[InstCombine] fold add(zext(xor X, C), C) --> sext X when C is INT_MIN in the source type

The pattern may look more obviously like a sext if written as:

  define i32 @g(i16 %x) {
    %zext = zext i16 %x to i32
    %xor = xor i32 %zext, 32768
    %add = add i32 %xor, -32768
    ret i32 %add
  }

We already have that fold in visitAdd().

Differential Revision: https://reviews.llvm.org/D22477

llvm-svn: 276035

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 221a2200717..8cb124d26b9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1047,6 +1047,16 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
     // X + (signbit) --> X ^ signbit
     if (Val->isSignBit())
       return BinaryOperator::CreateXor(LHS, RHS);
+
+    // Is this add the last step in a convoluted sext?
+    Value *X;
+    const APInt *C;
+    if (match(LHS, m_ZExt(m_Xor(m_Value(X), m_APInt(C)))) &&
+        C->isMinSignedValue() &&
+        C->sext(LHS->getType()->getScalarSizeInBits()) == *Val) {
+      // add(zext(xor i16 X, -32768), -32768) --> sext X
+      return CastInst::Create(Instruction::SExt, X, LHS->getType());
+    }
   }
 
   // FIXME: Use the match above instead of dyn_cast to allow these transforms