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[ValueTracking] Implement SignBitMustBeZero correctly for sqrt. 

Summary:
Previously we assumed that the result of sqrt(x) always had 0 as its
sign bit.  But sqrt(-0) == -0.

Reviewers: hfinkel, efriedma, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28928

llvm-svn: 293115
This commit is contained in:
Justin Lebar 2017-01-26 00:10:26 +00:00
parent a31f9dd69a
commit 7e3184c412
2 changed files with 66 additions and 4 deletions
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16
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -2645,7 +2645,8 @@ static bool cannotBeOrderedLessThanZeroImpl(const Value *V,
return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
Depth + 1);
case Instruction::Call:
Intrinsic::ID IID = getIntrinsicForCallSite(cast<CallInst>(I), TLI);
const auto *CI = cast<CallInst>(I);
Intrinsic::ID IID = getIntrinsicForCallSite(CI, TLI);
switch (IID) {
default:
break;
@ -2662,12 +2663,19 @@ static bool cannotBeOrderedLessThanZeroImpl(const Value *V,
case Intrinsic::exp:
case Intrinsic::exp2:
case Intrinsic::fabs:
case Intrinsic::sqrt:
return true;
case Intrinsic::sqrt:
// sqrt(x) is always >= -0 or NaN. Moreover, sqrt(x) == -0 iff x == -0.
if (!SignBitOnly)
return true;
return CI->hasNoNaNs() && (CI->hasNoSignedZeros() ||
CannotBeNegativeZero(CI->getOperand(0), TLI));
case Intrinsic::powi:
if (ConstantInt *CI = dyn_cast<ConstantInt>(I->getOperand(1))) {
if (ConstantInt *Exponent = dyn_cast<ConstantInt>(I->getOperand(1))) {
// powi(x,n) is non-negative if n is even.
if (CI->getBitWidth() <= 64 && CI->getSExtValue() % 2u == 0)
if (Exponent->getBitWidth() <= 64 && Exponent->getSExtValue() % 2u == 0)
return true;
}
return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,

54
llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
View File

@ -104,6 +104,7 @@ define float @PR22688(float %x) {
}
declare float @llvm.fabs.f32(float)
declare float @llvm.sqrt.f32(float)
; CHECK-LABEL: @fabs_select_positive_constants(
; CHECK: %select = select i1 %cmp, float 1.000000e+00, float 2.000000e+00
@ -195,3 +196,56 @@ define float @fabs_select_negnan_zero(float addrspace(1)* %out, i32 %c) {
%fabs = call float @llvm.fabs.f32(float %select)
ret float %fabs
}
; CHECK-LABEL: @fabs_sqrt
; CHECK: call float @llvm.sqrt.f32
; CHECK: call float @llvm.fabs.f32
define float @fabs_sqrt(float %a) {
; The fabs can't be eliminated because llvm.sqrt.f32 may return -0 or NaN with
; an arbitrary sign bit.
%sqrt = call float @llvm.sqrt.f32(float %a)
%fabs = call float @llvm.fabs.f32(float %sqrt)
ret float %fabs
}
; CHECK-LABEL: @fabs_sqrt_nnan
; CHECK: call nnan float @llvm.sqrt.f32
; CHECK: call float @llvm.fabs.f32
define float @fabs_sqrt_nnan(float %a) {
; The fabs can't be eliminated because the nnan sqrt may still return -0.
%sqrt = call nnan float @llvm.sqrt.f32(float %a)
%fabs = call float @llvm.fabs.f32(float %sqrt)
ret float %fabs
}
; CHECK-LABEL: @fabs_sqrt_nsz
; CHECK: call nsz float @llvm.sqrt.f32
; CHECK: call float @llvm.fabs.f32
define float @fabs_sqrt_nsz(float %a) {
; The fabs can't be eliminated because the nsz sqrt may still return NaN.
%sqrt = call nsz float @llvm.sqrt.f32(float %a)
%fabs = call float @llvm.fabs.f32(float %sqrt)
ret float %fabs
}
; CHECK-LABEL: @fabs_sqrt_nnan_nsz
; CHECK: call nnan nsz float @llvm.sqrt.f32
; CHECK-NOT: call float @llvm.fabs.f32
define float @fabs_sqrt_nnan_nsz(float %a) {
; The fabs can be eliminated because we're nsz and nnan.
%sqrt = call nnan nsz float @llvm.sqrt.f32(float %a)
%fabs = call float @llvm.fabs.f32(float %sqrt)
ret float %fabs
}
; CHECK-LABEL: @fabs_sqrt_nnan_fabs
; CHECK: call float @llvm.fabs.f32
; CHECK: call nnan float @llvm.sqrt.f32
; CHECK-NOT: call float @llvm.fabs.f32
define float @fabs_sqrt_nnan_fabs(float %a) {
; The second fabs can be eliminated because the operand to sqrt cannot be -0.
%b = call float @llvm.fabs.f32(float %a)
%sqrt = call nnan float @llvm.sqrt.f32(float %b)
%fabs = call float @llvm.fabs.f32(float %sqrt)
ret float %fabs
}