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

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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
This commit is contained in:
Sanjay Patel 2018-03-14 21:23:27 +00:00
parent 8c7fd1029e
commit 7475763bc6
2 changed files with 22 additions and 32 deletions
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24
lib/Analysis/InstructionSimplify.cpp
View File

@ -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;
}

30
test/Transforms/InstSimplify/fast-math.ll
View File

@ -46,50 +46,47 @@ define float @no_mul_zero_3(float %a) {
ret float %b
}
; FIXME: -X + X --> 0.0 (with nnan on the fadd)
; -X + X --> 0.0 (with nnan on the fadd)
define float @fadd_fnegx(float %x) {
; CHECK-LABEL: @fadd_fnegx(
; CHECK-NEXT: [[NEGX:%.*]] = fsub float -0.000000e+00, [[X:%.*]]
; CHECK-NEXT: [[R:%.*]] = fadd nnan float [[NEGX]], [[X]]
; CHECK-NEXT: ret float [[R]]
; CHECK-NEXT: ret float 0.000000e+00
;
%negx = fsub float -0.0, %x
%r = fadd nnan float %negx, %x
ret float %r
}
; FIXME: X + -X --> 0.0 (with nnan on the fadd)
; X + -X --> 0.0 (with nnan on the fadd)
define <2 x float> @fadd_fnegx_commute_vec(<2 x float> %x) {
; CHECK-LABEL: @fadd_fnegx_commute_vec(
; CHECK-NEXT: [[NEGX:%.*]] = fsub <2 x float> <float -0.000000e+00, float -0.000000e+00>, [[X:%.*]]
; CHECK-NEXT: [[R:%.*]] = fadd nnan <2 x float> [[X]], [[NEGX]]
; CHECK-NEXT: ret <2 x float> [[R]]
; CHECK-NEXT: ret <2 x float> zeroinitializer
;
%negx = fsub <2 x float> <float -0.0, float -0.0>, %x
%r = fadd nnan <2 x float> %x, %negx
ret <2 x float> %r
}
; FIXME: Could be NaN.
; https://bugs.llvm.org/show_bug.cgi?id=26958
; https://bugs.llvm.org/show_bug.cgi?id=27151
define float @fadd_fneg_nan(float %x) {
; CHECK-LABEL: @fadd_fneg_nan(
; CHECK-NEXT: ret float 0.000000e+00
; CHECK-NEXT: [[T:%.*]] = fsub nnan float -0.000000e+00, [[X:%.*]]
; CHECK-NEXT: [[COULD_BE_NAN:%.*]] = fadd ninf float [[T]], [[X]]
; CHECK-NEXT: ret float [[COULD_BE_NAN]]
;
%t = fsub nnan float -0.0, %x
%could_be_nan = fadd ninf float %t, %x
ret float %could_be_nan
}
; FIXME: Could be NaN.
define float @fadd_fneg_nan_commute(float %x) {
; CHECK-LABEL: @fadd_fneg_nan_commute(
; CHECK-NEXT: ret float 0.000000e+00
; CHECK-NEXT: [[T:%.*]] = fsub nnan ninf float -0.000000e+00, [[X:%.*]]
; CHECK-NEXT: [[COULD_BE_NAN:%.*]] = fadd float [[X]], [[T]]
; CHECK-NEXT: ret float [[COULD_BE_NAN]]
;
%t = fsub nnan ninf float -0.0, %x
%could_be_nan = fadd float %x, %t
@ -118,15 +115,12 @@ define <2 x float> @fadd_fsub_nnan_ninf_commute_vec(<2 x float> %x) {
ret <2 x float> %zero
}
; FIXME: Do fold this.
; 'ninf' is not required because 'nnan' allows us to assume
; that X is not INF/-INF (adding opposite INFs would be NaN).
; that X is not INF or -INF (adding opposite INFs would be NaN).
define float @fadd_fsub_nnan(float %x) {
; CHECK-LABEL: @fadd_fsub_nnan(
; CHECK-NEXT: [[SUB:%.*]] = fsub float 0.000000e+00, [[X:%.*]]
; CHECK-NEXT: [[ZERO:%.*]] = fadd nnan float [[SUB]], [[X]]
; CHECK-NEXT: ret float [[ZERO]]
; CHECK-NEXT: ret float 0.000000e+00
;
%sub = fsub float 0.0, %x
%zero = fadd nnan float %sub, %x