[InstCombine] peek through FP casts for sign-bit compares (PR36682)

This pattern came up in PR36682:
https://bugs.llvm.org/show_bug.cgi?id=36682
https://godbolt.org/g/LhuD9A

Equality checks are planned as a follow-up enhancement.

Differential Revision: https://reviews.llvm.org/D44367

llvm-svn: 328426

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 12894b7833b..61e1d928a99 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4506,7 +4506,16 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
         return New;
   }
 
+  // Sign-bit checks are preserved through signed floating-point casts:
+  // icmp slt (bitcast (sitofp X)),  0 --> icmp slt X,  0
+  // icmp sgt (bitcast (sitofp X)), -1 --> icmp sgt X, -1
   Value *X;
+  if (match(Op0, m_BitCast(m_SIToFP(m_Value(X))))) {
+    if (Pred == ICmpInst::ICMP_SLT && match(Op1, m_Zero()))
+      return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
+    if (Pred == ICmpInst::ICMP_SGT && match(Op1, m_AllOnes()))
+      return new ICmpInst(Pred, X, ConstantInt::getAllOnesValue(X->getType()));
+  }
 
   // Zero-equality checks are preserved through unsigned floating-point casts:
   // icmp eq (bitcast (uitofp X)), 0 --> icmp eq X, 0