1//===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
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// This pass reassociates n-ary add expressions and eliminates the redundancy
10// exposed by the reassociation.
11//
12// A motivating example:
13//
14//   void foo(int a, int b) {
15//     bar(a + b);
16//     bar((a + 2) + b);
17//   }
18//
19// An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
20// the above code to
21//
22//   int t = a + b;
23//   bar(t);
24//   bar(t + 2);
25//
26// However, the Reassociate pass is unable to do that because it processes each
27// instruction individually and believes (a + 2) + b is the best form according
28// to its rank system.
29//
30// To address this limitation, NaryReassociate reassociates an expression in a
31// form that reuses existing instructions. As a result, NaryReassociate can
32// reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
33// (a + b) is computed before.
34//
35// NaryReassociate works as follows. For every instruction in the form of (a +
36// b) + c, it checks whether a + c or b + c is already computed by a dominating
37// instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
38// c) + a and removes the redundancy accordingly. To efficiently look up whether
39// an expression is computed before, we store each instruction seen and its SCEV
40// into an SCEV-to-instruction map.
41//
42// Although the algorithm pattern-matches only ternary additions, it
43// automatically handles many >3-ary expressions by walking through the function
44// in the depth-first order. For example, given
45//
46//   (a + c) + d
47//   ((a + b) + c) + d
48//
49// NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
50// ((a + c) + b) + d into ((a + c) + d) + b.
51//
52// Finally, the above dominator-based algorithm may need to be run multiple
53// iterations before emitting optimal code. One source of this need is that we
54// only split an operand when it is used only once. The above algorithm can
55// eliminate an instruction and decrease the usage count of its operands. As a
56// result, an instruction that previously had multiple uses may become a
57// single-use instruction and thus eligible for split consideration. For
58// example,
59//
60//   ac = a + c
61//   ab = a + b
62//   abc = ab + c
63//   ab2 = ab + b
64//   ab2c = ab2 + c
65//
66// In the first iteration, we cannot reassociate abc to ac+b because ab is used
67// twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
68// result, ab2 becomes dead and ab will be used only once in the second
69// iteration.
70//
71// Limitations and TODO items:
72//
73// 1) We only considers n-ary adds and muls for now. This should be extended
74// and generalized.
75//
76//===----------------------------------------------------------------------===//
77
78#include "llvm/Transforms/Scalar/NaryReassociate.h"
79#include "llvm/ADT/DepthFirstIterator.h"
80#include "llvm/ADT/SmallVector.h"
81#include "llvm/Analysis/AssumptionCache.h"
82#include "llvm/Analysis/ScalarEvolution.h"
83#include "llvm/Analysis/TargetLibraryInfo.h"
84#include "llvm/Analysis/TargetTransformInfo.h"
85#include "llvm/Analysis/ValueTracking.h"
86#include "llvm/IR/BasicBlock.h"
87#include "llvm/IR/Constants.h"
88#include "llvm/IR/DataLayout.h"
89#include "llvm/IR/DerivedTypes.h"
90#include "llvm/IR/Dominators.h"
91#include "llvm/IR/Function.h"
92#include "llvm/IR/GetElementPtrTypeIterator.h"
93#include "llvm/IR/IRBuilder.h"
94#include "llvm/IR/InstrTypes.h"
95#include "llvm/IR/Instruction.h"
96#include "llvm/IR/Instructions.h"
97#include "llvm/IR/Module.h"
98#include "llvm/IR/Operator.h"
99#include "llvm/IR/PatternMatch.h"
100#include "llvm/IR/Type.h"
101#include "llvm/IR/Value.h"
102#include "llvm/IR/ValueHandle.h"
103#include "llvm/InitializePasses.h"
104#include "llvm/Pass.h"
105#include "llvm/Support/Casting.h"
106#include "llvm/Support/ErrorHandling.h"
107#include "llvm/Transforms/Scalar.h"
108#include "llvm/Transforms/Utils/Local.h"
109#include <cassert>
110#include <cstdint>
111
112using namespace llvm;
113using namespace PatternMatch;
114
115#define DEBUG_TYPE "nary-reassociate"
116
117namespace {
118
119class NaryReassociateLegacyPass : public FunctionPass {
120public:
121  static char ID;
122
123  NaryReassociateLegacyPass() : FunctionPass(ID) {
124    initializeNaryReassociateLegacyPassPass(*PassRegistry::getPassRegistry());
125  }
126
127  bool doInitialization(Module &M) override {
128    return false;
129  }
130
131  bool runOnFunction(Function &F) override;
132
133  void getAnalysisUsage(AnalysisUsage &AU) const override {
134    AU.addPreserved<DominatorTreeWrapperPass>();
135    AU.addPreserved<ScalarEvolutionWrapperPass>();
136    AU.addPreserved<TargetLibraryInfoWrapperPass>();
137    AU.addRequired<AssumptionCacheTracker>();
138    AU.addRequired<DominatorTreeWrapperPass>();
139    AU.addRequired<ScalarEvolutionWrapperPass>();
140    AU.addRequired<TargetLibraryInfoWrapperPass>();
141    AU.addRequired<TargetTransformInfoWrapperPass>();
142    AU.setPreservesCFG();
143  }
144
145private:
146  NaryReassociatePass Impl;
147};
148
149} // end anonymous namespace
150
151char NaryReassociateLegacyPass::ID = 0;
152
153INITIALIZE_PASS_BEGIN(NaryReassociateLegacyPass, "nary-reassociate",
154                      "Nary reassociation", false, false)
155INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
156INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
157INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
158INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
159INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
160INITIALIZE_PASS_END(NaryReassociateLegacyPass, "nary-reassociate",
161                    "Nary reassociation", false, false)
162
163FunctionPass *llvm::createNaryReassociatePass() {
164  return new NaryReassociateLegacyPass();
165}
166
167bool NaryReassociateLegacyPass::runOnFunction(Function &F) {
168  if (skipFunction(F))
169    return false;
170
171  auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
172  auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
173  auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
174  auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
175  auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
176
177  return Impl.runImpl(F, AC, DT, SE, TLI, TTI);
178}
179
180PreservedAnalyses NaryReassociatePass::run(Function &F,
181                                           FunctionAnalysisManager &AM) {
182  auto *AC = &AM.getResult<AssumptionAnalysis>(F);
183  auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
184  auto *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
185  auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
186  auto *TTI = &AM.getResult<TargetIRAnalysis>(F);
187
188  if (!runImpl(F, AC, DT, SE, TLI, TTI))
189    return PreservedAnalyses::all();
190
191  PreservedAnalyses PA;
192  PA.preserveSet<CFGAnalyses>();
193  PA.preserve<ScalarEvolutionAnalysis>();
194  return PA;
195}
196
197bool NaryReassociatePass::runImpl(Function &F, AssumptionCache *AC_,
198                                  DominatorTree *DT_, ScalarEvolution *SE_,
199                                  TargetLibraryInfo *TLI_,
200                                  TargetTransformInfo *TTI_) {
201  AC = AC_;
202  DT = DT_;
203  SE = SE_;
204  TLI = TLI_;
205  TTI = TTI_;
206  DL = &F.getParent()->getDataLayout();
207
208  bool Changed = false, ChangedInThisIteration;
209  do {
210    ChangedInThisIteration = doOneIteration(F);
211    Changed |= ChangedInThisIteration;
212  } while (ChangedInThisIteration);
213  return Changed;
214}
215
216// Whitelist the instruction types NaryReassociate handles for now.
217static bool isPotentiallyNaryReassociable(Instruction *I) {
218  switch (I->getOpcode()) {
219  case Instruction::Add:
220  case Instruction::GetElementPtr:
221  case Instruction::Mul:
222    return true;
223  default:
224    return false;
225  }
226}
227
228bool NaryReassociatePass::doOneIteration(Function &F) {
229  bool Changed = false;
230  SeenExprs.clear();
231  // Process the basic blocks in a depth first traversal of the dominator
232  // tree. This order ensures that all bases of a candidate are in Candidates
233  // when we process it.
234  for (const auto Node : depth_first(DT)) {
235    BasicBlock *BB = Node->getBlock();
236    for (auto I = BB->begin(); I != BB->end(); ++I) {
237      if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(&*I)) {
238        const SCEV *OldSCEV = SE->getSCEV(&*I);
239        if (Instruction *NewI = tryReassociate(&*I)) {
240          Changed = true;
241          SE->forgetValue(&*I);
242          I->replaceAllUsesWith(NewI);
243          WeakVH NewIExist = NewI;
244          // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
245          // entry will be replaced with nullptr if deleted.
246          RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
247          if (!NewIExist) {
248            // Rare occation where the new instruction (NewI) have been removed,
249            // probably due to parts of the input code was dead from the
250            // beginning, reset the iterator and start over from the beginning
251            I = BB->begin();
252            continue;
253          }
254          I = NewI->getIterator();
255        }
256        // Add the rewritten instruction to SeenExprs; the original instruction
257        // is deleted.
258        const SCEV *NewSCEV = SE->getSCEV(&*I);
259        SeenExprs[NewSCEV].push_back(WeakTrackingVH(&*I));
260        // Ideally, NewSCEV should equal OldSCEV because tryReassociate(I)
261        // is equivalent to I. However, ScalarEvolution::getSCEV may
262        // weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose
263        // we reassociate
264        //   I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4
265        // to
266        //   NewI = &a[sext(i)] + sext(j).
267        //
268        // ScalarEvolution computes
269        //   getSCEV(I)    = a + 4 * sext(i + j)
270        //   getSCEV(newI) = a + 4 * sext(i) + 4 * sext(j)
271        // which are different SCEVs.
272        //
273        // To alleviate this issue of ScalarEvolution not always capturing
274        // equivalence, we add I to SeenExprs[OldSCEV] as well so that we can
275        // map both SCEV before and after tryReassociate(I) to I.
276        //
277        // This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll.
278        if (NewSCEV != OldSCEV)
279          SeenExprs[OldSCEV].push_back(WeakTrackingVH(&*I));
280      }
281    }
282  }
283  return Changed;
284}
285
286Instruction *NaryReassociatePass::tryReassociate(Instruction *I) {
287  switch (I->getOpcode()) {
288  case Instruction::Add:
289  case Instruction::Mul:
290    return tryReassociateBinaryOp(cast<BinaryOperator>(I));
291  case Instruction::GetElementPtr:
292    return tryReassociateGEP(cast<GetElementPtrInst>(I));
293  default:
294    llvm_unreachable("should be filtered out by isPotentiallyNaryReassociable");
295  }
296}
297
298static bool isGEPFoldable(GetElementPtrInst *GEP,
299                          const TargetTransformInfo *TTI) {
300  SmallVector<const Value*, 4> Indices;
301  for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
302    Indices.push_back(*I);
303  return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
304                         Indices) == TargetTransformInfo::TCC_Free;
305}
306
307Instruction *NaryReassociatePass::tryReassociateGEP(GetElementPtrInst *GEP) {
308  // Not worth reassociating GEP if it is foldable.
309  if (isGEPFoldable(GEP, TTI))
310    return nullptr;
311
312  gep_type_iterator GTI = gep_type_begin(*GEP);
313  for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
314    if (GTI.isSequential()) {
315      if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I - 1,
316                                                  GTI.getIndexedType())) {
317        return NewGEP;
318      }
319    }
320  }
321  return nullptr;
322}
323
324bool NaryReassociatePass::requiresSignExtension(Value *Index,
325                                                GetElementPtrInst *GEP) {
326  unsigned PointerSizeInBits =
327      DL->getPointerSizeInBits(GEP->getType()->getPointerAddressSpace());
328  return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
329}
330
331GetElementPtrInst *
332NaryReassociatePass::tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
333                                              unsigned I, Type *IndexedType) {
334  Value *IndexToSplit = GEP->getOperand(I + 1);
335  if (SExtInst *SExt = dyn_cast<SExtInst>(IndexToSplit)) {
336    IndexToSplit = SExt->getOperand(0);
337  } else if (ZExtInst *ZExt = dyn_cast<ZExtInst>(IndexToSplit)) {
338    // zext can be treated as sext if the source is non-negative.
339    if (isKnownNonNegative(ZExt->getOperand(0), *DL, 0, AC, GEP, DT))
340      IndexToSplit = ZExt->getOperand(0);
341  }
342
343  if (AddOperator *AO = dyn_cast<AddOperator>(IndexToSplit)) {
344    // If the I-th index needs sext and the underlying add is not equipped with
345    // nsw, we cannot split the add because
346    //   sext(LHS + RHS) != sext(LHS) + sext(RHS).
347    if (requiresSignExtension(IndexToSplit, GEP) &&
348        computeOverflowForSignedAdd(AO, *DL, AC, GEP, DT) !=
349            OverflowResult::NeverOverflows)
350      return nullptr;
351
352    Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
353    // IndexToSplit = LHS + RHS.
354    if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I, LHS, RHS, IndexedType))
355      return NewGEP;
356    // Symmetrically, try IndexToSplit = RHS + LHS.
357    if (LHS != RHS) {
358      if (auto *NewGEP =
359              tryReassociateGEPAtIndex(GEP, I, RHS, LHS, IndexedType))
360        return NewGEP;
361    }
362  }
363  return nullptr;
364}
365
366GetElementPtrInst *
367NaryReassociatePass::tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
368                                              unsigned I, Value *LHS,
369                                              Value *RHS, Type *IndexedType) {
370  // Look for GEP's closest dominator that has the same SCEV as GEP except that
371  // the I-th index is replaced with LHS.
372  SmallVector<const SCEV *, 4> IndexExprs;
373  for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
374    IndexExprs.push_back(SE->getSCEV(*Index));
375  // Replace the I-th index with LHS.
376  IndexExprs[I] = SE->getSCEV(LHS);
377  if (isKnownNonNegative(LHS, *DL, 0, AC, GEP, DT) &&
378      DL->getTypeSizeInBits(LHS->getType()) <
379          DL->getTypeSizeInBits(GEP->getOperand(I)->getType())) {
380    // Zero-extend LHS if it is non-negative. InstCombine canonicalizes sext to
381    // zext if the source operand is proved non-negative. We should do that
382    // consistently so that CandidateExpr more likely appears before. See
383    // @reassociate_gep_assume for an example of this canonicalization.
384    IndexExprs[I] =
385        SE->getZeroExtendExpr(IndexExprs[I], GEP->getOperand(I)->getType());
386  }
387  const SCEV *CandidateExpr = SE->getGEPExpr(cast<GEPOperator>(GEP),
388                                             IndexExprs);
389
390  Value *Candidate = findClosestMatchingDominator(CandidateExpr, GEP);
391  if (Candidate == nullptr)
392    return nullptr;
393
394  IRBuilder<> Builder(GEP);
395  // Candidate does not necessarily have the same pointer type as GEP. Use
396  // bitcast or pointer cast to make sure they have the same type, so that the
397  // later RAUW doesn't complain.
398  Candidate = Builder.CreateBitOrPointerCast(Candidate, GEP->getType());
399  assert(Candidate->getType() == GEP->getType());
400
401  // NewGEP = (char *)Candidate + RHS * sizeof(IndexedType)
402  uint64_t IndexedSize = DL->getTypeAllocSize(IndexedType);
403  Type *ElementType = GEP->getResultElementType();
404  uint64_t ElementSize = DL->getTypeAllocSize(ElementType);
405  // Another less rare case: because I is not necessarily the last index of the
406  // GEP, the size of the type at the I-th index (IndexedSize) is not
407  // necessarily divisible by ElementSize. For example,
408  //
409  // #pragma pack(1)
410  // struct S {
411  //   int a[3];
412  //   int64 b[8];
413  // };
414  // #pragma pack()
415  //
416  // sizeof(S) = 100 is indivisible by sizeof(int64) = 8.
417  //
418  // TODO: bail out on this case for now. We could emit uglygep.
419  if (IndexedSize % ElementSize != 0)
420    return nullptr;
421
422  // NewGEP = &Candidate[RHS * (sizeof(IndexedType) / sizeof(Candidate[0])));
423  Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
424  if (RHS->getType() != IntPtrTy)
425    RHS = Builder.CreateSExtOrTrunc(RHS, IntPtrTy);
426  if (IndexedSize != ElementSize) {
427    RHS = Builder.CreateMul(
428        RHS, ConstantInt::get(IntPtrTy, IndexedSize / ElementSize));
429  }
430  GetElementPtrInst *NewGEP = cast<GetElementPtrInst>(
431      Builder.CreateGEP(GEP->getResultElementType(), Candidate, RHS));
432  NewGEP->setIsInBounds(GEP->isInBounds());
433  NewGEP->takeName(GEP);
434  return NewGEP;
435}
436
437Instruction *NaryReassociatePass::tryReassociateBinaryOp(BinaryOperator *I) {
438  Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
439  // There is no need to reassociate 0.
440  if (SE->getSCEV(I)->isZero())
441    return nullptr;
442  if (auto *NewI = tryReassociateBinaryOp(LHS, RHS, I))
443    return NewI;
444  if (auto *NewI = tryReassociateBinaryOp(RHS, LHS, I))
445    return NewI;
446  return nullptr;
447}
448
449Instruction *NaryReassociatePass::tryReassociateBinaryOp(Value *LHS, Value *RHS,
450                                                         BinaryOperator *I) {
451  Value *A = nullptr, *B = nullptr;
452  // To be conservative, we reassociate I only when it is the only user of (A op
453  // B).
454  if (LHS->hasOneUse() && matchTernaryOp(I, LHS, A, B)) {
455    // I = (A op B) op RHS
456    //   = (A op RHS) op B or (B op RHS) op A
457    const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
458    const SCEV *RHSExpr = SE->getSCEV(RHS);
459    if (BExpr != RHSExpr) {
460      if (auto *NewI =
461              tryReassociatedBinaryOp(getBinarySCEV(I, AExpr, RHSExpr), B, I))
462        return NewI;
463    }
464    if (AExpr != RHSExpr) {
465      if (auto *NewI =
466              tryReassociatedBinaryOp(getBinarySCEV(I, BExpr, RHSExpr), A, I))
467        return NewI;
468    }
469  }
470  return nullptr;
471}
472
473Instruction *NaryReassociatePass::tryReassociatedBinaryOp(const SCEV *LHSExpr,
474                                                          Value *RHS,
475                                                          BinaryOperator *I) {
476  // Look for the closest dominator LHS of I that computes LHSExpr, and replace
477  // I with LHS op RHS.
478  auto *LHS = findClosestMatchingDominator(LHSExpr, I);
479  if (LHS == nullptr)
480    return nullptr;
481
482  Instruction *NewI = nullptr;
483  switch (I->getOpcode()) {
484  case Instruction::Add:
485    NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
486    break;
487  case Instruction::Mul:
488    NewI = BinaryOperator::CreateMul(LHS, RHS, "", I);
489    break;
490  default:
491    llvm_unreachable("Unexpected instruction.");
492  }
493  NewI->takeName(I);
494  return NewI;
495}
496
497bool NaryReassociatePass::matchTernaryOp(BinaryOperator *I, Value *V,
498                                         Value *&Op1, Value *&Op2) {
499  switch (I->getOpcode()) {
500  case Instruction::Add:
501    return match(V, m_Add(m_Value(Op1), m_Value(Op2)));
502  case Instruction::Mul:
503    return match(V, m_Mul(m_Value(Op1), m_Value(Op2)));
504  default:
505    llvm_unreachable("Unexpected instruction.");
506  }
507  return false;
508}
509
510const SCEV *NaryReassociatePass::getBinarySCEV(BinaryOperator *I,
511                                               const SCEV *LHS,
512                                               const SCEV *RHS) {
513  switch (I->getOpcode()) {
514  case Instruction::Add:
515    return SE->getAddExpr(LHS, RHS);
516  case Instruction::Mul:
517    return SE->getMulExpr(LHS, RHS);
518  default:
519    llvm_unreachable("Unexpected instruction.");
520  }
521  return nullptr;
522}
523
524Instruction *
525NaryReassociatePass::findClosestMatchingDominator(const SCEV *CandidateExpr,
526                                                  Instruction *Dominatee) {
527  auto Pos = SeenExprs.find(CandidateExpr);
528  if (Pos == SeenExprs.end())
529    return nullptr;
530
531  auto &Candidates = Pos->second;
532  // Because we process the basic blocks in pre-order of the dominator tree, a
533  // candidate that doesn't dominate the current instruction won't dominate any
534  // future instruction either. Therefore, we pop it out of the stack. This
535  // optimization makes the algorithm O(n).
536  while (!Candidates.empty()) {
537    // Candidates stores WeakTrackingVHs, so a candidate can be nullptr if it's
538    // removed
539    // during rewriting.
540    if (Value *Candidate = Candidates.back()) {
541      Instruction *CandidateInstruction = cast<Instruction>(Candidate);
542      if (DT->dominates(CandidateInstruction, Dominatee))
543        return CandidateInstruction;
544    }
545    Candidates.pop_back();
546  }
547  return nullptr;
548}
549