[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
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index a95a775..2b8e23b 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4168,20 +4168,16 @@
(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;
}