[InstCombine] look through bswap/bitreverse for equality comparisons

I noticed this missed bswap optimization in the CGP memcmp() expansion, 
and then I saw that we don't have the fold in InstCombine.

Differential Revision: https://reviews.llvm.org/D34763

llvm-svn: 306980

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 58b8b2f5262..11507ae72b9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3438,6 +3438,15 @@ Instruction *InstCombiner::foldICmpEquality(ICmpInst &I) {
     }
   }
 
+  // If both operands are byte-swapped or bit-reversed, just compare the
+  // original values.
+  // TODO: Move this to a function similar to foldICmpIntrinsicWithConstant()
+  // and handle more intrinsics.
+  if ((match(Op0, m_BSwap(m_Value(A))) && match(Op1, m_BSwap(m_Value(B)))) ||
+      (match(Op0, m_Intrinsic<Intrinsic::bitreverse>(m_Value(A))) &&
+       match(Op1, m_Intrinsic<Intrinsic::bitreverse>(m_Value(B)))))
+    return new ICmpInst(Pred, A, B);
+
   return nullptr;
 }