[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

llvm-svn: 240798

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -504,6 +504,54 @@ static bool isInBoundsGep(Value *Ptr) {
   return false;
 }
 
+/// \brief Return true if an AddRec pointer \p Ptr is unsigned non-wrapping,
+/// i.e. monotonically increasing/decreasing.
+static bool isNoWrapAddRec(Value *Ptr, const SCEVAddRecExpr *AR,
+                           ScalarEvolution *SE, const Loop *L) {
+  // FIXME: This should probably only return true for NUW.
+  if (AR->getNoWrapFlags(SCEV::NoWrapMask))
+    return true;
+
+  // Scalar evolution does not propagate the non-wrapping flags to values that
+  // are derived from a non-wrapping induction variable because non-wrapping
+  // could be flow-sensitive.
+  //
+  // Look through the potentially overflowing instruction to try to prove
+  // non-wrapping for the *specific* value of Ptr.
+
+  // The arithmetic implied by an inbounds GEP can't overflow.
+  auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
+  if (!GEP || !GEP->isInBounds())
+    return false;
+
+  // Make sure there is only one non-const index and analyze that.
+  Value *NonConstIndex = nullptr;
+  for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
+    if (!isa<ConstantInt>(*Index)) {
+      if (NonConstIndex)
+        return false;
+      NonConstIndex = *Index;
+    }
+  if (!NonConstIndex)
+    // The recurrence is on the pointer, ignore for now.
+    return false;
+
+  // The index in GEP is signed.  It is non-wrapping if it's derived from a NSW
+  // AddRec using a NSW operation.
+  if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(NonConstIndex))
+    if (OBO->hasNoSignedWrap() &&
+        // Assume constant for other the operand so that the AddRec can be
+        // easily found.
+        isa<ConstantInt>(OBO->getOperand(1))) {
+      auto *OpScev = SE->getSCEV(OBO->getOperand(0));
+
+      if (auto *OpAR = dyn_cast<SCEVAddRecExpr>(OpScev))
+        return OpAR->getLoop() == L && OpAR->getNoWrapFlags(SCEV::FlagNSW);
+    }
+
+  return false;
+}
+
 /// \brief Check whether the access through \p Ptr has a constant stride.
 int llvm::isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
                        const ValueToValueMap &StridesMap) {
@@ -541,7 +589,7 @@ int llvm::isStridedPtr(ScalarEvolution *SE, Value *Ptr, const Loop *Lp,
   // to access the pointer value "0" which is undefined behavior in address
   // space 0, therefore we can also vectorize this case.
   bool IsInBoundsGEP = isInBoundsGep(Ptr);
-  bool IsNoWrapAddRec = AR->getNoWrapFlags(SCEV::NoWrapMask);
+  bool IsNoWrapAddRec = isNoWrapAddRec(Ptr, AR, SE, Lp);
   bool IsInAddressSpaceZero = PtrTy->getAddressSpace() == 0;
   if (!IsNoWrapAddRec && !IsInBoundsGEP && !IsInAddressSpaceZero) {
     DEBUG(dbgs() << "LAA: Bad stride - Pointer may wrap in the address space "

llvm/test/Analysis/LoopAccessAnalysis/non-wrapping-pointer.ll

@@ -0,0 +1,41 @@
+; RUN: opt -basicaa -loop-accesses -analyze < %s | FileCheck %s
+
+; For this loop:
+;   for (int i = 0; i < n; i++)
+;    A[2 * i] = A[2 * i] + B[i];
+;
+; , SCEV is unable to prove that A[2 * i] does not overflow.  However,
+; analyzing the IR helps us to conclude it and in turn allow dependence
+; analysis.
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; CHECK: Memory dependences are safe{{$}}
+
+define void @f(i16* noalias %a,
+               i16* noalias %b, i64 %N) {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %ind = phi i64 [ 0, %entry ], [ %inc, %for.body ]
+
+  %mul = mul nuw nsw i64 %ind, 2
+
+  %arrayidxA = getelementptr inbounds i16, i16* %a, i64 %mul
+  %loadA = load i16, i16* %arrayidxA, align 2
+
+  %arrayidxB = getelementptr inbounds i16, i16* %b, i64 %ind
+  %loadB = load i16, i16* %arrayidxB, align 2
+
+  %add = mul i16 %loadA, %loadB
+
+  store i16 %add, i16* %arrayidxA, align 2
+
+  %inc = add nuw nsw i64 %ind, 1
+  %exitcond = icmp eq i64 %inc, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}