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Fix really obscure bug in CannotBeNegativeZero() (PR22688) 

With a diabolically crafted test case, we could recurse
through this code and return true instead of false.

The larger engineering crime is the use of magic numbers. 
Added FIXME comments for those.

llvm-svn: 230515
This commit is contained in:
Sanjay Patel 2015-02-25 18:00:15 +00:00
parent d9b8bd6b86
commit 40eaa8df99
2 changed files with 33 additions and 1 deletions
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8
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -2000,8 +2000,11 @@ bool llvm::CannotBeNegativeZero(const Value *V, unsigned Depth) {
if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V))
return !CFP->getValueAPF().isNegZero();
// FIXME: Magic number! At the least, this should be given a name because it's
// used similarly in CannotBeOrderedLessThanZero(). A better fix may be to
// expose it as a parameter, so it can be used for testing / experimenting.
if (Depth == 6)
return 1; // Limit search depth.
return false; // Limit search depth.
const Operator *I = dyn_cast<Operator>(V);
if (!I) return false;
@ -2048,6 +2051,9 @@ bool llvm::CannotBeOrderedLessThanZero(const Value *V, unsigned Depth) {
if (const ConstantFP *CFP = dyn_cast<ConstantFP>(V))
return !CFP->getValueAPF().isNegative() || CFP->getValueAPF().isZero();
// FIXME: Magic number! At the least, this should be given a name because it's
// used similarly in CannotBeNegativeZero(). A better fix may be to
// expose it as a parameter, so it can be used for testing / experimenting.
if (Depth == 6)
return false; // Limit search depth.

26
llvm/test/Transforms/InstSimplify/floating-point-arithmetic.ll
View File

@ -33,3 +33,29 @@ define double @fmul_X_1(double %a) {
; CHECK: ret double %a
ret double %b
}
; We can't optimize away the fadd in this test because the input
; value to the function and subsequently to the fadd may be -0.0.
; In that one special case, the result of the fadd should be +0.0
; rather than the first parameter of the fadd.
; Fragile test warning: We need 6 sqrt calls to trigger the bug
; because the internal logic has a magic recursion limit of 6.
; This is presented without any explanation or ability to customize.
declare float @sqrtf(float)
define float @PR22688(float %x) {
%1 = call float @sqrtf(float %x)
%2 = call float @sqrtf(float %1)
%3 = call float @sqrtf(float %2)
%4 = call float @sqrtf(float %3)
%5 = call float @sqrtf(float %4)
%6 = call float @sqrtf(float %5)
%7 = fadd float %6, 0.0
ret float %7
; CHECK-LABEL: @PR22688(
; CHECK: fadd float %6, 0.0
}