[llvm] r293243 - [ValueTracking] Add comment that CannotBeOrderedLessThanZero does the wrong thing for powi.
Justin Lebar via llvm-commits
llvm-commits at lists.llvm.org
Thu Jan 26 16:58:34 PST 2017
Author: jlebar
Date: Thu Jan 26 18:58:34 2017
New Revision: 293243
URL: http://llvm.org/viewvc/llvm-project?rev=293243&view=rev
Log:
[ValueTracking] Add comment that CannotBeOrderedLessThanZero does the wrong thing for powi.
Summary:
CannotBeOrderedLessThanZero(powi(x, exp)) returns true if
CannotBeOrderedLessThanZero(x). But powi(-0, exp) is negative if exp is
odd, so we actually want to return SignBitMustBeZero(x).
Except that also isn't right, because we want to return true if x is
NaN, even if x has a negative sign bit.
What we really need in order to fix this is a consistent approach in
this function to handling the sign bit of NaNs. Without this it's very
difficult to say what the correct behavior here is.
Reviewers: hfinkel, efriedma, sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D28927
Modified:
llvm/trunk/lib/Analysis/ValueTracking.cpp
Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=293243&r1=293242&r2=293243&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Thu Jan 26 18:58:34 2017
@@ -2602,6 +2602,11 @@ static bool cannotBeOrderedLessThanZeroI
const TargetLibraryInfo *TLI,
bool SignBitOnly,
unsigned Depth) {
+ // TODO: This function does not do the right thing when SignBitOnly is true
+ // and we're lowering to a hypothetical IEEE 754-compliant-but-evil platform
+ // which flips the sign bits of NaNs. See
+ // https://llvm.org/bugs/show_bug.cgi?id=31702.
+
if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
return !CFP->getValueAPF().isNegative() ||
(!SignBitOnly && CFP->getValueAPF().isZero());
@@ -2678,8 +2683,22 @@ static bool cannotBeOrderedLessThanZeroI
if (Exponent->getBitWidth() <= 64 && Exponent->getSExtValue() % 2u == 0)
return true;
}
+ // TODO: This is not correct. Given that exp is an integer, here are the
+ // ways that pow can return a negative value:
+ //
+ // pow(x, exp) --> negative if exp is odd and x is negative.
+ // pow(-0, exp) --> -inf if exp is negative odd.
+ // pow(-0, exp) --> -0 if exp is positive odd.
+ // pow(-inf, exp) --> -0 if exp is negative odd.
+ // pow(-inf, exp) --> -inf if exp is positive odd.
+ //
+ // Therefore, if !SignBitOnly, we can return true if x >= +0 or x is NaN,
+ // but we must return false if x == -0. Unfortunately we do not currently
+ // have a way of expressing this constraint. See details in
+ // https://llvm.org/bugs/show_bug.cgi?id=31702.
return cannotBeOrderedLessThanZeroImpl(I->getOperand(0), TLI, SignBitOnly,
Depth + 1);
+
case Intrinsic::fma:
case Intrinsic::fmuladd:
// x*x+y is non-negative if y is non-negative.
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