[SCEV] Do not cache S -> V if S is not equivalent of V

SCEV tracks the correspondence of created SCEV to original instruction.
However during creation of SCEV it is possible that nuw/nsw/exact flags are
lost.

As a result during expansion of the SCEV the instruction with nuw/nsw/exact
will be used where it was expected and we produce poison incorreclty.

Reviewers: sanjoy, mkazantsev, sebpop, jbhateja
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D41578

llvm-svn: 322058

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 10b5c74e378..7a9fddfd10b 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -3774,6 +3774,24 @@ void ScalarEvolution::eraseValueFromMap(Value *V) {
   }
 }
 
+/// Check whether value has nuw/nsw/exact set but SCEV does not.
+/// TODO: In reality it is better to check the poison recursevely
+/// but this is better than nothing.
+static bool SCEVLostPoisonFlags(const SCEV *S, const Value *V) {
+  if (auto *I = dyn_cast<Instruction>(V)) {
+    if (isa<OverflowingBinaryOperator>(I)) {
+      if (auto *NS = dyn_cast<SCEVNAryExpr>(S)) {
+        if (I->hasNoSignedWrap() && !NS->hasNoSignedWrap())
+          return true;
+        if (I->hasNoUnsignedWrap() && !NS->hasNoUnsignedWrap())
+          return true;
+      }
+    } else if (isa<PossiblyExactOperator>(I) && I->isExact())
+      return true;
+  }
+  return false;
+}
+
 /// Return an existing SCEV if it exists, otherwise analyze the expression and
 /// create a new one.
 const SCEV *ScalarEvolution::getSCEV(Value *V) {
@@ -3787,7 +3805,7 @@ const SCEV *ScalarEvolution::getSCEV(Value *V) {
     // ValueExprMap before insert S->{V, 0} into ExprValueMap.
     std::pair<ValueExprMapType::iterator, bool> Pair =
         ValueExprMap.insert({SCEVCallbackVH(V, this), S});
-    if (Pair.second) {
+    if (Pair.second && !SCEVLostPoisonFlags(S, V)) {
       ExprValueMap[S].insert({V, nullptr});
 
       // If S == Stripped + Offset, add Stripped -> {V, Offset} into