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[LAA] Try to prove non-wrapping of pointers if SCEV cannot
Summary: Scalar evolution does not propagate the non-wrapping flags to values that are derived from a non-wrapping induction variable because the non-wrapping property could be flow-sensitive. This change is a first attempt to establish the non-wrapping property in some simple cases. The main idea is to look through the operations defining the pointer. As long as we arrive to a non-wrapping AddRec via a small chain of non-wrapping instruction, the pointer should not wrap either. I believe that this essentially is what Andy described in http://article.gmane.org/gmane.comp.compilers.llvm.cvs/220731 as the way forward. Reviewers: aschwaighofer, nadav, sanjoy, atrick Reviewed By: atrick Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D10472 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240798 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -504,6 +504,54 @@ static bool isInBoundsGep(Value *Ptr) {
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return false;
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}
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/// \brief Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
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/// i.e. monotonically increasing/decreasing.
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static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
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ScalarEvolution *SE, const Loop *L) {
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// FIXME: This should probably only return true for NUW.
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if (AR->getNoWrapFlags(SCEV::NoWrapMask))
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return true;
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// Scalar evolution does not propagate the non-wrapping flags to values that
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// are derived from a non-wrapping induction variable because non-wrapping
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// could be flow-sensitive.
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//
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// Look through the potentially overflowing instruction to try to prove
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// non-wrapping for the *specific* value of Ptr.
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// The arithmetic implied by an inbounds GEP can't overflow.
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auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
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if (!GEP || !GEP->isInBounds())
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return false;
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// Make sure there is only one non-const index and analyze that.
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Value *NonConstIndex = nullptr;
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for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
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if (!isa<ConstantInt>(*Index)) {
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if (NonConstIndex)
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return false;
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NonConstIndex = *Index;
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}
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if (!NonConstIndex)
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// The recurrence is on the pointer, ignore for now.
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return false;
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// The index in GEP is signed. It is non-wrapping if it's derived from a NSW
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// AddRec using a NSW operation.
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if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
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if (OBO->hasNoSignedWrap() &&
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// Assume constant for other the operand so that the AddRec can be
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// easily found.
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isa<ConstantInt>(OBO->getOperand(1))) {
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auto *OpScev = SE->getSCEV(OBO->getOperand(0));
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if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
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return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
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}
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return false;
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}
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/// \brief Check whether the access through \p Ptr has a constant stride.
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int llvm::isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
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const ValueToValueMap &StridesMap) {
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@ -541,7 +589,7 @@ int llvm::isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
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// to access the pointer value "0" which is undefined behavior in address
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// space 0, therefore we can also vectorize this case.
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bool IsInBoundsGEP = isInBoundsGep(Ptr);
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bool IsNoWrapAddRec = AR->getNoWrapFlags(SCEV::NoWrapMask);
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bool IsNoWrapAddRec = isNoWrapAddRec(Ptr, AR, SE, Lp);
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bool IsInAddressSpaceZero = PtrTy->getAddressSpace() == 0;
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if (!IsNoWrapAddRec && !IsInBoundsGEP && !IsInAddressSpaceZero) {
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DEBUG(dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "
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41
test/Analysis/LoopAccessAnalysis/non-wrapping-pointer.ll
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41
test/Analysis/LoopAccessAnalysis/non-wrapping-pointer.ll
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@ -0,0 +1,41 @@
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; RUN: opt -basicaa -loop-accesses -analyze < %s | FileCheck %s
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; For this loop:
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; for (int i = 0; i < n; i++)
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; A[2 * i] = A[2 * i] + B[i];
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;
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; , SCEV is unable to prove that A[2 * i] does not overflow. However,
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; analyzing the IR helps us to conclude it and in turn allow dependence
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; analysis.
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target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
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; CHECK: Memory dependences are safe{{$}}
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define void @f(i16* noalias %a,
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i16* noalias %b, i64 %N) {
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entry:
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br label %for.body
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for.body: ; preds = %for.body, %entry
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%ind = phi i64 [ 0, %entry ], [ %inc, %for.body ]
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%mul = mul nuw nsw i64 %ind, 2
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%arrayidxA = getelementptr inbounds i16, i16* %a, i64 %mul
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%loadA = load i16, i16* %arrayidxA, align 2
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%arrayidxB = getelementptr inbounds i16, i16* %b, i64 %ind
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%loadB = load i16, i16* %arrayidxB, align 2
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%add = mul i16 %loadA, %loadB
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store i16 %add, i16* %arrayidxA, align 2
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%inc = add nuw nsw i64 %ind, 1
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%exitcond = icmp eq i64 %inc, %N
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br i1 %exitcond, label %for.end, label %for.body
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for.end: ; preds = %for.body
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ret void
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}
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