[InstCombine] drop poison flags in SimplifyVectorDemandedElts

We established the (unfortunately complicated) rules for UB/poison
propagation with vector ops in:
D48893
D48987
D49047

It's clear from the affected tests that we are potentially creating 
poison where none existed before the transforms. For add/sub/mul,
the answer is simple: just drop the flags because the extra undef
vector lanes are generally more valuable for analysis and codegen.

llvm-svn: 343819

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index f8b57eeb388..936daa828a5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1592,8 +1592,11 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, APInt DemandedElts,
     simplifyAndSetOp(I, 0, DemandedElts, UndefElts);
     simplifyAndSetOp(I, 1, DemandedElts, UndefElts2);
 
-    // TODO: If this is a potentially poison-producing instruction, we need
-    // to drop the wrapping/exact flags?
+    // Any change to an instruction with potential poison must clear those flags
+    // because we can not guarantee those constraints now. Other analysis may
+    // determine that it is safe to re-apply the flags.
+    if (MadeChange)
+      BO->dropPoisonGeneratingFlags();
 
     // Output elements are undefined if both are undefined. Consider things
     // like undef & 0. The result is known zero, not undef.