[InstSimplify] fsub 0.0, (fsub -0.0, X) ==> X is only safe if signed zeros are ignored.

Only allow fsub -0.0, (fsub -0.0, X) ==> X without nsz. PR26746.

llvm-svn: 262212

2 files changed, 31 insertions, 10 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 61e7296d05f..aca19de4f17 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -833,14 +833,15 @@ static Value *SimplifyFSubInst(Value *Op0, Value *Op1, FastMathFlags FMF,
       (FMF.noSignedZeros() || CannotBeNegativeZero(Op0)))
     return Op0;
 
-  // fsub 0, (fsub -0.0, X) ==> X
+  // fsub -0.0, (fsub -0.0, X) ==> X
   Value *X;
-  if (match(Op0, m_AnyZero())) {
-    if (match(Op1, m_FSub(m_NegZero(), m_Value(X))))
-      return X;
-    if (FMF.noSignedZeros() && match(Op1, m_FSub(m_AnyZero(), m_Value(X))))
-      return X;
-  }
+  if (match(Op0, m_NegZero()) && match(Op1, m_FSub(m_NegZero(), m_Value(X))))
+    return X;
+
+  // fsub 0.0, (fsub 0.0, X) ==> X if signed zeros are ignored.
+  if (FMF.noSignedZeros() && match(Op0, m_NegZero()) &&
+      match(Op1, m_FSub(m_AnyZero(), m_Value(X))))
+    return X;
 
   // fsub nnan x, x ==> 0.0
   if (FMF.noNaNs() && Op0 == Op1)
diff --git a/llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll b/llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
index b0957a81773..1c29884753a 100644
--- a/llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
+++ b/llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
@@ -1,8 +1,8 @@
 ; RUN: opt < %s -instsimplify -S | FileCheck %s
 
-; fsub 0, (fsub 0, X) ==> X
-; CHECK-LABEL: @fsub_0_0_x(
-define float @fsub_0_0_x(float %a) {
+; fsub -0.0, (fsub -0.0, X) ==> X
+; CHECK-LABEL: @fsub_-0_-0_x(
+define float @fsub_-0_-0_x(float %a) {
   %t1 = fsub float -0.0, %a
   %ret = fsub float -0.0, %t1
 
@@ -10,6 +10,26 @@ define float @fsub_0_0_x(float %a) {
   ret float %ret
 }
 
+; fsub 0.0, (fsub -0.0, X) != X
+; CHECK-LABEL: @fsub_0_-0_x(
+define float @fsub_0_-0_x(float %a) {
+  %t1 = fsub float 0.0, %a
+  %ret = fsub float -0.0, %t1
+
+; CHECK-NOT: ret float %a
+  ret float %ret
+}
+
+; fsub -0.0, (fsub 0.0, X) != X
+; CHECK-LABEL: @fsub_-0_0_x(
+define float @fsub_-0_0_x(float %a) {
+  %t1 = fsub float -0.0, %a
+  %ret = fsub float 0.0, %t1
+
+; CHECK-NOT: ret float %a
+  ret float %ret
+}
+
 ; fsub X, 0 ==> X
 ; CHECK-LABEL: @fsub_x_0(
 define float @fsub_x_0(float %a) {