[InstCombine] peek through unsigned FP casts for zero-equality compares (PR36682)

Summary:
This pattern came up in PR36682 / D44390
https://bugs.llvm.org/show_bug.cgi?id=36682
https://reviews.llvm.org/D44390
https://godbolt.org/g/oKvT5H

See also D44416

Reviewers: spatel, majnemer, efriedma, arsenm

Reviewed By: spatel

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44424

llvm-svn: 327799

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 0cd72bce45d..12894b7833b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4506,6 +4506,15 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         return New;
   }
 
+  Value *X;
+
+  // Zero-equality checks are preserved through unsigned floating-point casts:
+  // icmp eq (bitcast (uitofp X)), 0 --> icmp eq X, 0
+  // icmp ne (bitcast (uitofp X)), 0 --> icmp ne X, 0
+  if (match(Op0, m_BitCast(m_UIToFP(m_Value(X)))))
+    if (I.isEquality() && match(Op1, m_Zero()))
+      return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
+
   // Test to see if the operands of the icmp are casted versions of other
   // values.  If the ptr->ptr cast can be stripped off both arguments, we do so
   // now.