[InstCombine] nsz: -(X - Y) --> Y - X

This restores part of the fold that was removed with rL73243 (PR4374).

llvm-svn: 329350

1 files changed, 11 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 578ba23604d..0541eb7b291 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1698,10 +1698,17 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
                                   SQ.getWithInstruction(&I)))
     return replaceInstUsesWith(I, V);
 
-  // Subtraction from -0.0 is the canonical form of fneg.
-  // fsub nsz 0, X ==> fsub nsz -0.0, X
-  if (I.getFastMathFlags().noSignedZeros() && match(Op0, m_PosZeroFP()))
-    return BinaryOperator::CreateFNegFMF(Op1, &I);
+  if (I.hasNoSignedZeros()) {
+    // Subtraction from -0.0 is the canonical form of fneg.
+    // fsub nsz 0, X ==> fsub nsz -0.0, X
+    if (match(Op0, m_PosZeroFP()))
+      return BinaryOperator::CreateFNegFMF(Op1, &I);
+
+    // With no-signed-zeros: -(X - Y) --> Y - X
+    Value *X, *Y;
+    if (match(Op0, m_NegZeroFP()) && match(Op1, m_FSub(m_Value(X), m_Value(Y))))
+      return BinaryOperator::CreateFSubFMF(Y, X, &I);
+  }
 
   if (isa<Constant>(Op0))
     if (SelectInst *SI = dyn_cast<SelectInst>(Op1))