[InstSimplify] fix folds for (0.0 - X) + X --> 0 (PR27151)

As shown in:
https://bugs.llvm.org/show_bug.cgi?id=27151
...the existing fold could miscompile when X is NaN.

The fold was also dependent on 'ninf' but that's not necessary.

From IEEE-754 (with default rounding which we can assume for these opcodes):
"When the sum of two operands with opposite signs (or the difference of two 
operands with like signs) is exactly zero, the sign of that sum (or difference) 
shall be +0...However, x + x = x − (−x) retains the same sign as x even when 
x is zero."

llvm-svn: 327575

1 files changed, 10 insertions, 14 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index a95a775ead6..2b8e23b9569 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4168,20 +4168,16 @@ static Value *SimplifyFAddInst(Value *Op0, Value *Op1, FastMathFlags FMF,
       (FMF.noSignedZeros() || CannotBeNegativeZero(Op0, Q.TLI)))
     return Op0;
 
-  // fadd [nnan ninf] X, (fsub [nnan ninf] 0, X) ==> 0
-  //   where nnan and ninf have to occur at least once somewhere in this
-  //   expression
-  Value *SubOp = nullptr;
-  if (match(Op1, m_FSub(m_AnyZero(), m_Specific(Op0))))
-    SubOp = Op1;
-  else if (match(Op0, m_FSub(m_AnyZero(), m_Specific(Op1))))
-    SubOp = Op0;
-  if (SubOp) {
-    Instruction *FSub = cast<Instruction>(SubOp);
-    if ((FMF.noNaNs() || FSub->hasNoNaNs()) &&
-        (FMF.noInfs() || FSub->hasNoInfs()))
-      return Constant::getNullValue(Op0->getType());
-  }
+  // With nnan: (+/-0.0 - X) + X --> 0.0 (and commuted variant)
+  // We don't have to explicitly exclude infinities (ninf): INF + -INF == NaN.
+  // Negative zeros are allowed because we always end up with positive zero:
+  // X = -0.0: (-0.0 - (-0.0)) + (-0.0) == ( 0.0) + (-0.0) == 0.0
+  // X = -0.0: ( 0.0 - (-0.0)) + (-0.0) == ( 0.0) + (-0.0) == 0.0
+  // X =  0.0: (-0.0 - ( 0.0)) + ( 0.0) == (-0.0) + ( 0.0) == 0.0
+  // X =  0.0: ( 0.0 - ( 0.0)) + ( 0.0) == ( 0.0) + ( 0.0) == 0.0
+  if (FMF.noNaNs() && (match(Op0, m_FSub(m_AnyZero(), m_Specific(Op1))) ||
+                       match(Op1, m_FSub(m_AnyZero(), m_Specific(Op0)))))
+    return ConstantFP::getNullValue(Op0->getType());
 
   return nullptr;
 }