[ValueTracking] Improve reverse assumption inference

Use isGuaranteedToTransferExecutionToSuccessor() instead of
isSafeToSpeculativelyExecute() when seeing whether we can propagate
the information in an assume backwards in isValidAssumeForContext().
The latter is more general - it also allows arbitrary loads/stores -
and is also the condition we want: if our assume is guaranteed to
execute, its condition not holding would be UB.

Original patch by arielb1.

Differential Revision: https://reviews.llvm.org/D37215

llvm-svn: 368723

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c09530cd697..2773b60bb75 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -558,12 +558,18 @@ bool llvm::isValidAssumeForContext(const Instruction *Inv,
         return true;
   }
 
+  // Don't let an assume affect itself - this would cause the problems
+  // `isEphemeralValueOf` is trying to prevent, and it would also make
+  // the loop below go out of bounds.
+  if (Inv == CxtI)
+    return false;
+
   // The context comes first, but they're both in the same block. Make sure
   // there is nothing in between that might interrupt the control flow.
   for (BasicBlock::const_iterator I =
          std::next(BasicBlock::const_iterator(CxtI)), IE(Inv);
        I != IE; ++I)
-    if (!isSafeToSpeculativelyExecute(&*I) && !isAssumeLikeIntrinsic(&*I))
+    if (!isGuaranteedToTransferExecutionToSuccessor(&*I))
       return false;
 
   return !isEphemeralValueOf(Inv, CxtI);