https://llvm.org/bugs/show_bug.cgi?id=26746
Bug ID: 26746
Summary: InstructionSimplify turns 0.0 to -0.0
Product: new-bugs
Version: 3.7
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected],
[email protected]
Classification: Unclassified
Given this input:
define float @foo(float %x) #0 {
entry:
%a = fsub float -0.0, %x
%b = fsub float 0.0, %a
ret float %b
}
opt -instcombine produces
define float @foo(float %x) #0 {
entry:
ret float %x
}
If %x = -0.0, the original function returns 0.0, but the optimized function
returns -0.0.
I believe this is the relevant code, from lib/Analysis/InstructionSimplify.cpp:
// fsub 0, (fsub -0.0, X) ==> X
Value *X;
if (match(Op0, m_AnyZero())) {
if (match(Op1, m_FSub(m_NegZero(), m_Value(X)))) // this clause
return X;
if (FMF.noSignedZeros() && match(Op1, m_FSub(m_AnyZero(), m_Value(X))))
return X;
}
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