134static cl::opt<unsigned> MemorySSAPartialStoreLimit(
135 "dse-memoryssa-partial-store-limit", cl::init(Val: 5), cl::Hidden,
136 cl::desc("The maximum number candidates that only partially overwrite the "
137 "killing MemoryDef to consider"
138 " (default = 5)"));
139
140static cl::opt<unsigned> MemorySSADefsPerBlockLimit(
141 "dse-memoryssa-defs-per-block-limit", cl::init(Val: 5000), cl::Hidden,
142 cl::desc("The number of MemoryDefs we consider as candidates to eliminated "
143 "other stores per basic block (default = 5000)"));
144
145static cl::opt<unsigned> MemorySSASameBBStepCost(
146 "dse-memoryssa-samebb-cost", cl::init(Val: 1), cl::Hidden,
147 cl::desc(
148 "The cost of a step in the same basic block as the killing MemoryDef"
149 "(default = 1)"));
150
151static cl::opt<unsigned>
152 MemorySSAOtherBBStepCost("dse-memoryssa-otherbb-cost", cl::init(Val: 5),
153 cl::Hidden,
154 cl::desc("The cost of a step in a different basic "
155 "block than the killing MemoryDef"
156 "(default = 5)"));
157
158static cl::opt<unsigned> MemorySSAPathCheckLimit(
159 "dse-memoryssa-path-check-limit", cl::init(Val: 50), cl::Hidden,
160 cl::desc("The maximum number of blocks to check when trying to prove that "
161 "all paths to an exit go through a killing block (default = 50)"));
162
163// This flags allows or disallows DSE to optimize MemorySSA during its
164// traversal. Note that DSE optimizing MemorySSA may impact other passes
165// downstream of the DSE invocation and can lead to issues not being
166// reproducible in isolation (i.e. when MemorySSA is built from scratch). In
167// those cases, the flag can be used to check if DSE's MemorySSA optimizations
168// impact follow-up passes.
169static cl::opt<bool>
170 OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(Val: true), cl::Hidden,
171 cl::desc("Allow DSE to optimize memory accesses."));
172
173// TODO: remove this flag.
174static cl::opt<bool> EnableInitializesImprovement(
175 "enable-dse-initializes-attr-improvement", cl::init(Val: true), cl::Hidden,
176 cl::desc("Enable the initializes attr improvement in DSE"));
177
178static cl::opt<unsigned> MaxDepthRecursion(
179 "dse-max-dom-cond-depth", cl::init(Val: 1024), cl::Hidden,
180 cl::desc("Max dominator tree recursion depth for eliminating redundant "
181 "stores via dominating conditions"));
182
183//===----------------------------------------------------------------------===//
184// Helper functions
185//===----------------------------------------------------------------------===//
186using OverlapIntervalsTy = std::map<int64_t, int64_t>;
187using InstOverlapIntervalsTy = MapVector<Instruction *, OverlapIntervalsTy>;
188
189/// Returns true if the end of this instruction can be safely shortened in
190/// length.
191static bool isShortenableAtTheEnd(Instruction *I) {
192 // Don't shorten stores for now
193 if (isa<StoreInst>(Val: I))
194 return false;
195
196 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: I)) {
197 switch (II->getIntrinsicID()) {
198 default: return false;
199 case Intrinsic::memset:
200 case Intrinsic::memcpy:
201 case Intrinsic::memcpy_element_unordered_atomic:
202 case Intrinsic::memset_element_unordered_atomic:
203 // Do shorten memory intrinsics.
204 // FIXME: Add memmove if it's also safe to transform.
205 return true;
206 }
207 }
208
209 // Don't shorten libcalls calls for now.
210
211 return false;
212}
213
214/// Returns true if the beginning of this instruction can be safely shortened
215/// in length.
216static bool isShortenableAtTheBeginning(Instruction *I) {
217 // FIXME: Handle only memset for now. Supporting memcpy/memmove should be
218 // easily done by offsetting the source address.
219 return isa<AnyMemSetInst>(Val: I);
220}
221
222static std::optional<TypeSize> getPointerSize(const Value *V,
223 const DataLayout &DL,
224 const TargetLibraryInfo &TLI,
225 const Function *F) {
226 uint64_t Size;
227 ObjectSizeOpts Opts;
228 Opts.NullIsUnknownSize = NullPointerIsDefined(F);
229
230 if (getObjectSize(Ptr: V, Size, DL, TLI: &TLI, Opts))
231 return TypeSize::getFixed(ExactSize: Size);
232 return std::nullopt;
233}
234
235namespace {
236
237enum OverwriteResult {
238 OW_Begin,
239 OW_Complete,
240 OW_End,
241 OW_PartialEarlierWithFullLater,
242 OW_MaybePartial,
243 OW_None,
244 OW_Unknown
245};
246
247} // end anonymous namespace
248
249/// Check if two instruction are masked stores that completely
250/// overwrite one another. More specifically, \p KillingI has to
251/// overwrite \p DeadI.
252static OverwriteResult isMaskedStoreOverwrite(const Instruction *KillingI,
253 const Instruction *DeadI,
254 BatchAAResults &AA) {
255 const auto *KillingII = dyn_cast<IntrinsicInst>(Val: KillingI);
256 const auto *DeadII = dyn_cast<IntrinsicInst>(Val: DeadI);
257 if (KillingII == nullptr || DeadII == nullptr)
258 return OW_Unknown;
259 if (KillingII->getIntrinsicID() != DeadII->getIntrinsicID())
260 return OW_Unknown;
261
262 switch (KillingII->getIntrinsicID()) {
263 case Intrinsic::masked_store:
264 case Intrinsic::vp_store: {
265 const DataLayout &DL = KillingII->getDataLayout();
266 auto *KillingTy = KillingII->getArgOperand(i: 0)->getType();
267 auto *DeadTy = DeadII->getArgOperand(i: 0)->getType();
268 if (DL.getTypeSizeInBits(Ty: KillingTy) != DL.getTypeSizeInBits(Ty: DeadTy))
269 return OW_Unknown;
270 // Element count.
271 if (cast<VectorType>(Val: KillingTy)->getElementCount() !=
272 cast<VectorType>(Val: DeadTy)->getElementCount())
273 return OW_Unknown;
274 // Pointers.
275 Value *KillingPtr = KillingII->getArgOperand(i: 1);
276 Value *DeadPtr = DeadII->getArgOperand(i: 1);
277 if (KillingPtr != DeadPtr && !AA.isMustAlias(V1: KillingPtr, V2: DeadPtr))
278 return OW_Unknown;
279 if (KillingII->getIntrinsicID() == Intrinsic::masked_store) {
280 // Masks.
281 // TODO: check that KillingII's mask is a superset of the DeadII's mask.
282 if (KillingII->getArgOperand(i: 2) != DeadII->getArgOperand(i: 2))
283 return OW_Unknown;
284 } else if (KillingII->getIntrinsicID() == Intrinsic::vp_store) {
285 // Masks.
286 // TODO: check that KillingII's mask is a superset of the DeadII's mask.
287 if (KillingII->getArgOperand(i: 2) != DeadII->getArgOperand(i: 2))
288 return OW_Unknown;
289 // Lengths.
290 if (KillingII->getArgOperand(i: 3) != DeadII->getArgOperand(i: 3))
291 return OW_Unknown;
292 }
293 return OW_Complete;
294 }
295 default:
296 return OW_Unknown;
297 }
298}
299
300/// Return 'OW_Complete' if a store to the 'KillingLoc' location completely
301/// overwrites a store to the 'DeadLoc' location, 'OW_End' if the end of the
302/// 'DeadLoc' location is completely overwritten by 'KillingLoc', 'OW_Begin'
303/// if the beginning of the 'DeadLoc' location is overwritten by 'KillingLoc'.
304/// 'OW_PartialEarlierWithFullLater' means that a dead (big) store was
305/// overwritten by a killing (smaller) store which doesn't write outside the big
306/// store's memory locations. Returns 'OW_Unknown' if nothing can be determined.
307/// NOTE: This function must only be called if both \p KillingLoc and \p
308/// DeadLoc belong to the same underlying object with valid \p KillingOff and
309/// \p DeadOff.
310static OverwriteResult isPartialOverwrite(const MemoryLocation &KillingLoc,
311 const MemoryLocation &DeadLoc,
312 int64_t KillingOff, int64_t DeadOff,
313 Instruction *DeadI,
314 InstOverlapIntervalsTy &IOL) {
315 const uint64_t KillingSize = KillingLoc.Size.getValue();
316 const uint64_t DeadSize = DeadLoc.Size.getValue();
317 // We may now overlap, although the overlap is not complete. There might also
318 // be other incomplete overlaps, and together, they might cover the complete
319 // dead store.
320 // Note: The correctness of this logic depends on the fact that this function
321 // is not even called providing DepWrite when there are any intervening reads.
322 if (EnablePartialOverwriteTracking &&
323 KillingOff < int64_t(DeadOff + DeadSize) &&
324 int64_t(KillingOff + KillingSize) >= DeadOff) {
325
326 // Insert our part of the overlap into the map.
327 auto &IM = IOL[DeadI];
328 LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: DeadLoc [" << DeadOff << ", "
329 << int64_t(DeadOff + DeadSize) << ") KillingLoc ["
330 << KillingOff << ", " << int64_t(KillingOff + KillingSize)
331 << ")\n");
332
333 // Make sure that we only insert non-overlapping intervals and combine
334 // adjacent intervals. The intervals are stored in the map with the ending
335 // offset as the key (in the half-open sense) and the starting offset as
336 // the value.
337 int64_t KillingIntStart = KillingOff;
338 int64_t KillingIntEnd = KillingOff + KillingSize;
339
340 // Find any intervals ending at, or after, KillingIntStart which start
341 // before KillingIntEnd.
342 auto ILI = IM.lower_bound(x: KillingIntStart);
343 if (ILI != IM.end() && ILI->second <= KillingIntEnd) {
344 // This existing interval is overlapped with the current store somewhere
345 // in [KillingIntStart, KillingIntEnd]. Merge them by erasing the existing
346 // intervals and adjusting our start and end.
347 KillingIntStart = std::min(a: KillingIntStart, b: ILI->second);
348 KillingIntEnd = std::max(a: KillingIntEnd, b: ILI->first);
349 ILI = IM.erase(position: ILI);
350
351 // Continue erasing and adjusting our end in case other previous
352 // intervals are also overlapped with the current store.
353 //
354 // |--- dead 1 ---| |--- dead 2 ---|
355 // |------- killing---------|
356 //
357 while (ILI != IM.end() && ILI->second <= KillingIntEnd) {
358 assert(ILI->second > KillingIntStart && "Unexpected interval");
359 KillingIntEnd = std::max(a: KillingIntEnd, b: ILI->first);
360 ILI = IM.erase(position: ILI);
361 }
362 }
363
364 IM[KillingIntEnd] = KillingIntStart;
365
366 ILI = IM.begin();
367 if (ILI->second <= DeadOff && ILI->first >= int64_t(DeadOff + DeadSize)) {
368 LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: DeadLoc ["
369 << DeadOff << ", " << int64_t(DeadOff + DeadSize)
370 << ") Composite KillingLoc [" << ILI->second << ", "
371 << ILI->first << ")\n");
372 ++NumCompletePartials;
373 return OW_Complete;
374 }
375 }
376
377 // Check for a dead store which writes to all the memory locations that
378 // the killing store writes to.
379 if (EnablePartialStoreMerging && KillingOff >= DeadOff &&
380 int64_t(DeadOff + DeadSize) > KillingOff &&
381 uint64_t(KillingOff - DeadOff) + KillingSize <= DeadSize) {
382 LLVM_DEBUG(dbgs() << "DSE: Partial overwrite a dead load [" << DeadOff
383 << ", " << int64_t(DeadOff + DeadSize)
384 << ") by a killing store [" << KillingOff << ", "
385 << int64_t(KillingOff + KillingSize) << ")\n");
386 // TODO: Maybe come up with a better name?
387 return OW_PartialEarlierWithFullLater;
388 }
389
390 // Another interesting case is if the killing store overwrites the end of the
391 // dead store.
392 //
393 // |--dead--|
394 // |-- killing --|
395 //
396 // In this case we may want to trim the size of dead store to avoid
397 // generating stores to addresses which will definitely be overwritten killing
398 // store.
399 if (!EnablePartialOverwriteTracking &&
400 (KillingOff > DeadOff && KillingOff < int64_t(DeadOff + DeadSize) &&
401 int64_t(KillingOff + KillingSize) >= int64_t(DeadOff + DeadSize)))
402 return OW_End;
403
404 // Finally, we also need to check if the killing store overwrites the
405 // beginning of the dead store.
406 //
407 // |--dead--|
408 // |-- killing --|
409 //
410 // In this case we may want to move the destination address and trim the size
411 // of dead store to avoid generating stores to addresses which will definitely
412 // be overwritten killing store.
413 if (!EnablePartialOverwriteTracking &&
414 (KillingOff <= DeadOff && int64_t(KillingOff + KillingSize) > DeadOff)) {
415 assert(int64_t(KillingOff + KillingSize) < int64_t(DeadOff + DeadSize) &&
416 "Expect to be handled as OW_Complete");
417 return OW_Begin;
418 }
419 // Otherwise, they don't completely overlap.
420 return OW_Unknown;
421}
422
423/// Returns true if the memory which is accessed by the second instruction is not
424/// modified between the first and the second instruction.
425/// Precondition: Second instruction must be dominated by the first
426/// instruction.
427static bool
428memoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI,
429 BatchAAResults &AA, const DataLayout &DL,
430 DominatorTree *DT) {
431 // Do a backwards scan through the CFG from SecondI to FirstI. Look for
432 // instructions which can modify the memory location accessed by SecondI.
433 //
434 // While doing the walk keep track of the address to check. It might be
435 // different in different basic blocks due to PHI translation.
436 using BlockAddressPair = std::pair<BasicBlock *, PHITransAddr>;
437 SmallVector<BlockAddressPair, 16> WorkList;
438 // Keep track of the address we visited each block with. Bail out if we
439 // visit a block with different addresses.
440 DenseMap<BasicBlock *, Value *> Visited;
441
442 BasicBlock::iterator FirstBBI(FirstI);
443 ++FirstBBI;
444 BasicBlock::iterator SecondBBI(SecondI);
445 BasicBlock *FirstBB = FirstI->getParent();
446 BasicBlock *SecondBB = SecondI->getParent();
447 MemoryLocation MemLoc;
448 if (auto *MemSet = dyn_cast<MemSetInst>(Val: SecondI))
449 MemLoc = MemoryLocation::getForDest(MI: MemSet);
450 else
451 MemLoc = MemoryLocation::get(Inst: SecondI);
452
453 auto *MemLocPtr = const_cast<Value *>(MemLoc.Ptr);
454
455 // Start checking the SecondBB.
456 WorkList.push_back(
457 Elt: std::make_pair(x&: SecondBB, y: PHITransAddr(MemLocPtr, DL, nullptr)));
458 bool isFirstBlock = true;
459
460 // Check all blocks going backward until we reach the FirstBB.
461 while (!WorkList.empty()) {
462 BlockAddressPair Current = WorkList.pop_back_val();
463 BasicBlock *B = Current.first;
464 PHITransAddr &Addr = Current.second;
465 Value *Ptr = Addr.getAddr();
466
467 // Ignore instructions before FirstI if this is the FirstBB.
468 BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
469
470 BasicBlock::iterator EI;
471 if (isFirstBlock) {
472 // Ignore instructions after SecondI if this is the first visit of SecondBB.
473 assert(B == SecondBB && "first block is not the store block");
474 EI = SecondBBI;
475 isFirstBlock = false;
476 } else {
477 // It's not SecondBB or (in case of a loop) the second visit of SecondBB.
478 // In this case we also have to look at instructions after SecondI.
479 EI = B->end();
480 }
481 for (; BI != EI; ++BI) {
482 Instruction *I = &*BI;
483 if (I->mayWriteToMemory() && I != SecondI)
484 if (isModSet(MRI: AA.getModRefInfo(I, OptLoc: MemLoc.getWithNewPtr(NewPtr: Ptr))))
485 return false;
486 }
487 if (B != FirstBB) {
488 assert(B != &FirstBB->getParent()->getEntryBlock() &&
489 "Should not hit the entry block because SI must be dominated by LI");
490 for (BasicBlock *Pred : predecessors(BB: B)) {
491 PHITransAddr PredAddr = Addr;
492 if (PredAddr.needsPHITranslationFromBlock(BB: B)) {
493 if (!PredAddr.isPotentiallyPHITranslatable())
494 return false;
495 if (!PredAddr.translateValue(CurBB: B, PredBB: Pred, DT, MustDominate: false))
496 return false;
497 }
498 Value *TranslatedPtr = PredAddr.getAddr();
499 auto Inserted = Visited.insert(KV: std::make_pair(x&: Pred, y&: TranslatedPtr));
500 if (!Inserted.second) {
501 // We already visited this block before. If it was with a different
502 // address - bail out!
503 if (TranslatedPtr != Inserted.first->second)
504 return false;
505 // ... otherwise just skip it.
506 continue;
507 }
508 WorkList.push_back(Elt: std::make_pair(x&: Pred, y&: PredAddr));
509 }
510 }
511 }
512 return true;
513}
514
515static void shortenAssignment(Instruction *Inst, Value *OriginalDest,
516 uint64_t OldOffsetInBits, uint64_t OldSizeInBits,
517 uint64_t NewSizeInBits, bool IsOverwriteEnd) {
518 const DataLayout &DL = Inst->getDataLayout();
519 uint64_t DeadSliceSizeInBits = OldSizeInBits - NewSizeInBits;
520 uint64_t DeadSliceOffsetInBits =
521 OldOffsetInBits + (IsOverwriteEnd ? NewSizeInBits : 0);
522 auto SetDeadFragExpr = [](auto *Assign,
523 DIExpression::FragmentInfo DeadFragment) {
524 // createFragmentExpression expects an offset relative to the existing
525 // fragment offset if there is one.
526 uint64_t RelativeOffset = DeadFragment.OffsetInBits -
527 Assign->getExpression()
528 ->getFragmentInfo()
529 .value_or(DIExpression::FragmentInfo(0, 0))
530 .OffsetInBits;
531 if (auto NewExpr = DIExpression::createFragmentExpression(
532 Expr: Assign->getExpression(), OffsetInBits: RelativeOffset, SizeInBits: DeadFragment.SizeInBits)) {
533 Assign->setExpression(*NewExpr);
534 return;
535 }
536 // Failed to create a fragment expression for this so discard the value,
537 // making this a kill location.
538 auto *Expr = *DIExpression::createFragmentExpression(
539 Expr: DIExpression::get(Context&: Assign->getContext(), Elements: {}), OffsetInBits: DeadFragment.OffsetInBits,
540 SizeInBits: DeadFragment.SizeInBits);
541 Assign->setExpression(Expr);
542 Assign->setKillLocation();
543 };
544
545 // A DIAssignID to use so that the inserted dbg.assign intrinsics do not
546 // link to any instructions. Created in the loop below (once).
547 DIAssignID *LinkToNothing = nullptr;
548 LLVMContext &Ctx = Inst->getContext();
549 auto GetDeadLink = [&Ctx, &LinkToNothing]() {
550 if (!LinkToNothing)
551 LinkToNothing = DIAssignID::getDistinct(Context&: Ctx);
552 return LinkToNothing;
553 };
554
555 // Insert an unlinked dbg.assign intrinsic for the dead fragment after each
556 // overlapping dbg.assign intrinsic.
557 for (DbgVariableRecord *Assign : at::getDVRAssignmentMarkers(Inst)) {
558 std::optional<DIExpression::FragmentInfo> NewFragment;
559 if (!at::calculateFragmentIntersect(DL, Dest: OriginalDest, SliceOffsetInBits: DeadSliceOffsetInBits,
560 SliceSizeInBits: DeadSliceSizeInBits, DVRAssign: Assign,
561 Result&: NewFragment) ||
562 !NewFragment) {
563 // We couldn't calculate the intersecting fragment for some reason. Be
564 // cautious and unlink the whole assignment from the store.
565 Assign->setKillAddress();
566 Assign->setAssignId(GetDeadLink());
567 continue;
568 }
569 // No intersect.
570 if (NewFragment->SizeInBits == 0)
571 continue;
572
573 // Fragments overlap: insert a new dbg.assign for this dead part.
574 auto *NewAssign = static_cast<decltype(Assign)>(Assign->clone());
575 NewAssign->insertAfter(InsertAfter: Assign->getIterator());
576 NewAssign->setAssignId(GetDeadLink());
577 if (NewFragment)
578 SetDeadFragExpr(NewAssign, *NewFragment);
579 NewAssign->setKillAddress();
580 }
581}
582
583/// Update the attributes given that a memory access is updated (the
584/// dereferenced pointer could be moved forward when shortening a
585/// mem intrinsic).
586static void adjustArgAttributes(AnyMemIntrinsic *Intrinsic, unsigned ArgNo,
587 uint64_t PtrOffset) {
588 // Remember old attributes.
589 AttributeSet OldAttrs = Intrinsic->getParamAttributes(ArgNo);
590
591 // Find attributes that should be kept, and remove the rest.
592 AttributeMask AttrsToRemove;
593 for (auto &Attr : OldAttrs) {
594 if (Attr.hasKindAsEnum()) {
595 switch (Attr.getKindAsEnum()) {
596 default:
597 break;
598 case Attribute::Alignment:
599 // Only keep alignment if PtrOffset satisfy the alignment.
600 if (isAligned(Lhs: Attr.getAlignment().valueOrOne(), SizeInBytes: PtrOffset))
601 continue;
602 break;
603 case Attribute::Dereferenceable:
604 case Attribute::DereferenceableOrNull:
605 // We could reduce the size of these attributes according to
606 // PtrOffset. But we simply drop these for now.
607 break;
608 case Attribute::NonNull:
609 case Attribute::NoUndef:
610 continue;
611 }
612 }
613 AttrsToRemove.addAttribute(A: Attr);
614 }
615
616 // Remove the attributes that should be dropped.
617 Intrinsic->removeParamAttrs(ArgNo, AttrsToRemove);
618}
619
620static bool tryToShorten(Instruction *DeadI, int64_t &DeadStart,
621 uint64_t &DeadSize, int64_t KillingStart,
622 uint64_t KillingSize, bool IsOverwriteEnd) {
623 auto *DeadIntrinsic = cast<AnyMemIntrinsic>(Val: DeadI);
624 Align PrefAlign = DeadIntrinsic->getDestAlign().valueOrOne();
625
626 // We assume that memet/memcpy operates in chunks of the "largest" native
627 // type size and aligned on the same value. That means optimal start and size
628 // of memset/memcpy should be modulo of preferred alignment of that type. That
629 // is it there is no any sense in trying to reduce store size any further
630 // since any "extra" stores comes for free anyway.
631 // On the other hand, maximum alignment we can achieve is limited by alignment
632 // of initial store.
633
634 // TODO: Limit maximum alignment by preferred (or abi?) alignment of the
635 // "largest" native type.
636 // Note: What is the proper way to get that value?
637 // Should TargetTransformInfo::getRegisterBitWidth be used or anything else?
638 // PrefAlign = std::min(DL.getPrefTypeAlign(LargestType), PrefAlign);
639
640 int64_t ToRemoveStart = 0;
641 uint64_t ToRemoveSize = 0;
642 // Compute start and size of the region to remove. Make sure 'PrefAlign' is
643 // maintained on the remaining store.
644 if (IsOverwriteEnd) {
645 // Calculate required adjustment for 'KillingStart' in order to keep
646 // remaining store size aligned on 'PerfAlign'.
647 uint64_t Off =
648 offsetToAlignment(Value: uint64_t(KillingStart - DeadStart), Alignment: PrefAlign);
649 ToRemoveStart = KillingStart + Off;
650 if (DeadSize <= uint64_t(ToRemoveStart - DeadStart))
651 return false;
652 ToRemoveSize = DeadSize - uint64_t(ToRemoveStart - DeadStart);
653 } else {
654 ToRemoveStart = DeadStart;
655 assert(KillingSize >= uint64_t(DeadStart - KillingStart) &&
656 "Not overlapping accesses?");
657 ToRemoveSize = KillingSize - uint64_t(DeadStart - KillingStart);
658 // Calculate required adjustment for 'ToRemoveSize'in order to keep
659 // start of the remaining store aligned on 'PerfAlign'.
660 uint64_t Off = offsetToAlignment(Value: ToRemoveSize, Alignment: PrefAlign);
661 if (Off != 0) {
662 if (ToRemoveSize <= (PrefAlign.value() - Off))
663 return false;
664 ToRemoveSize -= PrefAlign.value() - Off;
665 }
666 assert(isAligned(PrefAlign, ToRemoveSize) &&
667 "Should preserve selected alignment");
668 }
669
670 assert(ToRemoveSize > 0 && "Shouldn't reach here if nothing to remove");
671 assert(DeadSize > ToRemoveSize && "Can't remove more than original size");
672
673 uint64_t NewSize = DeadSize - ToRemoveSize;
674 if (DeadIntrinsic->isAtomic()) {
675 // When shortening an atomic memory intrinsic, the newly shortened
676 // length must remain an integer multiple of the element size.
677 const uint32_t ElementSize = DeadIntrinsic->getElementSizeInBytes();
678 if (0 != NewSize % ElementSize)
679 return false;
680 }
681
682 LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW "
683 << (IsOverwriteEnd ? "END" : "BEGIN") << ": " << *DeadI
684 << "\n KILLER [" << ToRemoveStart << ", "
685 << int64_t(ToRemoveStart + ToRemoveSize) << ")\n");
686
687 DeadIntrinsic->setLength(NewSize);
688 DeadIntrinsic->setDestAlignment(PrefAlign);
689
690 Value *OrigDest = DeadIntrinsic->getRawDest();
691 if (!IsOverwriteEnd) {
692 Value *Indices[1] = {
693 ConstantInt::get(Ty: DeadIntrinsic->getLength()->getType(), V: ToRemoveSize)};
694 Instruction *NewDestGEP = GetElementPtrInst::CreateInBounds(
695 PointeeType: Type::getInt8Ty(C&: DeadIntrinsic->getContext()), Ptr: OrigDest, IdxList: Indices, NameStr: "",
696 InsertBefore: DeadI->getIterator());
697 NewDestGEP->setDebugLoc(DeadIntrinsic->getDebugLoc());
698 DeadIntrinsic->setDest(NewDestGEP);
699 adjustArgAttributes(Intrinsic: DeadIntrinsic, ArgNo: 0, PtrOffset: ToRemoveSize);
700 }
701
702 // Update attached dbg.assign intrinsics. Assume 8-bit byte.
703 shortenAssignment(Inst: DeadI, OriginalDest: OrigDest, OldOffsetInBits: DeadStart * 8, OldSizeInBits: DeadSize * 8, NewSizeInBits: NewSize * 8,
704 IsOverwriteEnd);
705
706 // Finally update start and size of dead access.
707 if (!IsOverwriteEnd)
708 DeadStart += ToRemoveSize;
709 DeadSize = NewSize;
710
711 return true;
712}
713
714static bool tryToShortenEnd(Instruction *DeadI, OverlapIntervalsTy &IntervalMap,
715 int64_t &DeadStart, uint64_t &DeadSize) {
716 if (IntervalMap.empty() || !isShortenableAtTheEnd(I: DeadI))
717 return false;
718
719 OverlapIntervalsTy::iterator OII = --IntervalMap.end();
720 int64_t KillingStart = OII->second;
721 uint64_t KillingSize = OII->first - KillingStart;
722
723 assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
724
725 if (KillingStart > DeadStart &&
726 // Note: "KillingStart - KillingStart" is known to be positive due to
727 // preceding check.
728 (uint64_t)(KillingStart - DeadStart) < DeadSize &&
729 // Note: "DeadSize - (uint64_t)(KillingStart - DeadStart)" is known to
730 // be non negative due to preceding checks.
731 KillingSize >= DeadSize - (uint64_t)(KillingStart - DeadStart)) {
732 if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
733 IsOverwriteEnd: true)) {
734 IntervalMap.erase(position: OII);
735 return true;
736 }
737 }
738 return false;
739}
740
741static bool tryToShortenBegin(Instruction *DeadI,
742 OverlapIntervalsTy &IntervalMap,
743 int64_t &DeadStart, uint64_t &DeadSize) {
744 if (IntervalMap.empty() || !isShortenableAtTheBeginning(I: DeadI))
745 return false;
746
747 OverlapIntervalsTy::iterator OII = IntervalMap.begin();
748 int64_t KillingStart = OII->second;
749 uint64_t KillingSize = OII->first - KillingStart;
750
751 assert(OII->first - KillingStart >= 0 && "Size expected to be positive");
752
753 if (KillingStart <= DeadStart &&
754 // Note: "DeadStart - KillingStart" is known to be non negative due to
755 // preceding check.
756 KillingSize > (uint64_t)(DeadStart - KillingStart)) {
757 // Note: "KillingSize - (uint64_t)(DeadStart - DeadStart)" is known to
758 // be positive due to preceding checks.
759 assert(KillingSize - (uint64_t)(DeadStart - KillingStart) < DeadSize &&
760 "Should have been handled as OW_Complete");
761 if (tryToShorten(DeadI, DeadStart, DeadSize, KillingStart, KillingSize,
762 IsOverwriteEnd: false)) {
763 IntervalMap.erase(position: OII);
764 return true;
765 }
766 }
767 return false;
768}
769
770static Constant *
771tryToMergePartialOverlappingStores(StoreInst *KillingI, StoreInst *DeadI,
772 int64_t KillingOffset, int64_t DeadOffset,
773 const DataLayout &DL, BatchAAResults &AA,
774 DominatorTree *DT) {
775
776 if (DeadI && isa<ConstantInt>(Val: DeadI->getValueOperand()) &&
777 DL.typeSizeEqualsStoreSize(Ty: DeadI->getValueOperand()->getType()) &&
778 KillingI && isa<ConstantInt>(Val: KillingI->getValueOperand()) &&
779 DL.typeSizeEqualsStoreSize(Ty: KillingI->getValueOperand()->getType()) &&
780 memoryIsNotModifiedBetween(FirstI: DeadI, SecondI: KillingI, AA, DL, DT)) {
781 // If the store we find is:
782 // a) partially overwritten by the store to 'Loc'
783 // b) the killing store is fully contained in the dead one and
784 // c) they both have a constant value
785 // d) none of the two stores need padding
786 // Merge the two stores, replacing the dead store's value with a
787 // merge of both values.
788 // TODO: Deal with other constant types (vectors, etc), and probably
789 // some mem intrinsics (if needed)
790
791 APInt DeadValue = cast<ConstantInt>(Val: DeadI->getValueOperand())->getValue();
792 APInt KillingValue =
793 cast<ConstantInt>(Val: KillingI->getValueOperand())->getValue();
794 unsigned KillingBits = KillingValue.getBitWidth();
795 assert(DeadValue.getBitWidth() > KillingValue.getBitWidth());
796 KillingValue = KillingValue.zext(width: DeadValue.getBitWidth());
797
798 // Offset of the smaller store inside the larger store
799 unsigned BitOffsetDiff = (KillingOffset - DeadOffset) * 8;
800 unsigned LShiftAmount =
801 DL.isBigEndian() ? DeadValue.getBitWidth() - BitOffsetDiff - KillingBits
802 : BitOffsetDiff;
803 APInt Mask = APInt::getBitsSet(numBits: DeadValue.getBitWidth(), loBit: LShiftAmount,
804 hiBit: LShiftAmount + KillingBits);
805 // Clear the bits we'll be replacing, then OR with the smaller
806 // store, shifted appropriately.
807 APInt Merged = (DeadValue & ~Mask) | (KillingValue << LShiftAmount);
808 LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n Dead: " << *DeadI
809 << "\n Killing: " << *KillingI
810 << "\n Merged Value: " << Merged << '\n');
811 return ConstantInt::get(Ty: DeadI->getValueOperand()->getType(), V: Merged);
812 }
813 return nullptr;
814}
815
816// Returns true if \p I is an intrinsic that does not read or write memory.
817static bool isNoopIntrinsic(Instruction *I) {
818 if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: I)) {
819 switch (II->getIntrinsicID()) {
820 case Intrinsic::lifetime_start:
821 case Intrinsic::lifetime_end:
822 case Intrinsic::invariant_end:
823 case Intrinsic::launder_invariant_group:
824 case Intrinsic::assume:
825 return true;
826 case Intrinsic::dbg_declare:
827 case Intrinsic::dbg_label:
828 case Intrinsic::dbg_value:
829 llvm_unreachable("Intrinsic should not be modeled in MemorySSA");
830 default:
831 return false;
832 }
833 }
834 return false;
835}
836
837// Check if we can ignore \p D for DSE.
838static bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) {
839 Instruction *DI = D->getMemoryInst();
840 // Calls that only access inaccessible memory cannot read or write any memory
841 // locations we consider for elimination.
842 if (auto *CB = dyn_cast<CallBase>(Val: DI))
843 if (CB->onlyAccessesInaccessibleMemory())
844 return true;
845
846 // We can eliminate stores to locations not visible to the caller across
847 // throwing instructions.
848 if (DI->mayThrow() && !DefVisibleToCaller)
849 return true;
850
851 // We can remove the dead stores, irrespective of the fence and its ordering
852 // (release/acquire/seq_cst). Fences only constraints the ordering of
853 // already visible stores, it does not make a store visible to other
854 // threads. So, skipping over a fence does not change a store from being
855 // dead.
856 if (isa<FenceInst>(Val: DI))
857 return true;
858
859 // Skip intrinsics that do not really read or modify memory.
860 if (isNoopIntrinsic(I: DI))
861 return true;
862
863 return false;
864}
865
866namespace {
867
868// A memory location wrapper that represents a MemoryLocation, `MemLoc`,
869// defined by `MemDef`.
870struct MemoryLocationWrapper {
871 MemoryLocationWrapper(MemoryLocation MemLoc, MemoryDef *MemDef,
872 bool DefByInitializesAttr)
873 : MemLoc(MemLoc), MemDef(MemDef),
874 DefByInitializesAttr(DefByInitializesAttr) {
875 assert(MemLoc.Ptr && "MemLoc should be not null");
876 UnderlyingObject = getUnderlyingObject(V: MemLoc.Ptr);
877 DefInst = MemDef->getMemoryInst();
878 }
879
880 MemoryLocation MemLoc;
881 const Value *UnderlyingObject;
882 MemoryDef *MemDef;
883 Instruction *DefInst;
884 bool DefByInitializesAttr = false;
885};
886
887// A memory def wrapper that represents a MemoryDef and the MemoryLocation(s)
888// defined by this MemoryDef.
889struct MemoryDefWrapper {
890 MemoryDefWrapper(MemoryDef *MemDef,
891 ArrayRef<std::pair<MemoryLocation, bool>> MemLocations) {
892 DefInst = MemDef->getMemoryInst();
893 for (auto &[MemLoc, DefByInitializesAttr] : MemLocations)
894 DefinedLocations.push_back(
895 Elt: MemoryLocationWrapper(MemLoc, MemDef, DefByInitializesAttr));
896 }
897 Instruction *DefInst;
898 SmallVector<MemoryLocationWrapper, 1> DefinedLocations;
899};
900
901struct ArgumentInitInfo {
902 unsigned Idx;
903 bool IsDeadOrInvisibleOnUnwind;
904 ConstantRangeList Inits;
905};
906} // namespace
907
908static bool hasInitializesAttr(Instruction *I) {
909 CallBase *CB = dyn_cast<CallBase>(Val: I);
910 return CB && CB->getArgOperandWithAttribute(Kind: Attribute::Initializes);
911}
912
913// Return the intersected range list of the initializes attributes of "Args".
914// "Args" are call arguments that alias to each other.
915// If any argument in "Args" doesn't have dead_on_unwind attr and
916// "CallHasNoUnwindAttr" is false, return empty.
917static ConstantRangeList
918getIntersectedInitRangeList(ArrayRef<ArgumentInitInfo> Args,
919 bool CallHasNoUnwindAttr) {
920 if (Args.empty())
921 return {};
922
923 // To address unwind, the function should have nounwind attribute or the
924 // arguments have dead or invisible on unwind. Otherwise, return empty.
925 for (const auto &Arg : Args) {
926 if (!CallHasNoUnwindAttr && !Arg.IsDeadOrInvisibleOnUnwind)
927 return {};
928 if (Arg.Inits.empty())
929 return {};
930 }
931
932 ConstantRangeList IntersectedIntervals = Args.front().Inits;
933 for (auto &Arg : Args.drop_front())
934 IntersectedIntervals = IntersectedIntervals.intersectWith(CRL: Arg.Inits);
935
936 return IntersectedIntervals;
937}
938
939namespace {
940
941struct DSEState {
942 Function &F;
943 AliasAnalysis &AA;
944 EarliestEscapeAnalysis EA;
945
946 /// The single BatchAA instance that is used to cache AA queries. It will
947 /// not be invalidated over the whole run. This is safe, because:
948 /// 1. Only memory writes are removed, so the alias cache for memory
949 /// locations remains valid.
950 /// 2. No new instructions are added (only instructions removed), so cached
951 /// information for a deleted value cannot be accessed by a re-used new
952 /// value pointer.
953 BatchAAResults BatchAA;
954
955 MemorySSA &MSSA;
956 DominatorTree &DT;
957 PostDominatorTree &PDT;
958 const TargetLibraryInfo &TLI;
959 const DataLayout &DL;
960 const CycleInfo &CI;
961
962 // All MemoryDefs that potentially could kill other MemDefs.
963 SmallVector<MemoryDef *, 64> MemDefs;
964 // Any that should be skipped as they are already deleted
965 SmallPtrSet<MemoryAccess *, 4> SkipStores;
966 // Keep track whether a given object is captured before return or not.
967 DenseMap<const Value *, bool> CapturedBeforeReturn;
968 // Keep track of all of the objects that are invisible to the caller after
969 // the function returns.
970 DenseMap<const Value *, bool> InvisibleToCallerAfterRet;
971 DenseMap<const Value *, uint64_t> InvisibleToCallerAfterRetBounded;
972 // Keep track of blocks with throwing instructions not modeled in MemorySSA.
973 SmallPtrSet<BasicBlock *, 16> ThrowingBlocks;
974 // Post-order numbers for each basic block. Used to figure out if memory
975 // accesses are executed before another access.
976 DenseMap<BasicBlock *, unsigned> PostOrderNumbers;
977
978 /// Keep track of instructions (partly) overlapping with killing MemoryDefs per
979 /// basic block.
980 MapVector<BasicBlock *, InstOverlapIntervalsTy> IOLs;
981 // Check if there are root nodes that are terminated by UnreachableInst.
982 // Those roots pessimize post-dominance queries. If there are such roots,
983 // fall back to CFG scan starting from all non-unreachable roots.
984 bool AnyUnreachableExit;
985
986 // Whether or not we should iterate on removing dead stores at the end of the
987 // function due to removing a store causing a previously captured pointer to
988 // no longer be captured.
989 bool ShouldIterateEndOfFunctionDSE;
990
991 /// Dead instructions to be removed at the end of DSE.
992 SmallVector<Instruction *> ToRemove;
993
994 // Class contains self-reference, make sure it's not copied/moved.
995 DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
996 PostDominatorTree &PDT, const TargetLibraryInfo &TLI,
997 const CycleInfo &CI);
998 DSEState(const DSEState &) = delete;
999 DSEState &operator=(const DSEState &) = delete;
1000
1001 LocationSize strengthenLocationSize(const Instruction *I,
1002 LocationSize Size) const;
1003
1004 /// Return 'OW_Complete' if a store to the 'KillingLoc' location (by \p
1005 /// KillingI instruction) completely overwrites a store to the 'DeadLoc'
1006 /// location (by \p DeadI instruction).
1007 /// Return OW_MaybePartial if \p KillingI does not completely overwrite
1008 /// \p DeadI, but they both write to the same underlying object. In that
1009 /// case, use isPartialOverwrite to check if \p KillingI partially overwrites
1010 /// \p DeadI. Returns 'OR_None' if \p KillingI is known to not overwrite the
1011 /// \p DeadI. Returns 'OW_Unknown' if nothing can be determined.
1012 OverwriteResult isOverwrite(const Instruction *KillingI,
1013 const Instruction *DeadI,
1014 const MemoryLocation &KillingLoc,
1015 const MemoryLocation &DeadLoc,
1016 int64_t &KillingOff, int64_t &DeadOff);
1017
1018 bool isInvisibleToCallerAfterRet(const Value *V, const Value *Ptr,
1019 const LocationSize StoreSize);
1020
1021 bool isInvisibleToCallerOnUnwind(const Value *V);
1022
1023 std::optional<MemoryLocation> getLocForWrite(Instruction *I) const;
1024
1025 // Returns a list of <MemoryLocation, bool> pairs written by I.
1026 // The bool means whether the write is from Initializes attr.
1027 SmallVector<std::pair<MemoryLocation, bool>, 1>
1028 getLocForInst(Instruction *I, bool ConsiderInitializesAttr);
1029
1030 /// Assuming this instruction has a dead analyzable write, can we delete
1031 /// this instruction?
1032 bool isRemovable(Instruction *I);
1033
1034 /// Returns true if \p UseInst completely overwrites \p DefLoc
1035 /// (stored by \p DefInst).
1036 bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,
1037 Instruction *UseInst);
1038
1039 /// Returns true if \p Def is not read before returning from the function.
1040 bool isWriteAtEndOfFunction(MemoryDef *Def, const MemoryLocation &DefLoc);
1041
1042 /// If \p I is a memory terminator like llvm.lifetime.end or free, return a
1043 /// pair with the MemoryLocation terminated by \p I and a boolean flag
1044 /// indicating whether \p I is a free-like call.
1045 std::optional<std::pair<MemoryLocation, bool>>
1046 getLocForTerminator(Instruction *I) const;
1047
1048 /// Returns true if \p I is a memory terminator instruction like
1049 /// llvm.lifetime.end or free.
1050 bool isMemTerminatorInst(Instruction *I) const;
1051
1052 /// Returns true if \p MaybeTerm is a memory terminator for \p Loc from
1053 /// instruction \p AccessI.
1054 bool isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
1055 Instruction *MaybeTerm);
1056
1057 // Returns true if \p Use may read from \p DefLoc.
1058 bool isReadClobber(const MemoryLocation &DefLoc, Instruction *UseInst);
1059
1060 /// Returns true if a dependency between \p Current and \p KillingDef is
1061 /// guaranteed to be loop invariant for the loops that they are in. Either
1062 /// because they are known to be in the same block, in the same loop level or
1063 /// by guaranteeing that \p CurrentLoc only references a single MemoryLocation
1064 /// during execution of the containing function.
1065 bool isGuaranteedLoopIndependent(const Instruction *Current,
1066 const Instruction *KillingDef,
1067 const MemoryLocation &CurrentLoc);
1068
1069 /// Returns true if \p Ptr is guaranteed to be loop invariant for any possible
1070 /// loop. In particular, this guarantees that it only references a single
1071 /// MemoryLocation during execution of the containing function.
1072 bool isGuaranteedLoopInvariant(const Value *Ptr);
1073
1074 // Find a MemoryDef writing to \p KillingLoc and dominating \p StartAccess,
1075 // with no read access between them or on any other path to a function exit
1076 // block if \p KillingLoc is not accessible after the function returns. If
1077 // there is no such MemoryDef, return std::nullopt. The returned value may not
1078 // (completely) overwrite \p KillingLoc. Currently we bail out when we
1079 // encounter an aliasing MemoryUse (read).
1080 std::optional<MemoryAccess *>
1081 getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
1082 const MemoryLocation &KillingLoc, const Value *KillingUndObj,
1083 unsigned &ScanLimit, unsigned &WalkerStepLimit,
1084 bool IsMemTerm, unsigned &PartialLimit,
1085 bool IsInitializesAttrMemLoc);
1086
1087 /// Delete dead memory defs and recursively add their operands to ToRemove if
1088 /// they became dead.
1089 void
1090 deleteDeadInstruction(Instruction *SI,
1091 SmallPtrSetImpl<MemoryAccess *> *Deleted = nullptr);
1092
1093 // Check for any extra throws between \p KillingI and \p DeadI that block
1094 // DSE. This only checks extra maythrows (those that aren't MemoryDef's).
1095 // MemoryDef that may throw are handled during the walk from one def to the
1096 // next.
1097 bool mayThrowBetween(Instruction *KillingI, Instruction *DeadI,
1098 const Value *KillingUndObj);
1099
1100 // Check if \p DeadI acts as a DSE barrier for \p KillingI. The following
1101 // instructions act as barriers:
1102 // * A memory instruction that may throw and \p KillingI accesses a non-stack
1103 // object.
1104 // * Atomic stores stronger that monotonic.
1105 bool isDSEBarrier(const Value *KillingUndObj, Instruction *DeadI);
1106
1107 /// Eliminate writes to objects that are not visible in the caller and are not
1108 /// accessed before returning from the function.
1109 bool eliminateDeadWritesAtEndOfFunction();
1110
1111 /// If we have a zero initializing memset following a call to malloc,
1112 /// try folding it into a call to calloc.
1113 bool tryFoldIntoCalloc(MemoryDef *Def, const Value *DefUO);
1114
1115 /// \returns true if \p Def is a no-op store, either because it
1116 /// directly stores back a loaded value or stores zero to a calloced object.
1117 bool storeIsNoop(MemoryDef *Def, const Value *DefUO);
1118
1119 bool removePartiallyOverlappedStores(InstOverlapIntervalsTy &IOL);
1120
1121 /// Eliminates writes to locations where the value that is being written
1122 /// is already stored at the same location.
1123 bool eliminateRedundantStoresOfExistingValues();
1124
1125 /// If there is a dominating condition that implies the value being stored in
1126 /// a pointer, and such a condition appears in a node that dominates the
1127 /// store, then the store may be redundant if no write occurs in between.
1128 bool eliminateRedundantStoresViaDominatingConditions();
1129
1130 // Return the locations written by the initializes attribute.
1131 // Note that this function considers:
1132 // 1. Unwind edge: use "initializes" attribute only if the callee has
1133 // "nounwind" attribute, or the argument has "dead_on_unwind" attribute,
1134 // or the argument is invisible to caller on unwind. That is, we don't
1135 // perform incorrect DSE on unwind edges in the current function.
1136 // 2. Argument alias: for aliasing arguments, the "initializes" attribute is
1137 // the intersected range list of their "initializes" attributes.
1138 SmallVector<MemoryLocation, 1> getInitializesArgMemLoc(const Instruction *I);
1139
1140 // Try to eliminate dead defs that access `KillingLocWrapper.MemLoc` and are
1141 // killed by `KillingLocWrapper.MemDef`. Return whether
1142 // any changes were made, and whether `KillingLocWrapper.DefInst` was deleted.
1143 std::pair<bool, bool>
1144 eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper);
1145
1146 // Try to eliminate dead defs killed by `KillingDefWrapper` and return the
1147 // change state: whether make any change.
1148 bool eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper);
1149};
1150
1151} // end anonymous namespace
1152
1153static void pushMemUses(MemoryAccess *Acc,
1154 SmallVectorImpl<MemoryAccess *> &WorkList,
1155 SmallPtrSetImpl<MemoryAccess *> &Visited) {
1156 for (Use &U : Acc->uses()) {
1157 auto *MA = cast<MemoryAccess>(Val: U.getUser());
1158 if (Visited.insert(Ptr: MA).second)
1159 WorkList.push_back(Elt: MA);
1160 }
1161}
1162
1163// Return true if "Arg" is function local and isn't captured before "CB".
1164static bool isFuncLocalAndNotCaptured(Value *Arg, const CallBase *CB,
1165 EarliestEscapeAnalysis &EA) {
1166 const Value *UnderlyingObj = getUnderlyingObject(V: Arg);
1167 return isIdentifiedFunctionLocal(V: UnderlyingObj) &&
1168 capturesNothing(
1169 CC: EA.getCapturesBefore(Object: UnderlyingObj, I: CB, /*OrAt*/ true));
1170}
1171
1172DSEState::DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
1173 DominatorTree &DT, PostDominatorTree &PDT,
1174 const TargetLibraryInfo &TLI, const CycleInfo &CI)
1175 : F(F), AA(AA), EA(DT, nullptr, &CI), BatchAA(AA, &EA), MSSA(MSSA), DT(DT),
1176 PDT(PDT), TLI(TLI), DL(F.getDataLayout()), CI(CI) {
1177 // Collect blocks with throwing instructions not modeled in MemorySSA and
1178 // alloc-like objects.
1179 unsigned PO = 0;
1180 for (BasicBlock *BB : post_order(G: &F)) {
1181 PostOrderNumbers[BB] = PO++;
1182 for (Instruction &I : *BB) {
1183 MemoryAccess *MA = MSSA.getMemoryAccess(I: &I);
1184 if (I.mayThrow() && !MA)
1185 ThrowingBlocks.insert(Ptr: I.getParent());
1186
1187 auto *MD = dyn_cast_or_null<MemoryDef>(Val: MA);
1188 if (MD && MemDefs.size() < MemorySSADefsPerBlockLimit &&
1189 (getLocForWrite(I: &I) || isMemTerminatorInst(I: &I) ||
1190 (EnableInitializesImprovement && hasInitializesAttr(I: &I))))
1191 MemDefs.push_back(Elt: MD);
1192 }
1193 }
1194
1195 // Treat byval, inalloca or dead on return arguments the same as Allocas,
1196 // stores to them are dead at the end of the function.
1197 for (Argument &AI : F.args()) {
1198 if (AI.hasPassPointeeByValueCopyAttr()) {
1199 InvisibleToCallerAfterRet.insert(KV: {&AI, true});
1200 continue;
1201 }
1202
1203 if (!AI.getType()->isPointerTy())
1204 continue;
1205
1206 const DeadOnReturnInfo &Info = AI.getDeadOnReturnInfo();
1207 if (Info.coversAllReachableMemory())
1208 InvisibleToCallerAfterRet.insert(KV: {&AI, true});
1209 else if (uint64_t DeadBytes = Info.getNumberOfDeadBytes())
1210 InvisibleToCallerAfterRetBounded.insert(KV: {&AI, DeadBytes});
1211 }
1212
1213 AnyUnreachableExit = any_of(Range: PDT.roots(), P: [](const BasicBlock *E) {
1214 return isa<UnreachableInst>(Val: E->getTerminator());
1215 });
1216}
1217
1218LocationSize DSEState::strengthenLocationSize(const Instruction *I,
1219 LocationSize Size) const {
1220 if (auto *CB = dyn_cast<CallBase>(Val: I)) {
1221 LibFunc F;
1222 if (TLI.getLibFunc(CB: *CB, F) && TLI.has(F) &&
1223 (F == LibFunc_memset_chk || F == LibFunc_memcpy_chk)) {
1224 // Use the precise location size specified by the 3rd argument
1225 // for determining KillingI overwrites DeadLoc if it is a memset_chk
1226 // instruction. memset_chk will write either the amount specified as 3rd
1227 // argument or the function will immediately abort and exit the program.
1228 // NOTE: AA may determine NoAlias if it can prove that the access size
1229 // is larger than the allocation size due to that being UB. To avoid
1230 // returning potentially invalid NoAlias results by AA, limit the use of
1231 // the precise location size to isOverwrite.
1232 if (const auto *Len = dyn_cast<ConstantInt>(Val: CB->getArgOperand(i: 2)))
1233 return LocationSize::precise(Value: Len->getZExtValue());
1234 }
1235 }
1236 return Size;
1237}
1238
1239OverwriteResult DSEState::isOverwrite(const Instruction *KillingI,
1240 const Instruction *DeadI,
1241 const MemoryLocation &KillingLoc,
1242 const MemoryLocation &DeadLoc,
1243 int64_t &KillingOff, int64_t &DeadOff) {
1244 // AliasAnalysis does not always account for loops. Limit overwrite checks
1245 // to dependencies for which we can guarantee they are independent of any
1246 // loops they are in.
1247 if (!isGuaranteedLoopIndependent(Current: DeadI, KillingDef: KillingI, CurrentLoc: DeadLoc))
1248 return OW_Unknown;
1249
1250 LocationSize KillingLocSize =
1251 strengthenLocationSize(I: KillingI, Size: KillingLoc.Size);
1252 const Value *DeadPtr = DeadLoc.Ptr->stripPointerCasts();
1253 const Value *KillingPtr = KillingLoc.Ptr->stripPointerCasts();
1254 const Value *DeadUndObj = getUnderlyingObject(V: DeadPtr);
1255 const Value *KillingUndObj = getUnderlyingObject(V: KillingPtr);
1256
1257 // Check whether the killing store overwrites the whole object, in which
1258 // case the size/offset of the dead store does not matter.
1259 if (DeadUndObj == KillingUndObj && KillingLocSize.isPrecise() &&
1260 isIdentifiedObject(V: KillingUndObj)) {
1261 std::optional<TypeSize> KillingUndObjSize =
1262 getPointerSize(V: KillingUndObj, DL, TLI, F: &F);
1263 if (KillingUndObjSize && *KillingUndObjSize == KillingLocSize.getValue())
1264 return OW_Complete;
1265 }
1266
1267 // FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll
1268 // get imprecise values here, though (except for unknown sizes).
1269 if (!KillingLocSize.isPrecise() || !DeadLoc.Size.isPrecise()) {
1270 // In case no constant size is known, try to an IR values for the number
1271 // of bytes written and check if they match.
1272 const auto *KillingMemI = dyn_cast<MemIntrinsic>(Val: KillingI);
1273 const auto *DeadMemI = dyn_cast<MemIntrinsic>(Val: DeadI);
1274 if (KillingMemI && DeadMemI) {
1275 const Value *KillingV = KillingMemI->getLength();
1276 const Value *DeadV = DeadMemI->getLength();
1277 if (KillingV == DeadV && BatchAA.isMustAlias(LocA: DeadLoc, LocB: KillingLoc))
1278 return OW_Complete;
1279 }
1280
1281 // Masked stores have imprecise locations, but we can reason about them
1282 // to some extent.
1283 return isMaskedStoreOverwrite(KillingI, DeadI, AA&: BatchAA);
1284 }
1285
1286 const TypeSize KillingSize = KillingLocSize.getValue();
1287 const TypeSize DeadSize = DeadLoc.Size.getValue();
1288 // Bail on doing Size comparison which depends on AA for now
1289 // TODO: Remove AnyScalable once Alias Analysis deal with scalable vectors
1290 const bool AnyScalable = DeadSize.isScalable() || KillingLocSize.isScalable();
1291
1292 if (AnyScalable)
1293 return OW_Unknown;
1294 // Query the alias information
1295 AliasResult AAR = BatchAA.alias(LocA: KillingLoc, LocB: DeadLoc);
1296
1297 // If the start pointers are the same, we just have to compare sizes to see if
1298 // the killing store was larger than the dead store.
1299 if (AAR == AliasResult::MustAlias) {
1300 // Make sure that the KillingSize size is >= the DeadSize size.
1301 if (KillingSize >= DeadSize)
1302 return OW_Complete;
1303 }
1304
1305 // If we hit a partial alias we may have a full overwrite
1306 if (AAR == AliasResult::PartialAlias && AAR.hasOffset()) {
1307 int32_t Off = AAR.getOffset();
1308 if (Off >= 0 && (uint64_t)Off + DeadSize <= KillingSize)
1309 return OW_Complete;
1310 }
1311
1312 // If we can't resolve the same pointers to the same object, then we can't
1313 // analyze them at all.
1314 if (DeadUndObj != KillingUndObj) {
1315 // Non aliasing stores to different objects don't overlap. Note that
1316 // if the killing store is known to overwrite whole object (out of
1317 // bounds access overwrites whole object as well) then it is assumed to
1318 // completely overwrite any store to the same object even if they don't
1319 // actually alias (see next check).
1320 if (AAR == AliasResult::NoAlias)
1321 return OW_None;
1322 return OW_Unknown;
1323 }
1324
1325 // Okay, we have stores to two completely different pointers. Try to
1326 // decompose the pointer into a "base + constant_offset" form. If the base
1327 // pointers are equal, then we can reason about the two stores.
1328 DeadOff = 0;
1329 KillingOff = 0;
1330 const Value *DeadBasePtr =
1331 GetPointerBaseWithConstantOffset(Ptr: DeadPtr, Offset&: DeadOff, DL);
1332 const Value *KillingBasePtr =
1333 GetPointerBaseWithConstantOffset(Ptr: KillingPtr, Offset&: KillingOff, DL);
1334
1335 // If the base pointers still differ, we have two completely different
1336 // stores.
1337 if (DeadBasePtr != KillingBasePtr)
1338 return OW_Unknown;
1339
1340 // The killing access completely overlaps the dead store if and only if
1341 // both start and end of the dead one is "inside" the killing one:
1342 // |<->|--dead--|<->|
1343 // |-----killing------|
1344 // Accesses may overlap if and only if start of one of them is "inside"
1345 // another one:
1346 // |<->|--dead--|<-------->|
1347 // |-------killing--------|
1348 // OR
1349 // |-------dead-------|
1350 // |<->|---killing---|<----->|
1351 //
1352 // We have to be careful here as *Off is signed while *.Size is unsigned.
1353
1354 // Check if the dead access starts "not before" the killing one.
1355 if (DeadOff >= KillingOff) {
1356 // If the dead access ends "not after" the killing access then the
1357 // dead one is completely overwritten by the killing one.
1358 if (uint64_t(DeadOff - KillingOff) + DeadSize <= KillingSize)
1359 return OW_Complete;
1360 // If start of the dead access is "before" end of the killing access
1361 // then accesses overlap.
1362 else if ((uint64_t)(DeadOff - KillingOff) < KillingSize)
1363 return OW_MaybePartial;
1364 }
1365 // If start of the killing access is "before" end of the dead access then
1366 // accesses overlap.
1367 else if ((uint64_t)(KillingOff - DeadOff) < DeadSize) {
1368 return OW_MaybePartial;
1369 }
1370
1371 // Can reach here only if accesses are known not to overlap.
1372 return OW_None;
1373}
1374
1375bool DSEState::isInvisibleToCallerAfterRet(const Value *V, const Value *Ptr,
1376 const LocationSize StoreSize) {
1377 if (isa<AllocaInst>(Val: V))
1378 return true;
1379
1380 auto IBounded = InvisibleToCallerAfterRetBounded.find(Val: V);
1381 if (IBounded != InvisibleToCallerAfterRetBounded.end()) {
1382 int64_t ValueOffset;
1383 [[maybe_unused]] const Value *BaseValue =
1384 GetPointerBaseWithConstantOffset(Ptr, Offset&: ValueOffset, DL);
1385 // If we are not able to find a constant offset from the UO, we have to
1386 // pessimistically assume that the store writes to memory out of the
1387 // dead_on_return bounds.
1388 if (BaseValue != V)
1389 return false;
1390 // This store is only invisible after return if we are in bounds of the
1391 // range marked dead.
1392 if (StoreSize.hasValue() &&
1393 ValueOffset + StoreSize.getValue() <= IBounded->second &&
1394 ValueOffset >= 0)
1395 return true;
1396 }
1397 auto I = InvisibleToCallerAfterRet.insert(KV: {V, false});
1398 if (I.second && isInvisibleToCallerOnUnwind(V) && isNoAliasCall(V))
1399 I.first->second = capturesNothing(CC: PointerMayBeCaptured(
1400 V, /*ReturnCaptures=*/true, Mask: CaptureComponents::Provenance));
1401 return I.first->second;
1402}
1403
1404bool DSEState::isInvisibleToCallerOnUnwind(const Value *V) {
1405 bool RequiresNoCaptureBeforeUnwind;
1406 if (!isNotVisibleOnUnwind(Object: V, RequiresNoCaptureBeforeUnwind))
1407 return false;
1408 if (!RequiresNoCaptureBeforeUnwind)
1409 return true;
1410
1411 auto I = CapturedBeforeReturn.insert(KV: {V, true});
1412 if (I.second)
1413 // NOTE: This could be made more precise by PointerMayBeCapturedBefore
1414 // with the killing MemoryDef. But we refrain from doing so for now to
1415 // limit compile-time and this does not cause any changes to the number
1416 // of stores removed on a large test set in practice.
1417 I.first->second = capturesAnything(CC: PointerMayBeCaptured(
1418 V, /*ReturnCaptures=*/false, Mask: CaptureComponents::Provenance));
1419 return !I.first->second;
1420}
1421
1422std::optional<MemoryLocation> DSEState::getLocForWrite(Instruction *I) const {
1423 if (!I->mayWriteToMemory())
1424 return std::nullopt;
1425
1426 if (auto *CB = dyn_cast<CallBase>(Val: I))
1427 return MemoryLocation::getForDest(CI: CB, TLI);
1428
1429 return MemoryLocation::getOrNone(Inst: I);
1430}
1431
1432SmallVector<std::pair<MemoryLocation, bool>, 1>
1433DSEState::getLocForInst(Instruction *I, bool ConsiderInitializesAttr) {
1434 SmallVector<std::pair<MemoryLocation, bool>, 1> Locations;
1435 if (isMemTerminatorInst(I)) {
1436 if (auto Loc = getLocForTerminator(I))
1437 Locations.push_back(Elt: std::make_pair(x&: Loc->first, y: false));
1438 return Locations;
1439 }
1440
1441 if (auto Loc = getLocForWrite(I))
1442 Locations.push_back(Elt: std::make_pair(x&: *Loc, y: false));
1443
1444 if (ConsiderInitializesAttr) {
1445 for (auto &MemLoc : getInitializesArgMemLoc(I)) {
1446 Locations.push_back(Elt: std::make_pair(x&: MemLoc, y: true));
1447 }
1448 }
1449 return Locations;
1450}
1451
1452bool DSEState::isRemovable(Instruction *I) {
1453 assert(getLocForWrite(I) && "Must have analyzable write");
1454
1455 // Don't remove volatile/atomic stores.
1456 if (StoreInst *SI = dyn_cast<StoreInst>(Val: I))
1457 return SI->isUnordered();
1458
1459 if (auto *CB = dyn_cast<CallBase>(Val: I)) {
1460 // Don't remove volatile memory intrinsics.
1461 if (auto *MI = dyn_cast<MemIntrinsic>(Val: CB))
1462 return !MI->isVolatile();
1463
1464 // Never remove dead lifetime intrinsics, e.g. because they are followed
1465 // by a free.
1466 if (CB->isLifetimeStartOrEnd())
1467 return false;
1468
1469 return CB->use_empty() && CB->willReturn() && CB->doesNotThrow() &&
1470 !CB->isTerminator();
1471 }
1472
1473 return false;
1474}
1475
1476bool DSEState::isCompleteOverwrite(const MemoryLocation &DefLoc,
1477 Instruction *DefInst, Instruction *UseInst) {
1478 // UseInst has a MemoryDef associated in MemorySSA. It's possible for a
1479 // MemoryDef to not write to memory, e.g. a volatile load is modeled as a
1480 // MemoryDef.
1481 if (!UseInst->mayWriteToMemory())
1482 return false;
1483
1484 if (auto *CB = dyn_cast<CallBase>(Val: UseInst))
1485 if (CB->onlyAccessesInaccessibleMemory())
1486 return false;
1487
1488 int64_t InstWriteOffset, DepWriteOffset;
1489 if (auto CC = getLocForWrite(I: UseInst))
1490 return isOverwrite(KillingI: UseInst, DeadI: DefInst, KillingLoc: *CC, DeadLoc: DefLoc, KillingOff&: InstWriteOffset,
1491 DeadOff&: DepWriteOffset) == OW_Complete;
1492 return false;
1493}
1494
1495bool DSEState::isWriteAtEndOfFunction(MemoryDef *Def,
1496 const MemoryLocation &DefLoc) {
1497 LLVM_DEBUG(dbgs() << " Check if def " << *Def << " ("
1498 << *Def->getMemoryInst()
1499 << ") is at the end the function \n");
1500 SmallVector<MemoryAccess *, 4> WorkList;
1501 SmallPtrSet<MemoryAccess *, 8> Visited;
1502
1503 pushMemUses(Acc: Def, WorkList, Visited);
1504 for (unsigned I = 0; I < WorkList.size(); I++) {
1505 if (WorkList.size() >= MemorySSAScanLimit) {
1506 LLVM_DEBUG(dbgs() << " ... hit exploration limit.\n");
1507 return false;
1508 }
1509
1510 MemoryAccess *UseAccess = WorkList[I];
1511 if (isa<MemoryPhi>(Val: UseAccess)) {
1512 // AliasAnalysis does not account for loops. Limit elimination to
1513 // candidates for which we can guarantee they always store to the same
1514 // memory location.
1515 if (!isGuaranteedLoopInvariant(Ptr: DefLoc.Ptr))
1516 return false;
1517
1518 pushMemUses(Acc: cast<MemoryPhi>(Val: UseAccess), WorkList, Visited);
1519 continue;
1520 }
1521 // TODO: Checking for aliasing is expensive. Consider reducing the amount
1522 // of times this is called and/or caching it.
1523 Instruction *UseInst = cast<MemoryUseOrDef>(Val: UseAccess)->getMemoryInst();
1524 if (isReadClobber(DefLoc, UseInst)) {
1525 LLVM_DEBUG(dbgs() << " ... hit read clobber " << *UseInst << ".\n");
1526 return false;
1527 }
1528
1529 if (MemoryDef *UseDef = dyn_cast<MemoryDef>(Val: UseAccess))
1530 pushMemUses(Acc: UseDef, WorkList, Visited);
1531 }
1532 return true;
1533}
1534
1535std::optional<std::pair<MemoryLocation, bool>>
1536DSEState::getLocForTerminator(Instruction *I) const {
1537 if (auto *CB = dyn_cast<CallBase>(Val: I)) {
1538 if (CB->getIntrinsicID() == Intrinsic::lifetime_end)
1539 return {
1540 std::make_pair(x: MemoryLocation::getForArgument(Call: CB, ArgIdx: 0, TLI: &TLI), y: false)};
1541 if (Value *FreedOp = getFreedOperand(CB, TLI: &TLI))
1542 return {std::make_pair(x: MemoryLocation::getAfter(Ptr: FreedOp), y: true)};
1543 }
1544
1545 return std::nullopt;
1546}
1547
1548bool DSEState::isMemTerminatorInst(Instruction *I) const {
1549 auto *CB = dyn_cast<CallBase>(Val: I);
1550 return CB && (CB->getIntrinsicID() == Intrinsic::lifetime_end ||
1551 getFreedOperand(CB, TLI: &TLI) != nullptr);
1552}
1553
1554bool DSEState::isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI,
1555 Instruction *MaybeTerm) {
1556 std::optional<std::pair<MemoryLocation, bool>> MaybeTermLoc =
1557 getLocForTerminator(I: MaybeTerm);
1558
1559 if (!MaybeTermLoc)
1560 return false;
1561
1562 // If the terminator is a free-like call, all accesses to the underlying
1563 // object can be considered terminated.
1564 if (getUnderlyingObject(V: Loc.Ptr) !=
1565 getUnderlyingObject(V: MaybeTermLoc->first.Ptr))
1566 return false;
1567
1568 auto TermLoc = MaybeTermLoc->first;
1569 if (MaybeTermLoc->second) {
1570 const Value *LocUO = getUnderlyingObject(V: Loc.Ptr);
1571 return BatchAA.isMustAlias(V1: TermLoc.Ptr, V2: LocUO);
1572 }
1573 int64_t InstWriteOffset = 0;
1574 int64_t DepWriteOffset = 0;
1575 return isOverwrite(KillingI: MaybeTerm, DeadI: AccessI, KillingLoc: TermLoc, DeadLoc: Loc, KillingOff&: InstWriteOffset,
1576 DeadOff&: DepWriteOffset) == OW_Complete;
1577}
1578
1579bool DSEState::isReadClobber(const MemoryLocation &DefLoc,
1580 Instruction *UseInst) {
1581 if (isNoopIntrinsic(I: UseInst))
1582 return false;
1583
1584 // Monotonic or weaker atomic stores can be re-ordered and do not need to be
1585 // treated as read clobber.
1586 if (auto SI = dyn_cast<StoreInst>(Val: UseInst))
1587 return isStrongerThan(AO: SI->getOrdering(), Other: AtomicOrdering::Monotonic);
1588
1589 if (!UseInst->mayReadFromMemory())
1590 return false;
1591
1592 if (auto *CB = dyn_cast<CallBase>(Val: UseInst))
1593 if (CB->onlyAccessesInaccessibleMemory())
1594 return false;
1595
1596 return isRefSet(MRI: BatchAA.getModRefInfo(I: UseInst, OptLoc: DefLoc));
1597}
1598
1599bool DSEState::isGuaranteedLoopIndependent(const Instruction *Current,
1600 const Instruction *KillingDef,
1601 const MemoryLocation &CurrentLoc) {
1602 // If the dependency is within the same block or loop level (being careful
1603 // of irreducible loops), we know that AA will return a valid result for the
1604 // memory dependency. (Both at the function level, outside of any loop,
1605 // would also be valid but we currently disable that to limit compile time).
1606 if (Current->getParent() == KillingDef->getParent())
1607 return true;
1608 const Cycle *CurrentC = CI.getCycle(Block: Current->getParent());
1609 if (CurrentC && CurrentC == CI.getCycle(Block: KillingDef->getParent()))
1610 return true;
1611 // Otherwise check the memory location is invariant to any loops.
1612 return isGuaranteedLoopInvariant(Ptr: CurrentLoc.Ptr);
1613}
1614
1615bool DSEState::isGuaranteedLoopInvariant(const Value *Ptr) {
1616 Ptr = Ptr->stripPointerCasts();
1617 if (auto *GEP = dyn_cast<GEPOperator>(Val: Ptr))
1618 if (GEP->hasAllConstantIndices())
1619 Ptr = GEP->getPointerOperand()->stripPointerCasts();
1620
1621 if (auto *I = dyn_cast<Instruction>(Val: Ptr)) {
1622 return I->getParent()->isEntryBlock() || !CI.getCycle(Block: I->getParent());
1623 }
1624 return true;
1625}
1626
1627std::optional<MemoryAccess *> DSEState::getDomMemoryDef(
1628 MemoryDef *KillingDef, MemoryAccess *StartAccess,
1629 const MemoryLocation &KillingLoc, const Value *KillingUndObj,
1630 unsigned &ScanLimit, unsigned &WalkerStepLimit, bool IsMemTerm,
1631 unsigned &PartialLimit, bool IsInitializesAttrMemLoc) {
1632 if (ScanLimit == 0 || WalkerStepLimit == 0) {
1633 LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
1634 return std::nullopt;
1635 }
1636
1637 MemoryAccess *Current = StartAccess;
1638 Instruction *KillingI = KillingDef->getMemoryInst();
1639 LLVM_DEBUG(dbgs() << " trying to get dominating access\n");
1640
1641 // Only optimize defining access of KillingDef when directly starting at its
1642 // defining access. The defining access also must only access KillingLoc. At
1643 // the moment we only support instructions with a single write location, so
1644 // it should be sufficient to disable optimizations for instructions that
1645 // also read from memory.
1646 bool CanOptimize = OptimizeMemorySSA &&
1647 KillingDef->getDefiningAccess() == StartAccess &&
1648 !KillingI->mayReadFromMemory();
1649
1650 // Find the next clobbering Mod access for DefLoc, starting at StartAccess.
1651 std::optional<MemoryLocation> CurrentLoc;
1652 for (;; Current = cast<MemoryDef>(Val: Current)->getDefiningAccess()) {
1653 LLVM_DEBUG({
1654 dbgs() << " visiting " << *Current;
1655 if (!MSSA.isLiveOnEntryDef(Current) && isa<MemoryUseOrDef>(Current))
1656 dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst()
1657 << ")";
1658 dbgs() << "\n";
1659 });
1660
1661 // Reached TOP.
1662 if (MSSA.isLiveOnEntryDef(MA: Current)) {
1663 LLVM_DEBUG(dbgs() << " ... found LiveOnEntryDef\n");
1664 if (CanOptimize && Current != KillingDef->getDefiningAccess())
1665 // The first clobbering def is... none.
1666 KillingDef->setOptimized(Current);
1667 return std::nullopt;
1668 }
1669
1670 // Cost of a step. Accesses in the same block are more likely to be valid
1671 // candidates for elimination, hence consider them cheaper.
1672 unsigned StepCost = KillingDef->getBlock() == Current->getBlock()
1673 ? MemorySSASameBBStepCost
1674 : MemorySSAOtherBBStepCost;
1675 if (WalkerStepLimit <= StepCost) {
1676 LLVM_DEBUG(dbgs() << " ... hit walker step limit\n");
1677 return std::nullopt;
1678 }
1679 WalkerStepLimit -= StepCost;
1680
1681 // Return for MemoryPhis. They cannot be eliminated directly and the
1682 // caller is responsible for traversing them.
1683 if (isa<MemoryPhi>(Val: Current)) {
1684 LLVM_DEBUG(dbgs() << " ... found MemoryPhi\n");
1685 return Current;
1686 }
1687
1688 // Below, check if CurrentDef is a valid candidate to be eliminated by
1689 // KillingDef. If it is not, check the next candidate.
1690 MemoryDef *CurrentDef = cast<MemoryDef>(Val: Current);
1691 Instruction *CurrentI = CurrentDef->getMemoryInst();
1692
1693 if (canSkipDef(D: CurrentDef, DefVisibleToCaller: !isInvisibleToCallerOnUnwind(V: KillingUndObj))) {
1694 CanOptimize = false;
1695 continue;
1696 }
1697
1698 // Before we try to remove anything, check for any extra throwing
1699 // instructions that block us from DSEing
1700 if (mayThrowBetween(KillingI, DeadI: CurrentI, KillingUndObj)) {
1701 LLVM_DEBUG(dbgs() << " ... skip, may throw!\n");
1702 return std::nullopt;
1703 }
1704
1705 // Check for anything that looks like it will be a barrier to further
1706 // removal
1707 if (isDSEBarrier(KillingUndObj, DeadI: CurrentI)) {
1708 LLVM_DEBUG(dbgs() << " ... skip, barrier\n");
1709 return std::nullopt;
1710 }
1711
1712 // If Current is known to be on path that reads DefLoc or is a read
1713 // clobber, bail out, as the path is not profitable. We skip this check
1714 // for intrinsic calls, because the code knows how to handle memcpy
1715 // intrinsics.
1716 if (!isa<IntrinsicInst>(Val: CurrentI) && isReadClobber(DefLoc: KillingLoc, UseInst: CurrentI))
1717 return std::nullopt;
1718
1719 // Quick check if there are direct uses that are read-clobbers.
1720 if (any_of(Range: Current->uses(), P: [this, &KillingLoc, StartAccess](Use &U) {
1721 if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(Val: U.getUser()))
1722 return !MSSA.dominates(A: StartAccess, B: UseOrDef) &&
1723 isReadClobber(DefLoc: KillingLoc, UseInst: UseOrDef->getMemoryInst());
1724 return false;
1725 })) {
1726 LLVM_DEBUG(dbgs() << " ... found a read clobber\n");
1727 return std::nullopt;
1728 }
1729
1730 // If Current does not have an analyzable write location or is not
1731 // removable, skip it.
1732 CurrentLoc = getLocForWrite(I: CurrentI);
1733 if (!CurrentLoc || !isRemovable(I: CurrentI)) {
1734 CanOptimize = false;
1735 continue;
1736 }
1737
1738 // AliasAnalysis does not account for loops. Limit elimination to
1739 // candidates for which we can guarantee they always store to the same
1740 // memory location and not located in different loops.
1741 if (!isGuaranteedLoopIndependent(Current: CurrentI, KillingDef: KillingI, CurrentLoc: *CurrentLoc)) {
1742 LLVM_DEBUG(dbgs() << " ... not guaranteed loop independent\n");
1743 CanOptimize = false;
1744 continue;
1745 }
1746
1747 if (IsMemTerm) {
1748 // If the killing def is a memory terminator (e.g. lifetime.end), check
1749 // the next candidate if the current Current does not write the same
1750 // underlying object as the terminator.
1751 if (!isMemTerminator(Loc: *CurrentLoc, AccessI: CurrentI, MaybeTerm: KillingI)) {
1752 CanOptimize = false;
1753 continue;
1754 }
1755 } else {
1756 int64_t KillingOffset = 0;
1757 int64_t DeadOffset = 0;
1758 auto OR = isOverwrite(KillingI, DeadI: CurrentI, KillingLoc, DeadLoc: *CurrentLoc,
1759 KillingOff&: KillingOffset, DeadOff&: DeadOffset);
1760 if (CanOptimize) {
1761 // CurrentDef is the earliest write clobber of KillingDef. Use it as
1762 // optimized access. Do not optimize if CurrentDef is already the
1763 // defining access of KillingDef.
1764 if (CurrentDef != KillingDef->getDefiningAccess() &&
1765 (OR == OW_Complete || OR == OW_MaybePartial))
1766 KillingDef->setOptimized(CurrentDef);
1767
1768 // Once a may-aliasing def is encountered do not set an optimized
1769 // access.
1770 if (OR != OW_None)
1771 CanOptimize = false;
1772 }
1773
1774 // If Current does not write to the same object as KillingDef, check
1775 // the next candidate.
1776 if (OR == OW_Unknown || OR == OW_None)
1777 continue;
1778 else if (OR == OW_MaybePartial) {
1779 // If KillingDef only partially overwrites Current, check the next
1780 // candidate if the partial step limit is exceeded. This aggressively
1781 // limits the number of candidates for partial store elimination,
1782 // which are less likely to be removable in the end.
1783 if (PartialLimit <= 1) {
1784 WalkerStepLimit -= 1;
1785 LLVM_DEBUG(dbgs() << " ... reached partial limit ... continue with "
1786 "next access\n");
1787 continue;
1788 }
1789 PartialLimit -= 1;
1790 }
1791 }
1792 break;
1793 };
1794
1795 // Accesses to objects accessible after the function returns can only be
1796 // eliminated if the access is dead along all paths to the exit. Collect
1797 // the blocks with killing (=completely overwriting MemoryDefs) and check if
1798 // they cover all paths from MaybeDeadAccess to any function exit.
1799 SmallPtrSet<Instruction *, 16> KillingDefs;
1800 KillingDefs.insert(Ptr: KillingDef->getMemoryInst());
1801 MemoryAccess *MaybeDeadAccess = Current;
1802 MemoryLocation MaybeDeadLoc = *CurrentLoc;
1803 Instruction *MaybeDeadI = cast<MemoryDef>(Val: MaybeDeadAccess)->getMemoryInst();
1804 LLVM_DEBUG(dbgs() << " Checking for reads of " << *MaybeDeadAccess << " ("
1805 << *MaybeDeadI << ")\n");
1806
1807 SmallVector<MemoryAccess *, 32> WorkList;
1808 SmallPtrSet<MemoryAccess *, 32> Visited;
1809 pushMemUses(Acc: MaybeDeadAccess, WorkList, Visited);
1810
1811 // Check if DeadDef may be read.
1812 for (unsigned I = 0; I < WorkList.size(); I++) {
1813 MemoryAccess *UseAccess = WorkList[I];
1814
1815 LLVM_DEBUG(dbgs() << " " << *UseAccess);
1816 // Bail out if the number of accesses to check exceeds the scan limit.
1817 if (ScanLimit < (WorkList.size() - I)) {
1818 LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n");
1819 return std::nullopt;
1820 }
1821 --ScanLimit;
1822 NumDomMemDefChecks++;
1823
1824 if (isa<MemoryPhi>(Val: UseAccess)) {
1825 if (any_of(Range&: KillingDefs, P: [this, UseAccess](Instruction *KI) {
1826 return DT.properlyDominates(A: KI->getParent(), B: UseAccess->getBlock());
1827 })) {
1828 LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n");
1829 continue;
1830 }
1831 LLVM_DEBUG(dbgs() << "\n ... adding PHI uses\n");
1832 pushMemUses(Acc: UseAccess, WorkList, Visited);
1833 continue;
1834 }
1835
1836 Instruction *UseInst = cast<MemoryUseOrDef>(Val: UseAccess)->getMemoryInst();
1837 LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n");
1838
1839 if (any_of(Range&: KillingDefs, P: [this, UseInst](Instruction *KI) {
1840 return DT.dominates(Def: KI, User: UseInst);
1841 })) {
1842 LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing def\n");
1843 continue;
1844 }
1845
1846 // A memory terminator kills all preceeding MemoryDefs and all succeeding
1847 // MemoryAccesses. We do not have to check it's users.
1848 if (isMemTerminator(Loc: MaybeDeadLoc, AccessI: MaybeDeadI, MaybeTerm: UseInst)) {
1849 LLVM_DEBUG(
1850 dbgs()
1851 << " ... skipping, memterminator invalidates following accesses\n");
1852 continue;
1853 }
1854
1855 if (isNoopIntrinsic(I: cast<MemoryUseOrDef>(Val: UseAccess)->getMemoryInst())) {
1856 LLVM_DEBUG(dbgs() << " ... adding uses of intrinsic\n");
1857 pushMemUses(Acc: UseAccess, WorkList, Visited);
1858 continue;
1859 }
1860
1861 if (UseInst->mayThrow() && !isInvisibleToCallerOnUnwind(V: KillingUndObj)) {
1862 LLVM_DEBUG(dbgs() << " ... found throwing instruction\n");
1863 return std::nullopt;
1864 }
1865
1866 // Uses which may read the original MemoryDef mean we cannot eliminate the
1867 // original MD. Stop walk.
1868 // If KillingDef is a CallInst with "initializes" attribute, the reads in
1869 // the callee would be dominated by initializations, so it should be safe.
1870 bool IsKillingDefFromInitAttr = false;
1871 if (IsInitializesAttrMemLoc) {
1872 if (KillingI == UseInst &&
1873 KillingUndObj == getUnderlyingObject(V: MaybeDeadLoc.Ptr))
1874 IsKillingDefFromInitAttr = true;
1875 }
1876
1877 if (isReadClobber(DefLoc: MaybeDeadLoc, UseInst) && !IsKillingDefFromInitAttr) {
1878 LLVM_DEBUG(dbgs() << " ... found read clobber\n");
1879 return std::nullopt;
1880 }
1881
1882 // If this worklist walks back to the original memory access (and the
1883 // pointer is not guarenteed loop invariant) then we cannot assume that a
1884 // store kills itself.
1885 if (MaybeDeadAccess == UseAccess &&
1886 !isGuaranteedLoopInvariant(Ptr: MaybeDeadLoc.Ptr)) {
1887 LLVM_DEBUG(dbgs() << " ... found not loop invariant self access\n");
1888 return std::nullopt;
1889 }
1890 // Otherwise, for the KillingDef and MaybeDeadAccess we only have to check
1891 // if it reads the memory location.
1892 // TODO: It would probably be better to check for self-reads before
1893 // calling the function.
1894 if (KillingDef == UseAccess || MaybeDeadAccess == UseAccess) {
1895 LLVM_DEBUG(dbgs() << " ... skipping killing def/dom access\n");
1896 continue;
1897 }
1898
1899 // Check all uses for MemoryDefs, except for defs completely overwriting
1900 // the original location. Otherwise we have to check uses of *all*
1901 // MemoryDefs we discover, including non-aliasing ones. Otherwise we might
1902 // miss cases like the following
1903 // 1 = Def(LoE) ; <----- DeadDef stores [0,1]
1904 // 2 = Def(1) ; (2, 1) = NoAlias, stores [2,3]
1905 // Use(2) ; MayAlias 2 *and* 1, loads [0, 3].
1906 // (The Use points to the *first* Def it may alias)
1907 // 3 = Def(1) ; <---- Current (3, 2) = NoAlias, (3,1) = MayAlias,
1908 // stores [0,1]
1909 if (MemoryDef *UseDef = dyn_cast<MemoryDef>(Val: UseAccess)) {
1910 if (isCompleteOverwrite(DefLoc: MaybeDeadLoc, DefInst: MaybeDeadI, UseInst)) {
1911 BasicBlock *MaybeKillingBlock = UseInst->getParent();
1912 if (PostOrderNumbers.find(Val: MaybeKillingBlock)->second <
1913 PostOrderNumbers.find(Val: MaybeDeadAccess->getBlock())->second) {
1914 if (!isInvisibleToCallerAfterRet(V: KillingUndObj, Ptr: KillingLoc.Ptr,
1915 StoreSize: KillingLoc.Size)) {
1916 LLVM_DEBUG(dbgs()
1917 << " ... found killing def " << *UseInst << "\n");
1918 KillingDefs.insert(Ptr: UseInst);
1919 }
1920 } else {
1921 LLVM_DEBUG(dbgs()
1922 << " ... found preceeding def " << *UseInst << "\n");
1923 return std::nullopt;
1924 }
1925 } else
1926 pushMemUses(Acc: UseDef, WorkList, Visited);
1927 }
1928 }
1929
1930 // For accesses to locations visible after the function returns, make sure
1931 // that the location is dead (=overwritten) along all paths from
1932 // MaybeDeadAccess to the exit.
1933 if (!isInvisibleToCallerAfterRet(V: KillingUndObj, Ptr: KillingLoc.Ptr,
1934 StoreSize: KillingLoc.Size)) {
1935 SmallPtrSet<BasicBlock *, 16> KillingBlocks;
1936 for (Instruction *KD : KillingDefs)
1937 KillingBlocks.insert(Ptr: KD->getParent());
1938 assert(!KillingBlocks.empty() &&
1939 "Expected at least a single killing block");
1940
1941 // Find the common post-dominator of all killing blocks.
1942 BasicBlock *CommonPred = *KillingBlocks.begin();
1943 for (BasicBlock *BB : llvm::drop_begin(RangeOrContainer&: KillingBlocks)) {
1944 if (!CommonPred)
1945 break;
1946 CommonPred = PDT.findNearestCommonDominator(A: CommonPred, B: BB);
1947 }
1948
1949 // If the common post-dominator does not post-dominate MaybeDeadAccess,
1950 // there is a path from MaybeDeadAccess to an exit not going through a
1951 // killing block.
1952 if (!PDT.dominates(A: CommonPred, B: MaybeDeadAccess->getBlock())) {
1953 if (!AnyUnreachableExit)
1954 return std::nullopt;
1955
1956 // Fall back to CFG scan starting at all non-unreachable roots if not
1957 // all paths to the exit go through CommonPred.
1958 CommonPred = nullptr;
1959 }
1960
1961 // If CommonPred itself is in the set of killing blocks, we're done.
1962 if (KillingBlocks.count(Ptr: CommonPred))
1963 return {MaybeDeadAccess};
1964
1965 SetVector<BasicBlock *> WorkList;
1966 // If CommonPred is null, there are multiple exits from the function.
1967 // They all have to be added to the worklist.
1968 if (CommonPred)
1969 WorkList.insert(X: CommonPred);
1970 else
1971 for (BasicBlock *R : PDT.roots()) {
1972 if (!isa<UnreachableInst>(Val: R->getTerminator()))
1973 WorkList.insert(X: R);
1974 }
1975
1976 NumCFGTries++;
1977 // Check if all paths starting from an exit node go through one of the
1978 // killing blocks before reaching MaybeDeadAccess.
1979 for (unsigned I = 0; I < WorkList.size(); I++) {
1980 NumCFGChecks++;
1981 BasicBlock *Current = WorkList[I];
1982 if (KillingBlocks.count(Ptr: Current))
1983 continue;
1984 if (Current == MaybeDeadAccess->getBlock())
1985 return std::nullopt;
1986
1987 // MaybeDeadAccess is reachable from the entry, so we don't have to
1988 // explore unreachable blocks further.
1989 if (!DT.isReachableFromEntry(A: Current))
1990 continue;
1991
1992 WorkList.insert_range(R: predecessors(BB: Current));
1993
1994 if (WorkList.size() >= MemorySSAPathCheckLimit)
1995 return std::nullopt;
1996 }
1997 NumCFGSuccess++;
1998 }
1999
2000 // No aliasing MemoryUses of MaybeDeadAccess found, MaybeDeadAccess is
2001 // potentially dead.
2002 return {MaybeDeadAccess};
2003}
2004
2005void DSEState::deleteDeadInstruction(Instruction *SI,
2006 SmallPtrSetImpl<MemoryAccess *> *Deleted) {
2007 MemorySSAUpdater Updater(&MSSA);
2008 SmallVector<Instruction *, 32> NowDeadInsts;
2009 NowDeadInsts.push_back(Elt: SI);
2010 --NumFastOther;
2011
2012 while (!NowDeadInsts.empty()) {
2013 Instruction *DeadInst = NowDeadInsts.pop_back_val();
2014 ++NumFastOther;
2015
2016 // Try to preserve debug information attached to the dead instruction.
2017 salvageDebugInfo(I&: *DeadInst);
2018 salvageKnowledge(I: DeadInst);
2019
2020 // Remove the Instruction from MSSA.
2021 MemoryAccess *MA = MSSA.getMemoryAccess(I: DeadInst);
2022 bool IsMemDef = MA && isa<MemoryDef>(Val: MA);
2023 if (MA) {
2024 if (IsMemDef) {
2025 auto *MD = cast<MemoryDef>(Val: MA);
2026 SkipStores.insert(Ptr: MD);
2027 if (Deleted)
2028 Deleted->insert(Ptr: MD);
2029 if (auto *SI = dyn_cast<StoreInst>(Val: MD->getMemoryInst())) {
2030 if (SI->getValueOperand()->getType()->isPointerTy()) {
2031 const Value *UO = getUnderlyingObject(V: SI->getValueOperand());
2032 if (CapturedBeforeReturn.erase(Val: UO))
2033 ShouldIterateEndOfFunctionDSE = true;
2034 InvisibleToCallerAfterRet.erase(Val: UO);
2035 InvisibleToCallerAfterRetBounded.erase(Val: UO);
2036 }
2037 }
2038 }
2039
2040 Updater.removeMemoryAccess(MA);
2041 }
2042
2043 auto I = IOLs.find(Key: DeadInst->getParent());
2044 if (I != IOLs.end())
2045 I->second.erase(Key: DeadInst);
2046 // Remove its operands
2047 for (Use &O : DeadInst->operands())
2048 if (Instruction *OpI = dyn_cast<Instruction>(Val&: O)) {
2049 O.set(PoisonValue::get(T: O->getType()));
2050 if (isInstructionTriviallyDead(I: OpI, TLI: &TLI))
2051 NowDeadInsts.push_back(Elt: OpI);
2052 }
2053
2054 EA.removeInstruction(I: DeadInst);
2055 // Remove memory defs directly if they don't produce results, but only
2056 // queue other dead instructions for later removal. They may have been
2057 // used as memory locations that have been cached by BatchAA. Removing
2058 // them here may lead to newly created instructions to be allocated at the
2059 // same address, yielding stale cache entries.
2060 if (IsMemDef && DeadInst->getType()->isVoidTy())
2061 DeadInst->eraseFromParent();
2062 else
2063 ToRemove.push_back(Elt: DeadInst);
2064 }
2065}
2066
2067bool DSEState::mayThrowBetween(Instruction *KillingI, Instruction *DeadI,
2068 const Value *KillingUndObj) {
2069 // First see if we can ignore it by using the fact that KillingI is an
2070 // alloca/alloca like object that is not visible to the caller during
2071 // execution of the function.
2072 if (KillingUndObj && isInvisibleToCallerOnUnwind(V: KillingUndObj))
2073 return false;
2074
2075 if (KillingI->getParent() == DeadI->getParent())
2076 return ThrowingBlocks.count(Ptr: KillingI->getParent());
2077 return !ThrowingBlocks.empty();
2078}
2079
2080bool DSEState::isDSEBarrier(const Value *KillingUndObj, Instruction *DeadI) {
2081 // If DeadI may throw it acts as a barrier, unless we are to an
2082 // alloca/alloca like object that does not escape.
2083 if (DeadI->mayThrow() && !isInvisibleToCallerOnUnwind(V: KillingUndObj))
2084 return true;
2085
2086 // If DeadI is an atomic load/store stronger than monotonic, do not try to
2087 // eliminate/reorder it.
2088 if (DeadI->isAtomic()) {
2089 if (auto *LI = dyn_cast<LoadInst>(Val: DeadI))
2090 return isStrongerThanMonotonic(AO: LI->getOrdering());
2091 if (auto *SI = dyn_cast<StoreInst>(Val: DeadI))
2092 return isStrongerThanMonotonic(AO: SI->getOrdering());
2093 if (auto *ARMW = dyn_cast<AtomicRMWInst>(Val: DeadI))
2094 return isStrongerThanMonotonic(AO: ARMW->getOrdering());
2095 if (auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(Val: DeadI))
2096 return isStrongerThanMonotonic(AO: CmpXchg->getSuccessOrdering()) ||
2097 isStrongerThanMonotonic(AO: CmpXchg->getFailureOrdering());
2098 llvm_unreachable("other instructions should be skipped in MemorySSA");
2099 }
2100 return false;
2101}
2102
2103bool DSEState::eliminateDeadWritesAtEndOfFunction() {
2104 bool MadeChange = false;
2105 LLVM_DEBUG(
2106 dbgs() << "Trying to eliminate MemoryDefs at the end of the function\n");
2107 do {
2108 ShouldIterateEndOfFunctionDSE = false;
2109 for (MemoryDef *Def : llvm::reverse(C&: MemDefs)) {
2110 if (SkipStores.contains(Ptr: Def))
2111 continue;
2112
2113 Instruction *DefI = Def->getMemoryInst();
2114 auto DefLoc = getLocForWrite(I: DefI);
2115 if (!DefLoc || !isRemovable(I: DefI)) {
2116 LLVM_DEBUG(dbgs() << " ... could not get location for write or "
2117 "instruction not removable.\n");
2118 continue;
2119 }
2120
2121 // NOTE: Currently eliminating writes at the end of a function is
2122 // limited to MemoryDefs with a single underlying object, to save
2123 // compile-time. In practice it appears the case with multiple
2124 // underlying objects is very uncommon. If it turns out to be important,
2125 // we can use getUnderlyingObjects here instead.
2126 const Value *UO = getUnderlyingObject(V: DefLoc->Ptr);
2127 if (!isInvisibleToCallerAfterRet(V: UO, Ptr: DefLoc->Ptr, StoreSize: DefLoc->Size))
2128 continue;
2129
2130 if (isWriteAtEndOfFunction(Def, DefLoc: *DefLoc)) {
2131 // See through pointer-to-pointer bitcasts
2132 LLVM_DEBUG(dbgs() << " ... MemoryDef is not accessed until the end "
2133 "of the function\n");
2134 deleteDeadInstruction(SI: DefI);
2135 ++NumFastStores;
2136 MadeChange = true;
2137 }
2138 }
2139 } while (ShouldIterateEndOfFunctionDSE);
2140 return MadeChange;
2141}
2142
2143bool DSEState::eliminateRedundantStoresViaDominatingConditions() {
2144 bool MadeChange = false;
2145 LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs whose value being "
2146 "written is implied by a dominating condition\n");
2147
2148 using ConditionInfo = std::pair<Value *, Value *>;
2149 using ScopedHTType = ScopedHashTable<ConditionInfo, Instruction *>;
2150
2151 // We maintain a scoped hash table of the active dominating conditions for a
2152 // given node.
2153 ScopedHTType ActiveConditions;
2154 auto GetDominatingCondition = [&](BasicBlock *BB)
2155 -> std::optional<std::tuple<ConditionInfo, Instruction *, BasicBlock *>> {
2156 auto *BI = dyn_cast<CondBrInst>(Val: BB->getTerminator());
2157 if (!BI)
2158 return std::nullopt;
2159
2160 // In case both blocks are the same, it is not possible to determine
2161 // if optimization is possible. (We would not want to optimize a store
2162 // in the FalseBB if condition is true and vice versa.)
2163 if (BI->getSuccessor(i: 0) == BI->getSuccessor(i: 1))
2164 return std::nullopt;
2165
2166 Instruction *ICmpL;
2167 CmpPredicate Pred;
2168 Value *StorePtr, *StoreVal;
2169 if (!match(V: BI->getCondition(),
2170 P: m_c_ICmp(Pred, L: m_Instruction(I&: ICmpL, Match: m_Load(Op: m_Value(V&: StorePtr))),
2171 R: m_Value(V&: StoreVal))) ||
2172 !ICmpInst::isEquality(P: Pred))
2173 return std::nullopt;
2174
2175 // Ensure the replacement is allowed when comparing pointers, as
2176 // the equality compares addresses only, not pointers' provenance.
2177 if (StoreVal->getType()->isPointerTy() &&
2178 !canReplacePointersIfEqual(From: StoreVal, To: ICmpL, DL))
2179 return std::nullopt;
2180
2181 unsigned ImpliedSuccIdx = Pred == ICmpInst::ICMP_EQ ? 0 : 1;
2182 BasicBlock *ImpliedSucc = BI->getSuccessor(i: ImpliedSuccIdx);
2183 return {{ConditionInfo(StorePtr, StoreVal), ICmpL, ImpliedSucc}};
2184 };
2185
2186 auto VisitNode = [&](DomTreeNode *Node, unsigned Depth, auto &Self) -> void {
2187 if (Depth > MaxDepthRecursion)
2188 return;
2189
2190 BasicBlock *BB = Node->getBlock();
2191 // Check for redundant stores against active known conditions.
2192 if (auto *Accesses = MSSA.getBlockDefs(BB)) {
2193 for (auto &Access : make_early_inc_range(Range&: *Accesses)) {
2194 auto *Def = dyn_cast<MemoryDef>(Val: &Access);
2195 if (!Def)
2196 continue;
2197
2198 auto *SI = dyn_cast<StoreInst>(Val: Def->getMemoryInst());
2199 if (!SI || !SI->isUnordered())
2200 continue;
2201
2202 Instruction *LI = ActiveConditions.lookup(
2203 Key: {SI->getPointerOperand(), SI->getValueOperand()});
2204 if (!LI)
2205 continue;
2206
2207 // Found a dominating condition that may imply the value being stored.
2208 // Make sure there does not exist any clobbering access between the
2209 // load and the potential redundant store.
2210 MemoryAccess *LoadAccess = MSSA.getMemoryAccess(I: LI);
2211 MemoryAccess *ClobberingAccess =
2212 MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, AA&: BatchAA);
2213 if (MSSA.dominates(A: ClobberingAccess, B: LoadAccess)) {
2214 LLVM_DEBUG(dbgs()
2215 << "Removing No-Op Store:\n DEAD: " << *SI << '\n');
2216 deleteDeadInstruction(SI);
2217 NumRedundantStores++;
2218 MadeChange = true;
2219 }
2220 }
2221 }
2222
2223 // See whether this basic block establishes a dominating condition.
2224 auto MaybeCondition = GetDominatingCondition(BB);
2225
2226 for (DomTreeNode *Child : Node->children()) {
2227 // RAII scope for the active conditions.
2228 ScopedHTType::ScopeTy Scope(ActiveConditions);
2229 if (MaybeCondition) {
2230 const auto &[Cond, LI, ImpliedSucc] = *MaybeCondition;
2231 if (DT.dominates(BBE: BasicBlockEdge(BB, ImpliedSucc), BB: Child->getBlock())) {
2232 // Found a condition that holds for this child, dominated by the
2233 // current node via the equality edge. Propagate the condition to
2234 // the children by pushing it onto the table.
2235 ActiveConditions.insert(Key: Cond, Val: LI);
2236 }
2237 }
2238
2239 // Recursively visit the children of this node. Upon destruction, the no
2240 // longer active condition before visiting any sibling nodes is popped
2241 // from the active scope.
2242 Self(Child, Depth + 1, Self);
2243 }
2244 };
2245
2246 // Do a DFS walk of the dom-tree.
2247 VisitNode(DT.getRootNode(), 0, VisitNode);
2248
2249 return MadeChange;
2250}
2251
2252bool DSEState::tryFoldIntoCalloc(MemoryDef *Def, const Value *DefUO) {
2253 Instruction *DefI = Def->getMemoryInst();
2254 MemSetInst *MemSet = dyn_cast<MemSetInst>(Val: DefI);
2255 if (!MemSet)
2256 // TODO: Could handle zero store to small allocation as well.
2257 return false;
2258 Constant *StoredConstant = dyn_cast<Constant>(Val: MemSet->getValue());
2259 if (!StoredConstant || !StoredConstant->isNullValue())
2260 return false;
2261
2262 if (!isRemovable(I: DefI))
2263 // The memset might be volatile..
2264 return false;
2265
2266 if (F.hasFnAttribute(Kind: Attribute::SanitizeMemory) ||
2267 F.hasFnAttribute(Kind: Attribute::SanitizeAddress) ||
2268 F.hasFnAttribute(Kind: Attribute::SanitizeHWAddress) || F.getName() == "calloc")
2269 return false;
2270 auto *Malloc = const_cast<CallInst *>(dyn_cast<CallInst>(Val: DefUO));
2271 if (!Malloc)
2272 return false;
2273 auto *InnerCallee = Malloc->getCalledFunction();
2274 if (!InnerCallee)
2275 return false;
2276 LibFunc Func = NotLibFunc;
2277 StringRef ZeroedVariantName;
2278 if (!TLI.getLibFunc(FDecl: *InnerCallee, F&: Func) || !TLI.has(F: Func) ||
2279 Func != LibFunc_malloc) {
2280 Attribute Attr = Malloc->getFnAttr(Kind: "alloc-variant-zeroed");
2281 if (!Attr.isValid())
2282 return false;
2283 ZeroedVariantName = Attr.getValueAsString();
2284 if (ZeroedVariantName.empty())
2285 return false;
2286 }
2287
2288 // Gracefully handle malloc with unexpected memory attributes.
2289 auto *MallocDef = dyn_cast_or_null<MemoryDef>(Val: MSSA.getMemoryAccess(I: Malloc));
2290 if (!MallocDef)
2291 return false;
2292
2293 auto shouldCreateCalloc = [](CallInst *Malloc, CallInst *Memset) {
2294 // Check for br(icmp ptr, null), truebb, falsebb) pattern at the end
2295 // of malloc block
2296 auto *MallocBB = Malloc->getParent(), *MemsetBB = Memset->getParent();
2297 if (MallocBB == MemsetBB)
2298 return true;
2299 auto *Ptr = Memset->getArgOperand(i: 0);
2300 auto *TI = MallocBB->getTerminator();
2301 BasicBlock *TrueBB, *FalseBB;
2302 if (!match(V: TI, P: m_Br(C: m_SpecificICmp(MatchPred: ICmpInst::ICMP_EQ, L: m_Specific(V: Ptr),
2303 R: m_Zero()),
2304 T&: TrueBB, F&: FalseBB)))
2305 return false;
2306 if (MemsetBB != FalseBB)
2307 return false;
2308 return true;
2309 };
2310
2311 if (Malloc->getOperand(i_nocapture: 0) != MemSet->getLength())
2312 return false;
2313 if (!shouldCreateCalloc(Malloc, MemSet) || !DT.dominates(Def: Malloc, User: MemSet) ||
2314 !memoryIsNotModifiedBetween(FirstI: Malloc, SecondI: MemSet, AA&: BatchAA, DL, DT: &DT))
2315 return false;
2316 IRBuilder<> IRB(Malloc);
2317 assert(Func == LibFunc_malloc || !ZeroedVariantName.empty());
2318 Value *Calloc = nullptr;
2319 if (!ZeroedVariantName.empty()) {
2320 LLVMContext &Ctx = Malloc->getContext();
2321 AttributeList Attrs = InnerCallee->getAttributes();
2322 AllocFnKind AllocKind =
2323 Attrs.getFnAttr(Kind: Attribute::AllocKind).getAllocKind() |
2324 AllocFnKind::Zeroed;
2325 AllocKind &= ~AllocFnKind::Uninitialized;
2326 Attrs =
2327 Attrs.addFnAttribute(C&: Ctx, Attr: Attribute::getWithAllocKind(Context&: Ctx, Kind: AllocKind))
2328 .removeFnAttribute(C&: Ctx, Kind: "alloc-variant-zeroed");
2329 FunctionCallee ZeroedVariant = Malloc->getModule()->getOrInsertFunction(
2330 Name: ZeroedVariantName, T: InnerCallee->getFunctionType(), AttributeList: Attrs);
2331 cast<Function>(Val: ZeroedVariant.getCallee())
2332 ->setCallingConv(Malloc->getCallingConv());
2333 SmallVector<Value *, 3> Args;
2334 Args.append(in_start: Malloc->arg_begin(), in_end: Malloc->arg_end());
2335 CallInst *CI = IRB.CreateCall(Callee: ZeroedVariant, Args, Name: ZeroedVariantName);
2336 CI->setCallingConv(Malloc->getCallingConv());
2337 Calloc = CI;
2338 } else {
2339 Type *SizeTTy = Malloc->getArgOperand(i: 0)->getType();
2340 Calloc = emitCalloc(Num: ConstantInt::get(Ty: SizeTTy, V: 1), Size: Malloc->getArgOperand(i: 0),
2341 B&: IRB, TLI, AddrSpace: Malloc->getType()->getPointerAddressSpace());
2342 }
2343 if (!Calloc)
2344 return false;
2345
2346 MemorySSAUpdater Updater(&MSSA);
2347 auto *NewAccess = Updater.createMemoryAccessAfter(I: cast<Instruction>(Val: Calloc),
2348 Definition: nullptr, InsertPt: MallocDef);
2349 auto *NewAccessMD = cast<MemoryDef>(Val: NewAccess);
2350 Updater.insertDef(Def: NewAccessMD, /*RenameUses=*/true);
2351 Malloc->replaceAllUsesWith(V: Calloc);
2352 deleteDeadInstruction(SI: Malloc);
2353 return true;
2354}
2355
2356bool DSEState::storeIsNoop(MemoryDef *Def, const Value *DefUO) {
2357 Instruction *DefI = Def->getMemoryInst();
2358 StoreInst *Store = dyn_cast<StoreInst>(Val: DefI);
2359 MemSetInst *MemSet = dyn_cast<MemSetInst>(Val: DefI);
2360 Constant *StoredConstant = nullptr;
2361 if (Store)
2362 StoredConstant = dyn_cast<Constant>(Val: Store->getOperand(i_nocapture: 0));
2363 else if (MemSet)
2364 StoredConstant = dyn_cast<Constant>(Val: MemSet->getValue());
2365 else
2366 return false;
2367
2368 if (!isRemovable(I: DefI))
2369 return false;
2370
2371 if (StoredConstant) {
2372 Constant *InitC =
2373 getInitialValueOfAllocation(V: DefUO, TLI: &TLI, Ty: StoredConstant->getType());
2374 // If the clobbering access is LiveOnEntry, no instructions between them
2375 // can modify the memory location.
2376 if (InitC && InitC == StoredConstant)
2377 return MSSA.isLiveOnEntryDef(
2378 MA: MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, AA&: BatchAA));
2379 }
2380
2381 if (!Store)
2382 return false;
2383
2384 if (auto *LoadI = dyn_cast<LoadInst>(Val: Store->getOperand(i_nocapture: 0))) {
2385 if (LoadI->getPointerOperand() == Store->getOperand(i_nocapture: 1)) {
2386 // Get the defining access for the load.
2387 auto *LoadAccess = MSSA.getMemoryAccess(I: LoadI)->getDefiningAccess();
2388 // Fast path: the defining accesses are the same.
2389 if (LoadAccess == Def->getDefiningAccess())
2390 return true;
2391
2392 // Look through phi accesses. Recursively scan all phi accesses by
2393 // adding them to a worklist. Bail when we run into a memory def that
2394 // does not match LoadAccess.
2395 SetVector<MemoryAccess *> ToCheck;
2396 MemoryAccess *Current =
2397 MSSA.getWalker()->getClobberingMemoryAccess(Def, AA&: BatchAA);
2398 // We don't want to bail when we run into the store memory def. But,
2399 // the phi access may point to it. So, pretend like we've already
2400 // checked it.
2401 ToCheck.insert(X: Def);
2402 ToCheck.insert(X: Current);
2403 // Start at current (1) to simulate already having checked Def.
2404 for (unsigned I = 1; I < ToCheck.size(); ++I) {
2405 Current = ToCheck[I];
2406 if (auto PhiAccess = dyn_cast<MemoryPhi>(Val: Current)) {
2407 // Check all the operands.
2408 for (auto &Use : PhiAccess->incoming_values())
2409 ToCheck.insert(X: cast<MemoryAccess>(Val: &Use));
2410 continue;
2411 }
2412
2413 // If we found a memory def, bail. This happens when we have an
2414 // unrelated write in between an otherwise noop store.
2415 assert(isa<MemoryDef>(Current) && "Only MemoryDefs should reach here.");
2416 // TODO: Skip no alias MemoryDefs that have no aliasing reads.
2417 // We are searching for the definition of the store's destination.
2418 // So, if that is the same definition as the load, then this is a
2419 // noop. Otherwise, fail.
2420 if (LoadAccess != Current)
2421 return false;
2422 }
2423 return true;
2424 }
2425 }
2426
2427 return false;
2428}
2429
2430bool DSEState::removePartiallyOverlappedStores(InstOverlapIntervalsTy &IOL) {
2431 bool Changed = false;
2432 for (auto OI : IOL) {
2433 Instruction *DeadI = OI.first;
2434 MemoryLocation Loc = *getLocForWrite(I: DeadI);
2435 assert(isRemovable(DeadI) && "Expect only removable instruction");
2436
2437 const Value *Ptr = Loc.Ptr->stripPointerCasts();
2438 int64_t DeadStart = 0;
2439 uint64_t DeadSize = Loc.Size.getValue();
2440 GetPointerBaseWithConstantOffset(Ptr, Offset&: DeadStart, DL);
2441 OverlapIntervalsTy &IntervalMap = OI.second;
2442 Changed |= tryToShortenEnd(DeadI, IntervalMap, DeadStart, DeadSize);
2443 if (IntervalMap.empty())
2444 continue;
2445 Changed |= tryToShortenBegin(DeadI, IntervalMap, DeadStart, DeadSize);
2446 }
2447 return Changed;
2448}
2449
2450bool DSEState::eliminateRedundantStoresOfExistingValues() {
2451 bool MadeChange = false;
2452 LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs that write the "
2453 "already existing value\n");
2454 for (auto *Def : MemDefs) {
2455 if (SkipStores.contains(Ptr: Def) || MSSA.isLiveOnEntryDef(MA: Def))
2456 continue;
2457
2458 Instruction *DefInst = Def->getMemoryInst();
2459 auto MaybeDefLoc = getLocForWrite(I: DefInst);
2460 if (!MaybeDefLoc || !isRemovable(I: DefInst))
2461 continue;
2462
2463 MemoryDef *UpperDef;
2464 // To conserve compile-time, we avoid walking to the next clobbering def.
2465 // Instead, we just try to get the optimized access, if it exists. DSE
2466 // will try to optimize defs during the earlier traversal.
2467 if (Def->isOptimized())
2468 UpperDef = dyn_cast<MemoryDef>(Val: Def->getOptimized());
2469 else
2470 UpperDef = dyn_cast<MemoryDef>(Val: Def->getDefiningAccess());
2471 if (!UpperDef || MSSA.isLiveOnEntryDef(MA: UpperDef))
2472 continue;
2473
2474 Instruction *UpperInst = UpperDef->getMemoryInst();
2475 auto IsRedundantStore = [&]() {
2476 // We don't care about differences in call attributes here.
2477 if (DefInst->isIdenticalToWhenDefined(I: UpperInst,
2478 /*IntersectAttrs=*/true))
2479 return true;
2480 if (auto *MemSetI = dyn_cast<MemSetInst>(Val: UpperInst)) {
2481 if (auto *SI = dyn_cast<StoreInst>(Val: DefInst)) {
2482 // MemSetInst must have a write location.
2483 auto UpperLoc = getLocForWrite(I: UpperInst);
2484 if (!UpperLoc)
2485 return false;
2486 int64_t InstWriteOffset = 0;
2487 int64_t DepWriteOffset = 0;
2488 auto OR = isOverwrite(KillingI: UpperInst, DeadI: DefInst, KillingLoc: *UpperLoc, DeadLoc: *MaybeDefLoc,
2489 KillingOff&: InstWriteOffset, DeadOff&: DepWriteOffset);
2490 Value *StoredByte = isBytewiseValue(V: SI->getValueOperand(), DL);
2491 return StoredByte && StoredByte == MemSetI->getOperand(i_nocapture: 1) &&
2492 OR == OW_Complete;
2493 }
2494 }
2495 return false;
2496 };
2497
2498 if (!IsRedundantStore() || isReadClobber(DefLoc: *MaybeDefLoc, UseInst: DefInst))
2499 continue;
2500 LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *DefInst
2501 << '\n');
2502 deleteDeadInstruction(SI: DefInst);
2503 NumRedundantStores++;
2504 MadeChange = true;
2505 }
2506 return MadeChange;
2507}
2508
2509SmallVector<MemoryLocation, 1>
2510DSEState::getInitializesArgMemLoc(const Instruction *I) {
2511 const CallBase *CB = dyn_cast<CallBase>(Val: I);
2512 if (!CB)
2513 return {};
2514
2515 // Collect aliasing arguments and their initializes ranges.
2516 SmallMapVector<Value *, SmallVector<ArgumentInitInfo, 2>, 2> Arguments;
2517 for (unsigned Idx = 0, Count = CB->arg_size(); Idx < Count; ++Idx) {
2518 Value *CurArg = CB->getArgOperand(i: Idx);
2519 if (!CurArg->getType()->isPointerTy())
2520 continue;
2521
2522 ConstantRangeList Inits;
2523 Attribute InitializesAttr = CB->getParamAttr(ArgNo: Idx, Kind: Attribute::Initializes);
2524 // initializes on byval arguments refers to the callee copy, not the
2525 // original memory the caller passed in.
2526 if (InitializesAttr.isValid() && !CB->isByValArgument(ArgNo: Idx))
2527 Inits = InitializesAttr.getValueAsConstantRangeList();
2528
2529 // Check whether "CurArg" could alias with global variables. We require
2530 // either it's function local and isn't captured before or the "CB" only
2531 // accesses arg or inaccessible mem.
2532 if (!Inits.empty() && !CB->onlyAccessesInaccessibleMemOrArgMem() &&
2533 !isFuncLocalAndNotCaptured(Arg: CurArg, CB, EA))
2534 Inits = ConstantRangeList();
2535
2536 // We don't perform incorrect DSE on unwind edges in the current function,
2537 // and use the "initializes" attribute to kill dead stores if:
2538 // - The call does not throw exceptions, "CB->doesNotThrow()".
2539 // - Or the callee parameter has "dead_on_unwind" attribute.
2540 // - Or the argument is invisible to caller on unwind, and there are no
2541 // unwind edges from this call in the current function (e.g. `CallInst`).
2542 bool IsDeadOrInvisibleOnUnwind =
2543 CB->paramHasAttr(ArgNo: Idx, Kind: Attribute::DeadOnUnwind) ||
2544 (isa<CallInst>(Val: CB) && isInvisibleToCallerOnUnwind(V: CurArg));
2545 ArgumentInitInfo InitInfo{.Idx: Idx, .IsDeadOrInvisibleOnUnwind: IsDeadOrInvisibleOnUnwind, .Inits: Inits};
2546 bool FoundAliasing = false;
2547 for (auto &[Arg, AliasList] : Arguments) {
2548 auto AAR = BatchAA.alias(LocA: MemoryLocation::getBeforeOrAfter(Ptr: Arg),
2549 LocB: MemoryLocation::getBeforeOrAfter(Ptr: CurArg));
2550 if (AAR == AliasResult::NoAlias) {
2551 continue;
2552 } else if (AAR == AliasResult::MustAlias) {
2553 FoundAliasing = true;
2554 AliasList.push_back(Elt: InitInfo);
2555 } else {
2556 // For PartialAlias and MayAlias, there is an offset or may be an
2557 // unknown offset between the arguments and we insert an empty init
2558 // range to discard the entire initializes info while intersecting.
2559 FoundAliasing = true;
2560 AliasList.push_back(Elt: ArgumentInitInfo{.Idx: Idx, .IsDeadOrInvisibleOnUnwind: IsDeadOrInvisibleOnUnwind,
2561 .Inits: ConstantRangeList()});
2562 }
2563 }
2564 if (!FoundAliasing)
2565 Arguments[CurArg] = {InitInfo};
2566 }
2567
2568 SmallVector<MemoryLocation, 1> Locations;
2569 for (const auto &[_, Args] : Arguments) {
2570 auto IntersectedRanges =
2571 getIntersectedInitRangeList(Args, CallHasNoUnwindAttr: CB->doesNotThrow());
2572 if (IntersectedRanges.empty())
2573 continue;
2574
2575 for (const auto &Arg : Args) {
2576 for (const auto &Range : IntersectedRanges) {
2577 int64_t Start = Range.getLower().getSExtValue();
2578 int64_t End = Range.getUpper().getSExtValue();
2579 // For now, we only handle locations starting at offset 0.
2580 if (Start == 0)
2581 Locations.push_back(Elt: MemoryLocation(CB->getArgOperand(i: Arg.Idx),
2582 LocationSize::precise(Value: End - Start),
2583 CB->getAAMetadata()));
2584 }
2585 }
2586 }
2587 return Locations;
2588}
2589
2590std::pair<bool, bool>
2591DSEState::eliminateDeadDefs(const MemoryLocationWrapper &KillingLocWrapper) {
2592 bool Changed = false;
2593 bool DeletedKillingLoc = false;
2594 unsigned ScanLimit = MemorySSAScanLimit;
2595 unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit;
2596 unsigned PartialLimit = MemorySSAPartialStoreLimit;
2597 // Worklist of MemoryAccesses that may be killed by
2598 // "KillingLocWrapper.MemDef".
2599 SmallSetVector<MemoryAccess *, 8> ToCheck;
2600 // Track MemoryAccesses that have been deleted in the loop below, so we can
2601 // skip them. Don't use SkipStores for this, which may contain reused
2602 // MemoryAccess addresses.
2603 SmallPtrSet<MemoryAccess *, 8> Deleted;
2604 [[maybe_unused]] unsigned OrigNumSkipStores = SkipStores.size();
2605 ToCheck.insert(X: KillingLocWrapper.MemDef->getDefiningAccess());
2606
2607 // Check if MemoryAccesses in the worklist are killed by
2608 // "KillingLocWrapper.MemDef".
2609 for (unsigned I = 0; I < ToCheck.size(); I++) {
2610 MemoryAccess *Current = ToCheck[I];
2611 if (Deleted.contains(Ptr: Current))
2612 continue;
2613 std::optional<MemoryAccess *> MaybeDeadAccess = getDomMemoryDef(
2614 KillingDef: KillingLocWrapper.MemDef, StartAccess: Current, KillingLoc: KillingLocWrapper.MemLoc,
2615 KillingUndObj: KillingLocWrapper.UnderlyingObject, ScanLimit, WalkerStepLimit,
2616 IsMemTerm: isMemTerminatorInst(I: KillingLocWrapper.DefInst), PartialLimit,
2617 IsInitializesAttrMemLoc: KillingLocWrapper.DefByInitializesAttr);
2618
2619 if (!MaybeDeadAccess) {
2620 LLVM_DEBUG(dbgs() << " finished walk\n");
2621 continue;
2622 }
2623 MemoryAccess *DeadAccess = *MaybeDeadAccess;
2624 LLVM_DEBUG(dbgs() << " Checking if we can kill " << *DeadAccess);
2625 if (isa<MemoryPhi>(Val: DeadAccess)) {
2626 LLVM_DEBUG(dbgs() << "\n ... adding incoming values to worklist\n");
2627 for (Value *V : cast<MemoryPhi>(Val: DeadAccess)->incoming_values()) {
2628 MemoryAccess *IncomingAccess = cast<MemoryAccess>(Val: V);
2629 BasicBlock *IncomingBlock = IncomingAccess->getBlock();
2630 BasicBlock *PhiBlock = DeadAccess->getBlock();
2631
2632 // We only consider incoming MemoryAccesses that come before the
2633 // MemoryPhi. Otherwise we could discover candidates that do not
2634 // strictly dominate our starting def.
2635 if (PostOrderNumbers[IncomingBlock] > PostOrderNumbers[PhiBlock])
2636 ToCheck.insert(X: IncomingAccess);
2637 }
2638 continue;
2639 }
2640 // We cannot apply the initializes attribute to DeadAccess/DeadDef.
2641 // It would incorrectly consider a call instruction as redundant store
2642 // and remove this call instruction.
2643 // TODO: this conflates the existence of a MemoryLocation with being able
2644 // to delete the instruction. Fix isRemovable() to consider calls with
2645 // side effects that cannot be removed, e.g. calls with the initializes
2646 // attribute, and remove getLocForInst(ConsiderInitializesAttr = false).
2647 MemoryDefWrapper DeadDefWrapper(
2648 cast<MemoryDef>(Val: DeadAccess),
2649 getLocForInst(I: cast<MemoryDef>(Val: DeadAccess)->getMemoryInst(),
2650 /*ConsiderInitializesAttr=*/false));
2651 assert(DeadDefWrapper.DefinedLocations.size() == 1);
2652 MemoryLocationWrapper &DeadLocWrapper =
2653 DeadDefWrapper.DefinedLocations.front();
2654 LLVM_DEBUG(dbgs() << " (" << *DeadLocWrapper.DefInst << ")\n");
2655 ToCheck.insert(X: DeadLocWrapper.MemDef->getDefiningAccess());
2656 NumGetDomMemoryDefPassed++;
2657
2658 if (!DebugCounter::shouldExecute(Counter&: MemorySSACounter))
2659 continue;
2660 if (isMemTerminatorInst(I: KillingLocWrapper.DefInst)) {
2661 if (KillingLocWrapper.UnderlyingObject != DeadLocWrapper.UnderlyingObject)
2662 continue;
2663 LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
2664 << *DeadLocWrapper.DefInst << "\n KILLER: "
2665 << *KillingLocWrapper.DefInst << '\n');
2666 deleteDeadInstruction(SI: DeadLocWrapper.DefInst, Deleted: &Deleted);
2667 ++NumFastStores;
2668 Changed = true;
2669 } else {
2670 // Check if DeadI overwrites KillingI.
2671 int64_t KillingOffset = 0;
2672 int64_t DeadOffset = 0;
2673 OverwriteResult OR =
2674 isOverwrite(KillingI: KillingLocWrapper.DefInst, DeadI: DeadLocWrapper.DefInst,
2675 KillingLoc: KillingLocWrapper.MemLoc, DeadLoc: DeadLocWrapper.MemLoc,
2676 KillingOff&: KillingOffset, DeadOff&: DeadOffset);
2677 if (OR == OW_MaybePartial) {
2678 auto &IOL = IOLs[DeadLocWrapper.DefInst->getParent()];
2679 OR = isPartialOverwrite(KillingLoc: KillingLocWrapper.MemLoc, DeadLoc: DeadLocWrapper.MemLoc,
2680 KillingOff: KillingOffset, DeadOff: DeadOffset,
2681 DeadI: DeadLocWrapper.DefInst, IOL);
2682 }
2683 if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) {
2684 auto *DeadSI = dyn_cast<StoreInst>(Val: DeadLocWrapper.DefInst);
2685 auto *KillingSI = dyn_cast<StoreInst>(Val: KillingLocWrapper.DefInst);
2686 // We are re-using tryToMergePartialOverlappingStores, which requires
2687 // DeadSI to dominate KillingSI.
2688 // TODO: implement tryToMergeParialOverlappingStores using MemorySSA.
2689 if (DeadSI && KillingSI && DT.dominates(Def: DeadSI, User: KillingSI)) {
2690 if (Constant *Merged = tryToMergePartialOverlappingStores(
2691 KillingI: KillingSI, DeadI: DeadSI, KillingOffset, DeadOffset, DL, AA&: BatchAA,
2692 DT: &DT)) {
2693
2694 // Update stored value of earlier store to merged constant.
2695 DeadSI->setOperand(i_nocapture: 0, Val_nocapture: Merged);
2696 ++NumModifiedStores;
2697 Changed = true;
2698 DeletedKillingLoc = true;
2699
2700 // Remove killing store and remove any outstanding overlap
2701 // intervals for the updated store.
2702 deleteDeadInstruction(SI: KillingSI, Deleted: &Deleted);
2703 auto I = IOLs.find(Key: DeadSI->getParent());
2704 if (I != IOLs.end())
2705 I->second.erase(Key: DeadSI);
2706 break;
2707 }
2708 }
2709 }
2710 if (OR == OW_Complete) {
2711 LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
2712 << *DeadLocWrapper.DefInst << "\n KILLER: "
2713 << *KillingLocWrapper.DefInst << '\n');
2714 deleteDeadInstruction(SI: DeadLocWrapper.DefInst, Deleted: &Deleted);
2715 ++NumFastStores;
2716 Changed = true;
2717 }
2718 }
2719 }
2720
2721 assert(SkipStores.size() - OrigNumSkipStores == Deleted.size() &&
2722 "SkipStores and Deleted out of sync?");
2723
2724 return {Changed, DeletedKillingLoc};
2725}
2726
2727bool DSEState::eliminateDeadDefs(const MemoryDefWrapper &KillingDefWrapper) {
2728 if (KillingDefWrapper.DefinedLocations.empty()) {
2729 LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for "
2730 << *KillingDefWrapper.DefInst << "\n");
2731 return false;
2732 }
2733
2734 bool MadeChange = false;
2735 for (auto &KillingLocWrapper : KillingDefWrapper.DefinedLocations) {
2736 LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by "
2737 << *KillingLocWrapper.MemDef << " ("
2738 << *KillingLocWrapper.DefInst << ")\n");
2739 auto [Changed, DeletedKillingLoc] = eliminateDeadDefs(KillingLocWrapper);
2740 MadeChange |= Changed;
2741
2742 // Check if the store is a no-op.
2743 if (!DeletedKillingLoc && storeIsNoop(Def: KillingLocWrapper.MemDef,
2744 DefUO: KillingLocWrapper.UnderlyingObject)) {
2745 LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: "
2746 << *KillingLocWrapper.DefInst << '\n');
2747 deleteDeadInstruction(SI: KillingLocWrapper.DefInst);
2748 NumRedundantStores++;
2749 MadeChange = true;
2750 continue;
2751 }
2752 // Can we form a calloc from a memset/malloc pair?
2753 if (!DeletedKillingLoc &&
2754 tryFoldIntoCalloc(Def: KillingLocWrapper.MemDef,
2755 DefUO: KillingLocWrapper.UnderlyingObject)) {
2756 LLVM_DEBUG(dbgs() << "DSE: Remove memset after forming calloc:\n"
2757 << " DEAD: " << *KillingLocWrapper.DefInst << '\n');
2758 deleteDeadInstruction(SI: KillingLocWrapper.DefInst);
2759 MadeChange = true;
2760 continue;
2761 }
2762 }
2763 return MadeChange;
2764}
2765
2766static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
2767 DominatorTree &DT, PostDominatorTree &PDT,
2768 const TargetLibraryInfo &TLI,
2769 const CycleInfo &CI) {
2770 bool MadeChange = false;
2771 DSEState State(F, AA, MSSA, DT, PDT, TLI, CI);
2772 // For each store:
2773 for (unsigned I = 0; I < State.MemDefs.size(); I++) {
2774 MemoryDef *KillingDef = State.MemDefs[I];
2775 if (State.SkipStores.count(Ptr: KillingDef))
2776 continue;
2777
2778 MemoryDefWrapper KillingDefWrapper(
2779 KillingDef, State.getLocForInst(I: KillingDef->getMemoryInst(),
2780 ConsiderInitializesAttr: EnableInitializesImprovement));
2781 MadeChange |= State.eliminateDeadDefs(KillingDefWrapper);
2782 }
2783
2784 if (EnablePartialOverwriteTracking)
2785 for (auto &KV : State.IOLs)
2786 MadeChange |= State.removePartiallyOverlappedStores(IOL&: KV.second);
2787
2788 MadeChange |= State.eliminateRedundantStoresOfExistingValues();
2789 MadeChange |= State.eliminateDeadWritesAtEndOfFunction();
2790 MadeChange |= State.eliminateRedundantStoresViaDominatingConditions();
2791
2792 while (!State.ToRemove.empty()) {
2793 Instruction *DeadInst = State.ToRemove.pop_back_val();
2794 DeadInst->eraseFromParent();
2795 }
2796
2797 return MadeChange;
2798}
2799
2800//===----------------------------------------------------------------------===//
2801// DSE Pass
2802//===----------------------------------------------------------------------===//
2803PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
2804 AliasAnalysis &AA = AM.getResult<AAManager>(IR&: F);
2805 const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(IR&: F);
2806 DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
2807 MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(IR&: F).getMSSA();
2808 PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(IR&: F);
2809 CycleInfo &CI = AM.getResult<CycleAnalysis>(IR&: F);
2810
2811 bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, CI);
2812
2813#ifdef LLVM_ENABLE_STATS
2814 if (AreStatisticsEnabled())
2815 for (auto &I : instructions(F))
2816 NumRemainingStores += isa<StoreInst>(Val: &I);
2817#endif
2818
2819 if (!Changed)
2820 return PreservedAnalyses::all();
2821
2822 PreservedAnalyses PA;
2823 PA.preserveSet<CFGAnalyses>();
2824 PA.preserve<MemorySSAAnalysis>();
2825 return PA;
2826}
2827
2828namespace {
2829
2830/// A legacy pass for the legacy pass manager that wraps \c DSEPass.
2831class DSELegacyPass : public FunctionPass {
2832public:
2833 static char ID; // Pass identification, replacement for typeid
2834
2835 DSELegacyPass() : FunctionPass(ID) {
2836 initializeDSELegacyPassPass(*PassRegistry::getPassRegistry());
2837 }
2838
2839 bool runOnFunction(Function &F) override {
2840 if (skipFunction(F))
2841 return false;
2842
2843 AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
2844 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
2845 const TargetLibraryInfo &TLI =
2846 getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
2847 MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
2848 PostDominatorTree &PDT =
2849 getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
2850 CycleInfo &CI = getAnalysis<CycleInfoWrapperPass>().getResult();
2851
2852 bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, CI);
2853
2854#ifdef LLVM_ENABLE_STATS
2855 if (AreStatisticsEnabled())
2856 for (auto &I : instructions(F))
2857 NumRemainingStores += isa<StoreInst>(Val: &I);
2858#endif
2859
2860 return Changed;
2861 }
2862
2863 void getAnalysisUsage(AnalysisUsage &AU) const override {
2864 AU.setPreservesCFG();
2865 AU.addRequired<AAResultsWrapperPass>();
2866 AU.addRequired<TargetLibraryInfoWrapperPass>();
2867 AU.addPreserved<GlobalsAAWrapperPass>();
2868 AU.addRequired<DominatorTreeWrapperPass>();
2869 AU.addPreserved<DominatorTreeWrapperPass>();
2870 AU.addRequired<PostDominatorTreeWrapperPass>();
2871 AU.addRequired<MemorySSAWrapperPass>();
2872 AU.addPreserved<PostDominatorTreeWrapperPass>();
2873 AU.addPreserved<MemorySSAWrapperPass>();
2874 AU.addRequired<CycleInfoWrapperPass>();
2875 AU.addPreserved<CycleInfoWrapperPass>();
2876 AU.addRequired<AssumptionCacheTracker>();
2877 }
2878};
2879
2880} // end anonymous namespace
2881
2882char DSELegacyPass::ID = 0;
2883
2884INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false,
2885 false)
2886INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
2887INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
2888INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
2889INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
2890INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
2891INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
2892INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
2893INITIALIZE_PASS_DEPENDENCY(CycleInfoWrapperPass)
2894INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
2895INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
2896 false)
2897
2898LLVM_ABI FunctionPass *llvm::createDeadStoreEliminationPass() {
2899 return new DSELegacyPass();
2900}
2901