[InstCombine] Support (sub (sext x), (sext y)) --> (sext (sub x, y)) and (sub (zext x), (zext y)) --> (zext (sub x, y))

Summary:
If the sub doesn't overflow in the original type we can move it above the sext/zext.

This is similar to what we do for add. The overflow checking for sub is currently weaker than add, so the test cases are constructed for what is supported.

Reviewers: spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52075

llvm-svn: 342335

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index acb62b6ad9e..910ec835c5a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1697,6 +1697,9 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
     return SelectInst::Create(Cmp, Neg, A);
   }
 
+  if (Instruction *Ext = narrowMathIfNoOverflow(I))
+    return Ext;
+
   bool Changed = false;
   if (!I.hasNoSignedWrap() && willNotOverflowSignedSub(Op0, Op1, I)) {
     Changed = true;