1//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
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//! \file
10//! This pass performs merges of loads and stores on both sides of a
11// diamond (hammock). It hoists the loads and sinks the stores.
12//
13// The algorithm iteratively hoists two loads to the same address out of a
14// diamond (hammock) and merges them into a single load in the header. Similar
15// it sinks and merges two stores to the tail block (footer). The algorithm
16// iterates over the instructions of one side of the diamond and attempts to
17// find a matching load/store on the other side. New tail/footer block may be
18// insterted if the tail/footer block has more predecessors (not only the two
19// predecessors that are forming the diamond). It hoists / sinks when it thinks
20// it safe to do so. This optimization helps with eg. hiding load latencies,
21// triggering if-conversion, and reducing static code size.
22//
23// NOTE: This code no longer performs load hoisting, it is subsumed by GVNHoist.
24//
25//===----------------------------------------------------------------------===//
26//
27//
28// Example:
29// Diamond shaped code before merge:
30//
31// header:
32// br %cond, label %if.then, label %if.else
33// + +
34// + +
35// + +
36// if.then: if.else:
37// %lt = load %addr_l %le = load %addr_l
38// <use %lt> <use %le>
39// <...> <...>
40// store %st, %addr_s store %se, %addr_s
41// br label %if.end br label %if.end
42// + +
43// + +
44// + +
45// if.end ("footer"):
46// <...>
47//
48// Diamond shaped code after merge:
49//
50// header:
51// %l = load %addr_l
52// br %cond, label %if.then, label %if.else
53// + +
54// + +
55// + +
56// if.then: if.else:
57// <use %l> <use %l>
58// <...> <...>
59// br label %if.end br label %if.end
60// + +
61// + +
62// + +
63// if.end ("footer"):
64// %s.sink = phi [%st, if.then], [%se, if.else]
65// <...>
66// store %s.sink, %addr_s
67// <...>
68//
69//
70//===----------------------- TODO -----------------------------------------===//
71//
72// 1) Generalize to regions other than diamonds
73// 2) Be more aggressive merging memory operations
74// Note that both changes require register pressure control
75//
76//===----------------------------------------------------------------------===//
77
78#include "llvm/Transforms/Scalar/MergedLoadStoreMotion.h"
79#include "llvm/Analysis/AliasAnalysis.h"
80#include "llvm/Analysis/GlobalsModRef.h"
81#include "llvm/IR/IRBuilder.h"
82#include "llvm/IR/Instructions.h"
83#include "llvm/Support/Debug.h"
84#include "llvm/Support/raw_ostream.h"
85#include "llvm/Transforms/Scalar.h"
86#include "llvm/Transforms/Utils/BasicBlockUtils.h"
87#include "llvm/Transforms/Utils/Local.h"
88
89using namespace llvm;
90
91#define DEBUG_TYPE "mldst-motion"
92
93namespace {
94//===----------------------------------------------------------------------===//
95// MergedLoadStoreMotion Pass
96//===----------------------------------------------------------------------===//
97class MergedLoadStoreMotion {
98 AliasAnalysis *AA = nullptr;
99
100 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
101 // where Size0 and Size1 are the #instructions on the two sides of
102 // the diamond. The constant chosen here is arbitrary. Compiler Time
103 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
104 const int MagicCompileTimeControl = 250;
105
106 const bool SplitFooterBB;
107public:
108 MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {}
109 bool run(Function &F, AliasAnalysis &AA);
110
111private:
112 BasicBlock *getDiamondTail(BasicBlock *BB);
113 bool isDiamondHead(BasicBlock *BB);
114 // Routines for sinking stores
115 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
116 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
117 bool isStoreSinkBarrierInRange(const Instruction &Start,
118 const Instruction &End, MemoryLocation Loc);
119 bool canSinkStoresAndGEPs(StoreInst *S0, StoreInst *S1) const;
120 void sinkStoresAndGEPs(BasicBlock *BB, StoreInst *SinkCand,
121 StoreInst *ElseInst);
122 bool mergeStores(BasicBlock *BB);
123};
124} // end anonymous namespace
125
126///
127/// Return tail block of a diamond.
128///
129BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
130 assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
131 return BB->getTerminator()->getSuccessor(Idx: 0)->getSingleSuccessor();
132}
133
134///
135/// True when BB is the head of a diamond (hammock)
136///
137bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
138 if (!BB)
139 return false;
140 auto *BI = dyn_cast<BranchInst>(Val: BB->getTerminator());
141 if (!BI || !BI->isConditional())
142 return false;
143
144 BasicBlock *Succ0 = BI->getSuccessor(i: 0);
145 BasicBlock *Succ1 = BI->getSuccessor(i: 1);
146
147 if (!Succ0->getSinglePredecessor())
148 return false;
149 if (!Succ1->getSinglePredecessor())
150 return false;
151
152 BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
153 BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
154 // Ignore triangles.
155 if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
156 return false;
157 return true;
158}
159
160
161///
162/// True when instruction is a sink barrier for a store
163/// located in Loc
164///
165/// Whenever an instruction could possibly read or modify the
166/// value being stored or protect against the store from
167/// happening it is considered a sink barrier.
168///
169bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
170 const Instruction &End,
171 MemoryLocation Loc) {
172 for (const Instruction &Inst :
173 make_range(x: Start.getIterator(), y: End.getIterator()))
174 if (Inst.mayThrow())
175 return true;
176 return AA->canInstructionRangeModRef(I1: Start, I2: End, Loc, Mode: ModRefInfo::ModRef);
177}
178
179///
180/// Check if \p BB contains a store to the same address as \p SI
181///
182/// \return The store in \p when it is safe to sink. Otherwise return Null.
183///
184StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
185 StoreInst *Store0) {
186 LLVM_DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
187 BasicBlock *BB0 = Store0->getParent();
188 for (Instruction &Inst : reverse(C&: *BB1)) {
189 auto *Store1 = dyn_cast<StoreInst>(Val: &Inst);
190 if (!Store1)
191 continue;
192
193 MemoryLocation Loc0 = MemoryLocation::get(SI: Store0);
194 MemoryLocation Loc1 = MemoryLocation::get(SI: Store1);
195
196 if (AA->isMustAlias(LocA: Loc0, LocB: Loc1) &&
197 !isStoreSinkBarrierInRange(Start: *Store1->getNextNode(), End: BB1->back(), Loc: Loc1) &&
198 !isStoreSinkBarrierInRange(Start: *Store0->getNextNode(), End: BB0->back(), Loc: Loc0) &&
199 Store0->hasSameSpecialState(I2: Store1) &&
200 CastInst::isBitOrNoopPointerCastable(
201 SrcTy: Store0->getValueOperand()->getType(),
202 DestTy: Store1->getValueOperand()->getType(),
203 DL: Store0->getDataLayout()))
204 return Store1;
205 }
206 return nullptr;
207}
208
209///
210/// Create a PHI node in BB for the operands of S0 and S1
211///
212PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
213 StoreInst *S1) {
214 // Create a phi if the values mismatch.
215 Value *Opd1 = S0->getValueOperand();
216 Value *Opd2 = S1->getValueOperand();
217 if (Opd1 == Opd2)
218 return nullptr;
219
220 auto *NewPN = PHINode::Create(Ty: Opd1->getType(), NumReservedValues: 2, NameStr: Opd2->getName() + ".sink");
221 NewPN->insertBefore(InsertPos: BB->begin());
222 NewPN->applyMergedLocation(LocA: S0->getDebugLoc(), LocB: S1->getDebugLoc());
223 NewPN->addIncoming(V: Opd1, BB: S0->getParent());
224 NewPN->addIncoming(V: Opd2, BB: S1->getParent());
225 return NewPN;
226}
227
228///
229/// Check if 2 stores can be sunk, optionally together with corresponding GEPs.
230///
231bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0,
232 StoreInst *S1) const {
233 if (S0->getPointerOperand() == S1->getPointerOperand())
234 return true;
235 auto *GEP0 = dyn_cast<GetElementPtrInst>(Val: S0->getPointerOperand());
236 auto *GEP1 = dyn_cast<GetElementPtrInst>(Val: S1->getPointerOperand());
237 return GEP0 && GEP1 && GEP0->isIdenticalTo(I: GEP1) && GEP0->hasOneUse() &&
238 (GEP0->getParent() == S0->getParent()) && GEP1->hasOneUse() &&
239 (GEP1->getParent() == S1->getParent());
240}
241
242///
243/// Merge two stores to same address and sink into \p BB
244///
245/// Optionally also sinks GEP instruction computing the store address
246///
247void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0,
248 StoreInst *S1) {
249 Value *Ptr0 = S0->getPointerOperand();
250 Value *Ptr1 = S1->getPointerOperand();
251 // Only one definition?
252 LLVM_DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
253 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
254 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
255 // Hoist the instruction.
256 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
257 // Intersect optional metadata.
258 S0->andIRFlags(V: S1);
259
260 combineMetadataForCSE(K: S0, J: S1, DoesKMove: true);
261 S0->applyMergedLocation(LocA: S0->getDebugLoc(), LocB: S1->getDebugLoc());
262 S0->mergeDIAssignID(SourceInstructions: S1);
263
264 // Insert bitcast for conflicting typed stores (or just use original value if
265 // same type).
266 IRBuilder<> Builder(S0);
267 auto Cast = Builder.CreateBitOrPointerCast(V: S0->getValueOperand(),
268 DestTy: S1->getValueOperand()->getType());
269 S0->setOperand(i_nocapture: 0, Val_nocapture: Cast);
270
271 // Create the new store to be inserted at the join point.
272 StoreInst *SNew = cast<StoreInst>(Val: S0->clone());
273 SNew->insertBefore(InsertPos: InsertPt);
274 // New PHI operand? Use it.
275 if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
276 SNew->setOperand(i_nocapture: 0, Val_nocapture: NewPN);
277 S0->eraseFromParent();
278 S1->eraseFromParent();
279
280 if (Ptr0 != Ptr1) {
281 auto *GEP0 = cast<GetElementPtrInst>(Val: Ptr0);
282 auto *GEP1 = cast<GetElementPtrInst>(Val: Ptr1);
283 Instruction *GEPNew = GEP0->clone();
284 GEPNew->insertBefore(InsertPos: SNew->getIterator());
285 GEPNew->applyMergedLocation(LocA: GEP0->getDebugLoc(), LocB: GEP1->getDebugLoc());
286 SNew->setOperand(i_nocapture: 1, Val_nocapture: GEPNew);
287 GEP0->replaceAllUsesWith(V: GEPNew);
288 GEP0->eraseFromParent();
289 GEP1->replaceAllUsesWith(V: GEPNew);
290 GEP1->eraseFromParent();
291 }
292}
293
294///
295/// True when two stores are equivalent and can sink into the footer
296///
297/// Starting from a diamond head block, iterate over the instructions in one
298/// successor block and try to match a store in the second successor.
299///
300bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) {
301
302 bool MergedStores = false;
303 BasicBlock *TailBB = getDiamondTail(BB: HeadBB);
304 BasicBlock *SinkBB = TailBB;
305 assert(SinkBB && "Footer of a diamond cannot be empty");
306
307 succ_iterator SI = succ_begin(BB: HeadBB);
308 assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors");
309 BasicBlock *Pred0 = *SI;
310 ++SI;
311 assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor");
312 BasicBlock *Pred1 = *SI;
313 // tail block of a diamond/hammock?
314 if (Pred0 == Pred1)
315 return false; // No.
316 // bail out early if we can not merge into the footer BB
317 if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(N: 3))
318 return false;
319 // #Instructions in Pred1 for Compile Time Control
320 auto InstsNoDbg = Pred1->instructionsWithoutDebug();
321 int Size1 = std::distance(first: InstsNoDbg.begin(), last: InstsNoDbg.end());
322 int NStores = 0;
323
324 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
325 RBI != RBE;) {
326
327 Instruction *I = &*RBI;
328 ++RBI;
329
330 // Don't sink non-simple (atomic, volatile) stores.
331 auto *S0 = dyn_cast<StoreInst>(Val: I);
332 if (!S0 || !S0->isSimple())
333 continue;
334
335 ++NStores;
336 if (NStores * Size1 >= MagicCompileTimeControl)
337 break;
338 if (StoreInst *S1 = canSinkFromBlock(BB1: Pred1, Store0: S0)) {
339 if (!canSinkStoresAndGEPs(S0, S1))
340 // Don't attempt to sink below stores that had to stick around
341 // But after removal of a store and some of its feeding
342 // instruction search again from the beginning since the iterator
343 // is likely stale at this point.
344 break;
345
346 if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(N: 3)) {
347 // We have more than 2 predecessors. Insert a new block
348 // postdominating 2 predecessors we're going to sink from.
349 SinkBB = SplitBlockPredecessors(BB: TailBB, Preds: {Pred0, Pred1}, Suffix: ".sink.split");
350 if (!SinkBB)
351 break;
352 }
353
354 MergedStores = true;
355 sinkStoresAndGEPs(BB: SinkBB, S0, S1);
356 RBI = Pred0->rbegin();
357 RBE = Pred0->rend();
358 LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
359 }
360 }
361 return MergedStores;
362}
363
364bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) {
365 this->AA = &AA;
366
367 bool Changed = false;
368 LLVM_DEBUG(dbgs() << "Instruction Merger\n");
369
370 // Merge unconditional branches, allowing PRE to catch more
371 // optimization opportunities.
372 // This loop doesn't care about newly inserted/split blocks
373 // since they never will be diamond heads.
374 for (BasicBlock &BB : make_early_inc_range(Range&: F))
375 // Hoist equivalent loads and sink stores
376 // outside diamonds when possible
377 if (isDiamondHead(BB: &BB))
378 Changed |= mergeStores(HeadBB: &BB);
379 return Changed;
380}
381
382PreservedAnalyses
383MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) {
384 MergedLoadStoreMotion Impl(Options.SplitFooterBB);
385 auto &AA = AM.getResult<AAManager>(IR&: F);
386 if (!Impl.run(F, AA))
387 return PreservedAnalyses::all();
388
389 PreservedAnalyses PA;
390 if (!Options.SplitFooterBB)
391 PA.preserveSet<CFGAnalyses>();
392 return PA;
393}
394
395void MergedLoadStoreMotionPass::printPipeline(
396 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
397 static_cast<PassInfoMixin<MergedLoadStoreMotionPass> *>(this)->printPipeline(
398 OS, MapClassName2PassName);
399 OS << '<';
400 OS << (Options.SplitFooterBB ? "" : "no-") << "split-footer-bb";
401 OS << '>';
402}
403