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/InitializePasses.h"
84 #include "llvm/Support/Debug.h"
85 #include "llvm/Support/raw_ostream.h"
86 #include "llvm/Transforms/Scalar.h"
87 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
88 
89 using namespace llvm;
90 
91 #define DEBUG_TYPE "mldst-motion"
92 
93 namespace {
94 //===----------------------------------------------------------------------===//
95 //                         MergedLoadStoreMotion Pass
96 //===----------------------------------------------------------------------===//
97 class 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;
107 public:
108   MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {}
109   bool run(Function &F, AliasAnalysis &AA);
110 
111 private:
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 ///
129 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
130   assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
131   return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();
132 }
133 
134 ///
135 /// True when BB is the head of a diamond (hammock)
136 ///
137 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
138   if (!BB)
139     return false;
140   auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
141   if (!BI || !BI->isConditional())
142     return false;
143 
144   BasicBlock *Succ0 = BI->getSuccessor(0);
145   BasicBlock *Succ1 = BI->getSuccessor(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 ///
169 bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
170                                                       const Instruction &End,
171                                                       MemoryLocation Loc) {
172   for (const Instruction &Inst :
173        make_range(Start.getIterator(), End.getIterator()))
174     if (Inst.mayThrow())
175       return true;
176   return AA->canInstructionRangeModRef(Start, End, Loc, 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 ///
184 StoreInst *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(*BB1)) {
189     auto *Store1 = dyn_cast<StoreInst>(&Inst);
190     if (!Store1)
191       continue;
192 
193     MemoryLocation Loc0 = MemoryLocation::get(Store0);
194     MemoryLocation Loc1 = MemoryLocation::get(Store1);
195 
196     if (AA->isMustAlias(Loc0, Loc1) &&
197         !isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&
198         !isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0) &&
199         Store0->hasSameSpecialState(Store1) &&
200         CastInst::isBitOrNoopPointerCastable(
201             Store0->getValueOperand()->getType(),
202             Store1->getValueOperand()->getType(),
203             Store0->getModule()->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 ///
212 PHINode *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(Opd1->getType(), 2, Opd2->getName() + ".sink",
221                                 &BB->front());
222   NewPN->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());
223   NewPN->addIncoming(Opd1, S0->getParent());
224   NewPN->addIncoming(Opd2, S1->getParent());
225   return NewPN;
226 }
227 
228 ///
229 /// Check if 2 stores can be sunk, optionally together with corresponding GEPs.
230 ///
231 bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0,
232                                                  StoreInst *S1) const {
233   if (S0->getPointerOperand() == S1->getPointerOperand())
234     return true;
235   auto *GEP0 = dyn_cast<GetElementPtrInst>(S0->getPointerOperand());
236   auto *GEP1 = dyn_cast<GetElementPtrInst>(S1->getPointerOperand());
237   return GEP0 && GEP1 && GEP0->isIdenticalTo(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 ///
247 void 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(S1);
259   S0->dropUnknownNonDebugMetadata();
260   S0->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());
261   S0->mergeDIAssignID(S1);
262 
263   // Insert bitcast for conflicting typed stores (or just use original value if
264   // same type).
265   IRBuilder<> Builder(S0);
266   auto Cast = Builder.CreateBitOrPointerCast(S0->getValueOperand(),
267                                              S1->getValueOperand()->getType());
268   S0->setOperand(0, Cast);
269 
270   // Create the new store to be inserted at the join point.
271   StoreInst *SNew = cast<StoreInst>(S0->clone());
272   SNew->insertBefore(&*InsertPt);
273   // New PHI operand? Use it.
274   if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
275     SNew->setOperand(0, NewPN);
276   S0->eraseFromParent();
277   S1->eraseFromParent();
278 
279   if (Ptr0 != Ptr1) {
280     auto *GEP0 = cast<GetElementPtrInst>(Ptr0);
281     auto *GEP1 = cast<GetElementPtrInst>(Ptr1);
282     Instruction *GEPNew = GEP0->clone();
283     GEPNew->insertBefore(SNew);
284     GEPNew->applyMergedLocation(GEP0->getDebugLoc(), GEP1->getDebugLoc());
285     SNew->setOperand(1, GEPNew);
286     GEP0->replaceAllUsesWith(GEPNew);
287     GEP0->eraseFromParent();
288     GEP1->replaceAllUsesWith(GEPNew);
289     GEP1->eraseFromParent();
290   }
291 }
292 
293 ///
294 /// True when two stores are equivalent and can sink into the footer
295 ///
296 /// Starting from a diamond head block, iterate over the instructions in one
297 /// successor block and try to match a store in the second successor.
298 ///
299 bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) {
300 
301   bool MergedStores = false;
302   BasicBlock *TailBB = getDiamondTail(HeadBB);
303   BasicBlock *SinkBB = TailBB;
304   assert(SinkBB && "Footer of a diamond cannot be empty");
305 
306   succ_iterator SI = succ_begin(HeadBB);
307   assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors");
308   BasicBlock *Pred0 = *SI;
309   ++SI;
310   assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor");
311   BasicBlock *Pred1 = *SI;
312   // tail block  of a diamond/hammock?
313   if (Pred0 == Pred1)
314     return false; // No.
315   // bail out early if we can not merge into the footer BB
316   if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(3))
317     return false;
318   // #Instructions in Pred1 for Compile Time Control
319   auto InstsNoDbg = Pred1->instructionsWithoutDebug();
320   int Size1 = std::distance(InstsNoDbg.begin(), InstsNoDbg.end());
321   int NStores = 0;
322 
323   for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
324        RBI != RBE;) {
325 
326     Instruction *I = &*RBI;
327     ++RBI;
328 
329     // Don't sink non-simple (atomic, volatile) stores.
330     auto *S0 = dyn_cast<StoreInst>(I);
331     if (!S0 || !S0->isSimple())
332       continue;
333 
334     ++NStores;
335     if (NStores * Size1 >= MagicCompileTimeControl)
336       break;
337     if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
338       if (!canSinkStoresAndGEPs(S0, S1))
339         // Don't attempt to sink below stores that had to stick around
340         // But after removal of a store and some of its feeding
341         // instruction search again from the beginning since the iterator
342         // is likely stale at this point.
343         break;
344 
345       if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(3)) {
346         // We have more than 2 predecessors. Insert a new block
347         // postdominating 2 predecessors we're going to sink from.
348         SinkBB = SplitBlockPredecessors(TailBB, {Pred0, Pred1}, ".sink.split");
349         if (!SinkBB)
350           break;
351       }
352 
353       MergedStores = true;
354       sinkStoresAndGEPs(SinkBB, S0, S1);
355       RBI = Pred0->rbegin();
356       RBE = Pred0->rend();
357       LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
358     }
359   }
360   return MergedStores;
361 }
362 
363 bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) {
364   this->AA = &AA;
365 
366   bool Changed = false;
367   LLVM_DEBUG(dbgs() << "Instruction Merger\n");
368 
369   // Merge unconditional branches, allowing PRE to catch more
370   // optimization opportunities.
371   // This loop doesn't care about newly inserted/split blocks
372   // since they never will be diamond heads.
373   for (BasicBlock &BB : make_early_inc_range(F))
374     // Hoist equivalent loads and sink stores
375     // outside diamonds when possible
376     if (isDiamondHead(&BB))
377       Changed |= mergeStores(&BB);
378   return Changed;
379 }
380 
381 namespace {
382 class MergedLoadStoreMotionLegacyPass : public FunctionPass {
383   const bool SplitFooterBB;
384 public:
385   static char ID; // Pass identification, replacement for typeid
386   MergedLoadStoreMotionLegacyPass(bool SplitFooterBB = false)
387       : FunctionPass(ID), SplitFooterBB(SplitFooterBB) {
388     initializeMergedLoadStoreMotionLegacyPassPass(
389         *PassRegistry::getPassRegistry());
390   }
391 
392   ///
393   /// Run the transformation for each function
394   ///
395   bool runOnFunction(Function &F) override {
396     if (skipFunction(F))
397       return false;
398     MergedLoadStoreMotion Impl(SplitFooterBB);
399     return Impl.run(F, getAnalysis<AAResultsWrapperPass>().getAAResults());
400   }
401 
402 private:
403   void getAnalysisUsage(AnalysisUsage &AU) const override {
404     if (!SplitFooterBB)
405       AU.setPreservesCFG();
406     AU.addRequired<AAResultsWrapperPass>();
407     AU.addPreserved<GlobalsAAWrapperPass>();
408   }
409 };
410 
411 char MergedLoadStoreMotionLegacyPass::ID = 0;
412 } // anonymous namespace
413 
414 ///
415 /// createMergedLoadStoreMotionPass - The public interface to this file.
416 ///
417 FunctionPass *llvm::createMergedLoadStoreMotionPass(bool SplitFooterBB) {
418   return new MergedLoadStoreMotionLegacyPass(SplitFooterBB);
419 }
420 
421 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
422                       "MergedLoadStoreMotion", false, false)
423 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
424 INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
425                     "MergedLoadStoreMotion", false, false)
426 
427 PreservedAnalyses
428 MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) {
429   MergedLoadStoreMotion Impl(Options.SplitFooterBB);
430   auto &AA = AM.getResult<AAManager>(F);
431   if (!Impl.run(F, AA))
432     return PreservedAnalyses::all();
433 
434   PreservedAnalyses PA;
435   if (!Options.SplitFooterBB)
436     PA.preserveSet<CFGAnalyses>();
437   return PA;
438 }
439 
440 void MergedLoadStoreMotionPass::printPipeline(
441     raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
442   static_cast<PassInfoMixin<MergedLoadStoreMotionPass> *>(this)->printPipeline(
443       OS, MapClassName2PassName);
444   OS << '<';
445   OS << (Options.SplitFooterBB ? "" : "no-") << "split-footer-bb";
446   OS << '>';
447 }
448