[InstCombine] fold cmp+select using select operand equivalence

As discussed in PR42696:
https://bugs.llvm.org/show_bug.cgi?id=42696
...but won't help that case yet.

We have an odd situation where a select operand equivalence fold was
implemented in InstSimplify when it could have been done more generally
in InstCombine if we allow dropping of {nsw,nuw,exact} from a binop operand.

Here's an example:
https://rise4fun.com/Alive/Xplr

  %cmp = icmp eq i32 %x, 2147483647
  %add = add nsw i32 %x, 1
  %sel = select i1 %cmp, i32 -2147483648, i32 %add
  =>
  %sel = add i32 %x, 1

I've left the InstSimplify code in place for now, but my guess is that we'd
prefer to remove that as a follow-up to save on code duplication and
compile-time.

Differential Revision: https://reviews.llvm.org/D65576

llvm-svn: 367695

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 04ee1e8a90d..85002aafa85 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3529,6 +3529,9 @@ static const Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
     //   %sel = select i1 %cmp, i32 -2147483648, i32 %add
     //
     // We can't replace %sel with %add unless we strip away the flags.
+    // TODO: This is an unusual limitation because better analysis results in
+    //       worse simplification. InstCombine can do this fold more generally
+    //       by dropping the flags. Remove this fold to save compile-time?
     if (isa<OverflowingBinaryOperator>(B))
       if (Q.IIQ.hasNoSignedWrap(B) || Q.IIQ.hasNoUnsignedWrap(B))
         return nullptr;