[InstCombine] sub X, sext(bool Y) -> add X, zext(bool Y)

Prefer add/zext because they are better supported in terms of value-tracking.

Note that the backend should be prepared for this IR canonicalization 
(including vector types) after:
https://reviews.llvm.org/rL284015

Differential Revision: https://reviews.llvm.org/D25135

llvm-svn: 284241

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index a490a91535d..85fa7c0d4fc 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1642,6 +1642,17 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
       if (Value *XNeg = dyn_castNegVal(X))
         return BinaryOperator::CreateShl(XNeg, Y);
 
+    // Subtracting -1/0 is the same as adding 1/0:
+    // sub [nsw] Op0, sext(bool Y) -> add [nsw] Op0, zext(bool Y)
+    // 'nuw' is dropped in favor of the canonical form.
+    if (match(Op1, m_SExt(m_Value(Y))) &&
+        Y->getType()->getScalarSizeInBits() == 1) {
+      Value *Zext = Builder->CreateZExt(Y, I.getType());
+      BinaryOperator *Add = BinaryOperator::CreateAdd(Op0, Zext);
+      Add->setHasNoSignedWrap(I.hasNoSignedWrap());
+      return Add;
+    }
+
     // X - A*-B -> X + A*B
     // X - -A*B -> X + A*B
     Value *A, *B;