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