[llvm] 89c4b29 - [GuardWidening] Fix a nasty cast bug in c2eccc6

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2022-06-07T13:27:13-07:00
New Revision: 89c4b29e8d35ec352019d828e546bea3850403df

URL: https://github.com/llvm/llvm-project/commit/89c4b29e8d35ec352019d828e546bea3850403df
DIFF: https://github.com/llvm/llvm-project/commit/89c4b29e8d35ec352019d828e546bea3850403df.diff

LOG: [GuardWidening] Fix a nasty cast bug in c2eccc6

c2eccc6 introduced a call to etHasNoUnsignedWrap which implicitly assumes that Inst is a OverflowingBinaryOperator.  This is frequently untrue, but was not caught because cast<Ty>(X) has been broken, see https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033 for context.

I considered reverting this, but since doing so re-introduces a nasty miscompile of its own, I decided to fix forward instead.

I'll note that this is a particularly nasty form of the cast<Ty>(X) issue.  Because the cast was succeeding unexpected, we were writing data to instructions which weren't OBOs.  This could result in near arbitrary data or memory corruption.  I'm a bit shocked that the sanitizers didn't find this TBH.

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/GuardWidening.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Scalar/GuardWidening.cpp b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
index 5032f3106d50c..af6062d142f07 100644
--- a/llvm/lib/Transforms/Scalar/GuardWidening.cpp
+++ b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
@@ -495,9 +495,8 @@ void GuardWideningImpl::makeAvailableAt(Value *V, Instruction *Loc) const {
     makeAvailableAt(Op, Loc);
 
   Inst->moveBefore(Loc);
-  // If we moved instruction before guard we must clean nuw, nsw flags.
-  Inst->setHasNoUnsignedWrap(false);
-  Inst->setHasNoSignedWrap(false);
+  // If we moved instruction before guard we must clean poison generating flags.
+  Inst->dropPoisonGeneratingFlags();
 }
 
 bool GuardWideningImpl::widenCondCommon(Value *Cond0, Value *Cond1,