1 //===- MergeICmps.cpp - Optimize chains of integer comparisons ------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass turns chains of integer comparisons into memcmp (the memcmp is
11 // later typically inlined as a chain of efficient hardware comparisons). This
12 // typically benefits c++ member or nonmember operator==().
13 //
14 // The basic idea is to replace a larger chain of integer comparisons loaded
15 // from contiguous memory locations into a smaller chain of such integer
16 // comparisons. Benefits are double:
17 // - There are less jumps, and therefore less opportunities for mispredictions
18 // and I-cache misses.
19 // - Code size is smaller, both because jumps are removed and because the
20 // encoding of a 2*n byte compare is smaller than that of two n-byte
21 // compares.
22
23 //===----------------------------------------------------------------------===//
24
25 #include <algorithm>
26 #include <numeric>
27 #include <utility>
28 #include <vector>
29 #include "llvm/Analysis/Loads.h"
30 #include "llvm/Analysis/TargetLibraryInfo.h"
31 #include "llvm/Analysis/TargetTransformInfo.h"
32 #include "llvm/IR/Function.h"
33 #include "llvm/IR/IRBuilder.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Transforms/Scalar.h"
36 #include "llvm/Transforms/Utils/BuildLibCalls.h"
37
38 using namespace llvm;
39
40 namespace {
41
42 #define DEBUG_TYPE "mergeicmps"
43
44 // A BCE atom.
45 struct BCEAtom {
BCEAtom__anonc0097c020111::BCEAtom46 BCEAtom() : GEP(nullptr), LoadI(nullptr), Offset() {}
47
Base__anonc0097c020111::BCEAtom48 const Value *Base() const { return GEP ? GEP->getPointerOperand() : nullptr; }
49
operator <__anonc0097c020111::BCEAtom50 bool operator<(const BCEAtom &O) const {
51 assert(Base() && "invalid atom");
52 assert(O.Base() && "invalid atom");
53 // Just ordering by (Base(), Offset) is sufficient. However because this
54 // means that the ordering will depend on the addresses of the base
55 // values, which are not reproducible from run to run. To guarantee
56 // stability, we use the names of the values if they exist; we sort by:
57 // (Base.getName(), Base(), Offset).
58 const int NameCmp = Base()->getName().compare(O.Base()->getName());
59 if (NameCmp == 0) {
60 if (Base() == O.Base()) {
61 return Offset.slt(O.Offset);
62 }
63 return Base() < O.Base();
64 }
65 return NameCmp < 0;
66 }
67
68 GetElementPtrInst *GEP;
69 LoadInst *LoadI;
70 APInt Offset;
71 };
72
73 // If this value is a load from a constant offset w.r.t. a base address, and
74 // there are no other users of the load or address, returns the base address and
75 // the offset.
visitICmpLoadOperand(Value * const Val)76 BCEAtom visitICmpLoadOperand(Value *const Val) {
77 BCEAtom Result;
78 if (auto *const LoadI = dyn_cast<LoadInst>(Val)) {
79 LLVM_DEBUG(dbgs() << "load\n");
80 if (LoadI->isUsedOutsideOfBlock(LoadI->getParent())) {
81 LLVM_DEBUG(dbgs() << "used outside of block\n");
82 return {};
83 }
84 if (LoadI->isVolatile()) {
85 LLVM_DEBUG(dbgs() << "volatile\n");
86 return {};
87 }
88 Value *const Addr = LoadI->getOperand(0);
89 if (auto *const GEP = dyn_cast<GetElementPtrInst>(Addr)) {
90 LLVM_DEBUG(dbgs() << "GEP\n");
91 if (LoadI->isUsedOutsideOfBlock(LoadI->getParent())) {
92 LLVM_DEBUG(dbgs() << "used outside of block\n");
93 return {};
94 }
95 const auto &DL = GEP->getModule()->getDataLayout();
96 if (!isDereferenceablePointer(GEP, DL)) {
97 LLVM_DEBUG(dbgs() << "not dereferenceable\n");
98 // We need to make sure that we can do comparison in any order, so we
99 // require memory to be unconditionnally dereferencable.
100 return {};
101 }
102 Result.Offset = APInt(DL.getPointerTypeSizeInBits(GEP->getType()), 0);
103 if (GEP->accumulateConstantOffset(DL, Result.Offset)) {
104 Result.GEP = GEP;
105 Result.LoadI = LoadI;
106 }
107 }
108 }
109 return Result;
110 }
111
112 // A basic block with a comparison between two BCE atoms.
113 // The block might do extra work besides the atom comparison, in which case
114 // doesOtherWork() returns true. Under some conditions, the block can be
115 // split into the atom comparison part and the "other work" part
116 // (see canSplit()).
117 // Note: the terminology is misleading: the comparison is symmetric, so there
118 // is no real {l/r}hs. What we want though is to have the same base on the
119 // left (resp. right), so that we can detect consecutive loads. To ensure this
120 // we put the smallest atom on the left.
121 class BCECmpBlock {
122 public:
BCECmpBlock()123 BCECmpBlock() {}
124
BCECmpBlock(BCEAtom L,BCEAtom R,int SizeBits)125 BCECmpBlock(BCEAtom L, BCEAtom R, int SizeBits)
126 : Lhs_(L), Rhs_(R), SizeBits_(SizeBits) {
127 if (Rhs_ < Lhs_) std::swap(Rhs_, Lhs_);
128 }
129
IsValid() const130 bool IsValid() const {
131 return Lhs_.Base() != nullptr && Rhs_.Base() != nullptr;
132 }
133
134 // Assert the block is consistent: If valid, it should also have
135 // non-null members besides Lhs_ and Rhs_.
AssertConsistent() const136 void AssertConsistent() const {
137 if (IsValid()) {
138 assert(BB);
139 assert(CmpI);
140 assert(BranchI);
141 }
142 }
143
Lhs() const144 const BCEAtom &Lhs() const { return Lhs_; }
Rhs() const145 const BCEAtom &Rhs() const { return Rhs_; }
SizeBits() const146 int SizeBits() const { return SizeBits_; }
147
148 // Returns true if the block does other works besides comparison.
149 bool doesOtherWork() const;
150
151 // Returns true if the non-BCE-cmp instructions can be separated from BCE-cmp
152 // instructions in the block.
153 bool canSplit() const;
154
155 // Return true if this all the relevant instructions in the BCE-cmp-block can
156 // be sunk below this instruction. By doing this, we know we can separate the
157 // BCE-cmp-block instructions from the non-BCE-cmp-block instructions in the
158 // block.
159 bool canSinkBCECmpInst(const Instruction *, DenseSet<Instruction *> &) const;
160
161 // We can separate the BCE-cmp-block instructions and the non-BCE-cmp-block
162 // instructions. Split the old block and move all non-BCE-cmp-insts into the
163 // new parent block.
164 void split(BasicBlock *NewParent) const;
165
166 // The basic block where this comparison happens.
167 BasicBlock *BB = nullptr;
168 // The ICMP for this comparison.
169 ICmpInst *CmpI = nullptr;
170 // The terminating branch.
171 BranchInst *BranchI = nullptr;
172 // The block requires splitting.
173 bool RequireSplit = false;
174
175 private:
176 BCEAtom Lhs_;
177 BCEAtom Rhs_;
178 int SizeBits_ = 0;
179 };
180
canSinkBCECmpInst(const Instruction * Inst,DenseSet<Instruction * > & BlockInsts) const181 bool BCECmpBlock::canSinkBCECmpInst(const Instruction *Inst,
182 DenseSet<Instruction *> &BlockInsts) const {
183 // If this instruction has side effects and its in middle of the BCE cmp block
184 // instructions, then bail for now.
185 // TODO: use alias analysis to tell whether there is real interference.
186 if (Inst->mayHaveSideEffects())
187 return false;
188 // Make sure this instruction does not use any of the BCE cmp block
189 // instructions as operand.
190 for (auto BI : BlockInsts) {
191 if (is_contained(Inst->operands(), BI))
192 return false;
193 }
194 return true;
195 }
196
split(BasicBlock * NewParent) const197 void BCECmpBlock::split(BasicBlock *NewParent) const {
198 DenseSet<Instruction *> BlockInsts(
199 {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
200 llvm::SmallVector<Instruction *, 4> OtherInsts;
201 for (Instruction &Inst : *BB) {
202 if (BlockInsts.count(&Inst))
203 continue;
204 assert(canSinkBCECmpInst(&Inst, BlockInsts) && "Split unsplittable block");
205 // This is a non-BCE-cmp-block instruction. And it can be separated
206 // from the BCE-cmp-block instruction.
207 OtherInsts.push_back(&Inst);
208 }
209
210 // Do the actual spliting.
211 for (Instruction *Inst : reverse(OtherInsts)) {
212 Inst->moveBefore(&*NewParent->begin());
213 }
214 }
215
canSplit() const216 bool BCECmpBlock::canSplit() const {
217 DenseSet<Instruction *> BlockInsts(
218 {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
219 for (Instruction &Inst : *BB) {
220 if (!BlockInsts.count(&Inst)) {
221 if (!canSinkBCECmpInst(&Inst, BlockInsts))
222 return false;
223 }
224 }
225 return true;
226 }
227
doesOtherWork() const228 bool BCECmpBlock::doesOtherWork() const {
229 AssertConsistent();
230 // All the instructions we care about in the BCE cmp block.
231 DenseSet<Instruction *> BlockInsts(
232 {Lhs_.GEP, Rhs_.GEP, Lhs_.LoadI, Rhs_.LoadI, CmpI, BranchI});
233 // TODO(courbet): Can we allow some other things ? This is very conservative.
234 // We might be able to get away with anything does not have any side
235 // effects outside of the basic block.
236 // Note: The GEPs and/or loads are not necessarily in the same block.
237 for (const Instruction &Inst : *BB) {
238 if (!BlockInsts.count(&Inst))
239 return true;
240 }
241 return false;
242 }
243
244 // Visit the given comparison. If this is a comparison between two valid
245 // BCE atoms, returns the comparison.
visitICmp(const ICmpInst * const CmpI,const ICmpInst::Predicate ExpectedPredicate)246 BCECmpBlock visitICmp(const ICmpInst *const CmpI,
247 const ICmpInst::Predicate ExpectedPredicate) {
248 // The comparison can only be used once:
249 // - For intermediate blocks, as a branch condition.
250 // - For the final block, as an incoming value for the Phi.
251 // If there are any other uses of the comparison, we cannot merge it with
252 // other comparisons as we would create an orphan use of the value.
253 if (!CmpI->hasOneUse()) {
254 LLVM_DEBUG(dbgs() << "cmp has several uses\n");
255 return {};
256 }
257 if (CmpI->getPredicate() == ExpectedPredicate) {
258 LLVM_DEBUG(dbgs() << "cmp "
259 << (ExpectedPredicate == ICmpInst::ICMP_EQ ? "eq" : "ne")
260 << "\n");
261 auto Lhs = visitICmpLoadOperand(CmpI->getOperand(0));
262 if (!Lhs.Base()) return {};
263 auto Rhs = visitICmpLoadOperand(CmpI->getOperand(1));
264 if (!Rhs.Base()) return {};
265 return BCECmpBlock(std::move(Lhs), std::move(Rhs),
266 CmpI->getOperand(0)->getType()->getScalarSizeInBits());
267 }
268 return {};
269 }
270
271 // Visit the given comparison block. If this is a comparison between two valid
272 // BCE atoms, returns the comparison.
visitCmpBlock(Value * const Val,BasicBlock * const Block,const BasicBlock * const PhiBlock)273 BCECmpBlock visitCmpBlock(Value *const Val, BasicBlock *const Block,
274 const BasicBlock *const PhiBlock) {
275 if (Block->empty()) return {};
276 auto *const BranchI = dyn_cast<BranchInst>(Block->getTerminator());
277 if (!BranchI) return {};
278 LLVM_DEBUG(dbgs() << "branch\n");
279 if (BranchI->isUnconditional()) {
280 // In this case, we expect an incoming value which is the result of the
281 // comparison. This is the last link in the chain of comparisons (note
282 // that this does not mean that this is the last incoming value, blocks
283 // can be reordered).
284 auto *const CmpI = dyn_cast<ICmpInst>(Val);
285 if (!CmpI) return {};
286 LLVM_DEBUG(dbgs() << "icmp\n");
287 auto Result = visitICmp(CmpI, ICmpInst::ICMP_EQ);
288 Result.CmpI = CmpI;
289 Result.BranchI = BranchI;
290 return Result;
291 } else {
292 // In this case, we expect a constant incoming value (the comparison is
293 // chained).
294 const auto *const Const = dyn_cast<ConstantInt>(Val);
295 LLVM_DEBUG(dbgs() << "const\n");
296 if (!Const->isZero()) return {};
297 LLVM_DEBUG(dbgs() << "false\n");
298 auto *const CmpI = dyn_cast<ICmpInst>(BranchI->getCondition());
299 if (!CmpI) return {};
300 LLVM_DEBUG(dbgs() << "icmp\n");
301 assert(BranchI->getNumSuccessors() == 2 && "expecting a cond branch");
302 BasicBlock *const FalseBlock = BranchI->getSuccessor(1);
303 auto Result = visitICmp(
304 CmpI, FalseBlock == PhiBlock ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE);
305 Result.CmpI = CmpI;
306 Result.BranchI = BranchI;
307 return Result;
308 }
309 return {};
310 }
311
enqueueBlock(std::vector<BCECmpBlock> & Comparisons,BCECmpBlock & Comparison)312 static inline void enqueueBlock(std::vector<BCECmpBlock> &Comparisons,
313 BCECmpBlock &Comparison) {
314 LLVM_DEBUG(dbgs() << "Block '" << Comparison.BB->getName()
315 << "': Found cmp of " << Comparison.SizeBits()
316 << " bits between " << Comparison.Lhs().Base() << " + "
317 << Comparison.Lhs().Offset << " and "
318 << Comparison.Rhs().Base() << " + "
319 << Comparison.Rhs().Offset << "\n");
320 LLVM_DEBUG(dbgs() << "\n");
321 Comparisons.push_back(Comparison);
322 }
323
324 // A chain of comparisons.
325 class BCECmpChain {
326 public:
327 BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi);
328
size() const329 int size() const { return Comparisons_.size(); }
330
331 #ifdef MERGEICMPS_DOT_ON
332 void dump() const;
333 #endif // MERGEICMPS_DOT_ON
334
335 bool simplify(const TargetLibraryInfo *const TLI);
336
337 private:
IsContiguous(const BCECmpBlock & First,const BCECmpBlock & Second)338 static bool IsContiguous(const BCECmpBlock &First,
339 const BCECmpBlock &Second) {
340 return First.Lhs().Base() == Second.Lhs().Base() &&
341 First.Rhs().Base() == Second.Rhs().Base() &&
342 First.Lhs().Offset + First.SizeBits() / 8 == Second.Lhs().Offset &&
343 First.Rhs().Offset + First.SizeBits() / 8 == Second.Rhs().Offset;
344 }
345
346 // Merges the given comparison blocks into one memcmp block and update
347 // branches. Comparisons are assumed to be continguous. If NextBBInChain is
348 // null, the merged block will link to the phi block.
349 void mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
350 BasicBlock *const NextBBInChain, PHINode &Phi,
351 const TargetLibraryInfo *const TLI);
352
353 PHINode &Phi_;
354 std::vector<BCECmpBlock> Comparisons_;
355 // The original entry block (before sorting);
356 BasicBlock *EntryBlock_;
357 };
358
BCECmpChain(const std::vector<BasicBlock * > & Blocks,PHINode & Phi)359 BCECmpChain::BCECmpChain(const std::vector<BasicBlock *> &Blocks, PHINode &Phi)
360 : Phi_(Phi) {
361 assert(!Blocks.empty() && "a chain should have at least one block");
362 // Now look inside blocks to check for BCE comparisons.
363 std::vector<BCECmpBlock> Comparisons;
364 for (size_t BlockIdx = 0; BlockIdx < Blocks.size(); ++BlockIdx) {
365 BasicBlock *const Block = Blocks[BlockIdx];
366 assert(Block && "invalid block");
367 BCECmpBlock Comparison = visitCmpBlock(Phi.getIncomingValueForBlock(Block),
368 Block, Phi.getParent());
369 Comparison.BB = Block;
370 if (!Comparison.IsValid()) {
371 LLVM_DEBUG(dbgs() << "chain with invalid BCECmpBlock, no merge.\n");
372 return;
373 }
374 if (Comparison.doesOtherWork()) {
375 LLVM_DEBUG(dbgs() << "block '" << Comparison.BB->getName()
376 << "' does extra work besides compare\n");
377 if (Comparisons.empty()) {
378 // This is the initial block in the chain, in case this block does other
379 // work, we can try to split the block and move the irrelevant
380 // instructions to the predecessor.
381 //
382 // If this is not the initial block in the chain, splitting it wont
383 // work.
384 //
385 // As once split, there will still be instructions before the BCE cmp
386 // instructions that do other work in program order, i.e. within the
387 // chain before sorting. Unless we can abort the chain at this point
388 // and start anew.
389 //
390 // NOTE: we only handle block with single predecessor for now.
391 if (Comparison.canSplit()) {
392 LLVM_DEBUG(dbgs()
393 << "Split initial block '" << Comparison.BB->getName()
394 << "' that does extra work besides compare\n");
395 Comparison.RequireSplit = true;
396 enqueueBlock(Comparisons, Comparison);
397 } else {
398 LLVM_DEBUG(dbgs()
399 << "ignoring initial block '" << Comparison.BB->getName()
400 << "' that does extra work besides compare\n");
401 }
402 continue;
403 }
404 // TODO(courbet): Right now we abort the whole chain. We could be
405 // merging only the blocks that don't do other work and resume the
406 // chain from there. For example:
407 // if (a[0] == b[0]) { // bb1
408 // if (a[1] == b[1]) { // bb2
409 // some_value = 3; //bb3
410 // if (a[2] == b[2]) { //bb3
411 // do a ton of stuff //bb4
412 // }
413 // }
414 // }
415 //
416 // This is:
417 //
418 // bb1 --eq--> bb2 --eq--> bb3* -eq--> bb4 --+
419 // \ \ \ \
420 // ne ne ne \
421 // \ \ \ v
422 // +------------+-----------+----------> bb_phi
423 //
424 // We can only merge the first two comparisons, because bb3* does
425 // "other work" (setting some_value to 3).
426 // We could still merge bb1 and bb2 though.
427 return;
428 }
429 enqueueBlock(Comparisons, Comparison);
430 }
431
432 // It is possible we have no suitable comparison to merge.
433 if (Comparisons.empty()) {
434 LLVM_DEBUG(dbgs() << "chain with no BCE basic blocks, no merge\n");
435 return;
436 }
437 EntryBlock_ = Comparisons[0].BB;
438 Comparisons_ = std::move(Comparisons);
439 #ifdef MERGEICMPS_DOT_ON
440 errs() << "BEFORE REORDERING:\n\n";
441 dump();
442 #endif // MERGEICMPS_DOT_ON
443 // Reorder blocks by LHS. We can do that without changing the
444 // semantics because we are only accessing dereferencable memory.
445 llvm::sort(Comparisons_.begin(), Comparisons_.end(),
446 [](const BCECmpBlock &a, const BCECmpBlock &b) {
447 return a.Lhs() < b.Lhs();
448 });
449 #ifdef MERGEICMPS_DOT_ON
450 errs() << "AFTER REORDERING:\n\n";
451 dump();
452 #endif // MERGEICMPS_DOT_ON
453 }
454
455 #ifdef MERGEICMPS_DOT_ON
dump() const456 void BCECmpChain::dump() const {
457 errs() << "digraph dag {\n";
458 errs() << " graph [bgcolor=transparent];\n";
459 errs() << " node [color=black,style=filled,fillcolor=lightyellow];\n";
460 errs() << " edge [color=black];\n";
461 for (size_t I = 0; I < Comparisons_.size(); ++I) {
462 const auto &Comparison = Comparisons_[I];
463 errs() << " \"" << I << "\" [label=\"%"
464 << Comparison.Lhs().Base()->getName() << " + "
465 << Comparison.Lhs().Offset << " == %"
466 << Comparison.Rhs().Base()->getName() << " + "
467 << Comparison.Rhs().Offset << " (" << (Comparison.SizeBits() / 8)
468 << " bytes)\"];\n";
469 const Value *const Val = Phi_.getIncomingValueForBlock(Comparison.BB);
470 if (I > 0) errs() << " \"" << (I - 1) << "\" -> \"" << I << "\";\n";
471 errs() << " \"" << I << "\" -> \"Phi\" [label=\"" << *Val << "\"];\n";
472 }
473 errs() << " \"Phi\" [label=\"Phi\"];\n";
474 errs() << "}\n\n";
475 }
476 #endif // MERGEICMPS_DOT_ON
477
simplify(const TargetLibraryInfo * const TLI)478 bool BCECmpChain::simplify(const TargetLibraryInfo *const TLI) {
479 // First pass to check if there is at least one merge. If not, we don't do
480 // anything and we keep analysis passes intact.
481 {
482 bool AtLeastOneMerged = false;
483 for (size_t I = 1; I < Comparisons_.size(); ++I) {
484 if (IsContiguous(Comparisons_[I - 1], Comparisons_[I])) {
485 AtLeastOneMerged = true;
486 break;
487 }
488 }
489 if (!AtLeastOneMerged) return false;
490 }
491
492 // Remove phi references to comparison blocks, they will be rebuilt as we
493 // merge the blocks.
494 for (const auto &Comparison : Comparisons_) {
495 Phi_.removeIncomingValue(Comparison.BB, false);
496 }
497
498 // If entry block is part of the chain, we need to make the first block
499 // of the chain the new entry block of the function.
500 BasicBlock *Entry = &Comparisons_[0].BB->getParent()->getEntryBlock();
501 for (size_t I = 1; I < Comparisons_.size(); ++I) {
502 if (Entry == Comparisons_[I].BB) {
503 BasicBlock *NEntryBB = BasicBlock::Create(Entry->getContext(), "",
504 Entry->getParent(), Entry);
505 BranchInst::Create(Entry, NEntryBB);
506 break;
507 }
508 }
509
510 // Point the predecessors of the chain to the first comparison block (which is
511 // the new entry point) and update the entry block of the chain.
512 if (EntryBlock_ != Comparisons_[0].BB) {
513 EntryBlock_->replaceAllUsesWith(Comparisons_[0].BB);
514 EntryBlock_ = Comparisons_[0].BB;
515 }
516
517 // Effectively merge blocks.
518 int NumMerged = 1;
519 for (size_t I = 1; I < Comparisons_.size(); ++I) {
520 if (IsContiguous(Comparisons_[I - 1], Comparisons_[I])) {
521 ++NumMerged;
522 } else {
523 // Merge all previous comparisons and start a new merge block.
524 mergeComparisons(
525 makeArrayRef(Comparisons_).slice(I - NumMerged, NumMerged),
526 Comparisons_[I].BB, Phi_, TLI);
527 NumMerged = 1;
528 }
529 }
530 mergeComparisons(makeArrayRef(Comparisons_)
531 .slice(Comparisons_.size() - NumMerged, NumMerged),
532 nullptr, Phi_, TLI);
533
534 return true;
535 }
536
mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,BasicBlock * const NextBBInChain,PHINode & Phi,const TargetLibraryInfo * const TLI)537 void BCECmpChain::mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
538 BasicBlock *const NextBBInChain,
539 PHINode &Phi,
540 const TargetLibraryInfo *const TLI) {
541 assert(!Comparisons.empty());
542 const auto &FirstComparison = *Comparisons.begin();
543 BasicBlock *const BB = FirstComparison.BB;
544 LLVMContext &Context = BB->getContext();
545
546 if (Comparisons.size() >= 2) {
547 // If there is one block that requires splitting, we do it now, i.e.
548 // just before we know we will collapse the chain. The instructions
549 // can be executed before any of the instructions in the chain.
550 auto C = std::find_if(Comparisons.begin(), Comparisons.end(),
551 [](const BCECmpBlock &B) { return B.RequireSplit; });
552 if (C != Comparisons.end())
553 C->split(EntryBlock_);
554
555 LLVM_DEBUG(dbgs() << "Merging " << Comparisons.size() << " comparisons\n");
556 const auto TotalSize =
557 std::accumulate(Comparisons.begin(), Comparisons.end(), 0,
558 [](int Size, const BCECmpBlock &C) {
559 return Size + C.SizeBits();
560 }) /
561 8;
562
563 // Incoming edges do not need to be updated, and both GEPs are already
564 // computing the right address, we just need to:
565 // - replace the two loads and the icmp with the memcmp
566 // - update the branch
567 // - update the incoming values in the phi.
568 FirstComparison.BranchI->eraseFromParent();
569 FirstComparison.CmpI->eraseFromParent();
570 FirstComparison.Lhs().LoadI->eraseFromParent();
571 FirstComparison.Rhs().LoadI->eraseFromParent();
572
573 IRBuilder<> Builder(BB);
574 const auto &DL = Phi.getModule()->getDataLayout();
575 Value *const MemCmpCall = emitMemCmp(
576 FirstComparison.Lhs().GEP, FirstComparison.Rhs().GEP,
577 ConstantInt::get(DL.getIntPtrType(Context), TotalSize),
578 Builder, DL, TLI);
579 Value *const MemCmpIsZero = Builder.CreateICmpEQ(
580 MemCmpCall, ConstantInt::get(Type::getInt32Ty(Context), 0));
581
582 // Add a branch to the next basic block in the chain.
583 if (NextBBInChain) {
584 Builder.CreateCondBr(MemCmpIsZero, NextBBInChain, Phi.getParent());
585 Phi.addIncoming(ConstantInt::getFalse(Context), BB);
586 } else {
587 Builder.CreateBr(Phi.getParent());
588 Phi.addIncoming(MemCmpIsZero, BB);
589 }
590
591 // Delete merged blocks.
592 for (size_t I = 1; I < Comparisons.size(); ++I) {
593 BasicBlock *CBB = Comparisons[I].BB;
594 CBB->replaceAllUsesWith(BB);
595 CBB->eraseFromParent();
596 }
597 } else {
598 assert(Comparisons.size() == 1);
599 // There are no blocks to merge, but we still need to update the branches.
600 LLVM_DEBUG(dbgs() << "Only one comparison, updating branches\n");
601 if (NextBBInChain) {
602 if (FirstComparison.BranchI->isConditional()) {
603 LLVM_DEBUG(dbgs() << "conditional -> conditional\n");
604 // Just update the "true" target, the "false" target should already be
605 // the phi block.
606 assert(FirstComparison.BranchI->getSuccessor(1) == Phi.getParent());
607 FirstComparison.BranchI->setSuccessor(0, NextBBInChain);
608 Phi.addIncoming(ConstantInt::getFalse(Context), BB);
609 } else {
610 LLVM_DEBUG(dbgs() << "unconditional -> conditional\n");
611 // Replace the unconditional branch by a conditional one.
612 FirstComparison.BranchI->eraseFromParent();
613 IRBuilder<> Builder(BB);
614 Builder.CreateCondBr(FirstComparison.CmpI, NextBBInChain,
615 Phi.getParent());
616 Phi.addIncoming(FirstComparison.CmpI, BB);
617 }
618 } else {
619 if (FirstComparison.BranchI->isConditional()) {
620 LLVM_DEBUG(dbgs() << "conditional -> unconditional\n");
621 // Replace the conditional branch by an unconditional one.
622 FirstComparison.BranchI->eraseFromParent();
623 IRBuilder<> Builder(BB);
624 Builder.CreateBr(Phi.getParent());
625 Phi.addIncoming(FirstComparison.CmpI, BB);
626 } else {
627 LLVM_DEBUG(dbgs() << "unconditional -> unconditional\n");
628 Phi.addIncoming(FirstComparison.CmpI, BB);
629 }
630 }
631 }
632 }
633
getOrderedBlocks(PHINode & Phi,BasicBlock * const LastBlock,int NumBlocks)634 std::vector<BasicBlock *> getOrderedBlocks(PHINode &Phi,
635 BasicBlock *const LastBlock,
636 int NumBlocks) {
637 // Walk up from the last block to find other blocks.
638 std::vector<BasicBlock *> Blocks(NumBlocks);
639 assert(LastBlock && "invalid last block");
640 BasicBlock *CurBlock = LastBlock;
641 for (int BlockIndex = NumBlocks - 1; BlockIndex > 0; --BlockIndex) {
642 if (CurBlock->hasAddressTaken()) {
643 // Somebody is jumping to the block through an address, all bets are
644 // off.
645 LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
646 << " has its address taken\n");
647 return {};
648 }
649 Blocks[BlockIndex] = CurBlock;
650 auto *SinglePredecessor = CurBlock->getSinglePredecessor();
651 if (!SinglePredecessor) {
652 // The block has two or more predecessors.
653 LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
654 << " has two or more predecessors\n");
655 return {};
656 }
657 if (Phi.getBasicBlockIndex(SinglePredecessor) < 0) {
658 // The block does not link back to the phi.
659 LLVM_DEBUG(dbgs() << "skip: block " << BlockIndex
660 << " does not link back to the phi\n");
661 return {};
662 }
663 CurBlock = SinglePredecessor;
664 }
665 Blocks[0] = CurBlock;
666 return Blocks;
667 }
668
processPhi(PHINode & Phi,const TargetLibraryInfo * const TLI)669 bool processPhi(PHINode &Phi, const TargetLibraryInfo *const TLI) {
670 LLVM_DEBUG(dbgs() << "processPhi()\n");
671 if (Phi.getNumIncomingValues() <= 1) {
672 LLVM_DEBUG(dbgs() << "skip: only one incoming value in phi\n");
673 return false;
674 }
675 // We are looking for something that has the following structure:
676 // bb1 --eq--> bb2 --eq--> bb3 --eq--> bb4 --+
677 // \ \ \ \
678 // ne ne ne \
679 // \ \ \ v
680 // +------------+-----------+----------> bb_phi
681 //
682 // - The last basic block (bb4 here) must branch unconditionally to bb_phi.
683 // It's the only block that contributes a non-constant value to the Phi.
684 // - All other blocks (b1, b2, b3) must have exactly two successors, one of
685 // them being the phi block.
686 // - All intermediate blocks (bb2, bb3) must have only one predecessor.
687 // - Blocks cannot do other work besides the comparison, see doesOtherWork()
688
689 // The blocks are not necessarily ordered in the phi, so we start from the
690 // last block and reconstruct the order.
691 BasicBlock *LastBlock = nullptr;
692 for (unsigned I = 0; I < Phi.getNumIncomingValues(); ++I) {
693 if (isa<ConstantInt>(Phi.getIncomingValue(I))) continue;
694 if (LastBlock) {
695 // There are several non-constant values.
696 LLVM_DEBUG(dbgs() << "skip: several non-constant values\n");
697 return false;
698 }
699 if (!isa<ICmpInst>(Phi.getIncomingValue(I)) ||
700 cast<ICmpInst>(Phi.getIncomingValue(I))->getParent() !=
701 Phi.getIncomingBlock(I)) {
702 // Non-constant incoming value is not from a cmp instruction or not
703 // produced by the last block. We could end up processing the value
704 // producing block more than once.
705 //
706 // This is an uncommon case, so we bail.
707 LLVM_DEBUG(
708 dbgs()
709 << "skip: non-constant value not from cmp or not from last block.\n");
710 return false;
711 }
712 LastBlock = Phi.getIncomingBlock(I);
713 }
714 if (!LastBlock) {
715 // There is no non-constant block.
716 LLVM_DEBUG(dbgs() << "skip: no non-constant block\n");
717 return false;
718 }
719 if (LastBlock->getSingleSuccessor() != Phi.getParent()) {
720 LLVM_DEBUG(dbgs() << "skip: last block non-phi successor\n");
721 return false;
722 }
723
724 const auto Blocks =
725 getOrderedBlocks(Phi, LastBlock, Phi.getNumIncomingValues());
726 if (Blocks.empty()) return false;
727 BCECmpChain CmpChain(Blocks, Phi);
728
729 if (CmpChain.size() < 2) {
730 LLVM_DEBUG(dbgs() << "skip: only one compare block\n");
731 return false;
732 }
733
734 return CmpChain.simplify(TLI);
735 }
736
737 class MergeICmps : public FunctionPass {
738 public:
739 static char ID;
740
MergeICmps()741 MergeICmps() : FunctionPass(ID) {
742 initializeMergeICmpsPass(*PassRegistry::getPassRegistry());
743 }
744
runOnFunction(Function & F)745 bool runOnFunction(Function &F) override {
746 if (skipFunction(F)) return false;
747 const auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
748 const auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
749 auto PA = runImpl(F, &TLI, &TTI);
750 return !PA.areAllPreserved();
751 }
752
753 private:
getAnalysisUsage(AnalysisUsage & AU) const754 void getAnalysisUsage(AnalysisUsage &AU) const override {
755 AU.addRequired<TargetLibraryInfoWrapperPass>();
756 AU.addRequired<TargetTransformInfoWrapperPass>();
757 }
758
759 PreservedAnalyses runImpl(Function &F, const TargetLibraryInfo *TLI,
760 const TargetTransformInfo *TTI);
761 };
762
runImpl(Function & F,const TargetLibraryInfo * TLI,const TargetTransformInfo * TTI)763 PreservedAnalyses MergeICmps::runImpl(Function &F, const TargetLibraryInfo *TLI,
764 const TargetTransformInfo *TTI) {
765 LLVM_DEBUG(dbgs() << "MergeICmpsPass: " << F.getName() << "\n");
766
767 // We only try merging comparisons if the target wants to expand memcmp later.
768 // The rationale is to avoid turning small chains into memcmp calls.
769 if (!TTI->enableMemCmpExpansion(true)) return PreservedAnalyses::all();
770
771 // If we don't have memcmp avaiable we can't emit calls to it.
772 if (!TLI->has(LibFunc_memcmp))
773 return PreservedAnalyses::all();
774
775 bool MadeChange = false;
776
777 for (auto BBIt = ++F.begin(); BBIt != F.end(); ++BBIt) {
778 // A Phi operation is always first in a basic block.
779 if (auto *const Phi = dyn_cast<PHINode>(&*BBIt->begin()))
780 MadeChange |= processPhi(*Phi, TLI);
781 }
782
783 if (MadeChange) return PreservedAnalyses::none();
784 return PreservedAnalyses::all();
785 }
786
787 } // namespace
788
789 char MergeICmps::ID = 0;
790 INITIALIZE_PASS_BEGIN(MergeICmps, "mergeicmps",
791 "Merge contiguous icmps into a memcmp", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)792 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
793 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
794 INITIALIZE_PASS_END(MergeICmps, "mergeicmps",
795 "Merge contiguous icmps into a memcmp", false, false)
796
797 Pass *llvm::createMergeICmpsPass() { return new MergeICmps(); }
798