[llvm] r329350 - [InstCombine] nsz: -(X - Y) --> Y - X

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Thu Apr  5 14:37:17 2018
New Revision: 329350

URL: http://llvm.org/viewvc/llvm-project?rev=329350&view=rev
Log:
[InstCombine] nsz: -(X - Y) --> Y - X

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

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/trunk/test/Transforms/InstCombine/fsub.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=329350&r1=329349&r2=329350&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Thu Apr  5 14:37:17 2018
@@ -1698,10 +1698,17 @@ Instruction *InstCombiner::visitFSub(Bin
                                   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))

Modified: llvm/trunk/test/Transforms/InstCombine/fsub.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/fsub.ll?rev=329350&r1=329349&r2=329350&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/fsub.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/fsub.ll Thu Apr  5 14:37:17 2018
@@ -19,8 +19,7 @@ define float @test1(float %x, float %y)
 
 define float @neg_sub(float %x, float %y) {
 ; CHECK-LABEL: @neg_sub(
-; CHECK-NEXT:    [[T1:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[T2:%.*]] = fsub nsz float -0.000000e+00, [[T1]]
+; CHECK-NEXT:    [[T2:%.*]] = fsub nsz float [[Y:%.*]], [[X:%.*]]
 ; CHECK-NEXT:    ret float [[T2]]
 ;
   %t1 = fsub float %x, %y