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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstructionSimplify turns NaN to 0.0"
href="https://llvm.org/bugs/show_bug.cgi?id=27151">27151</a>
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<th>Summary</th>
<td>InstructionSimplify turns NaN to 0.0
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>davemm@cs.rutgers.edu
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, santosh.nagarakatte@gmail.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LLVM optimizes this code:
define float @foo(float %x) #0 {
entry:
%0 = fsub nnan ninf float 0.0, %x
%1 = fadd float %0, %x
ret float %1
}
to this:
define float @foo(float %x) #0 {
entry:
ret float 0.000000e+00
}
My reading of the language reference is that the original program will return
NaN when %x is NaN (or +/-infinity). Specifically, the nnan flag on %0 means
that it will return an undefined value when %x is NaN, but it is required to
have defined behavior. That means %1 is calculated as normal, which means it
must return NaN, since %x is NaN.
>From the reference:
---
No NaNs - Allow optimizations to assume the arguments and result are not NaN.
Such optimizations are required to retain defined behavior over NaNs, but the
value of the result is undefined.
---
Counter argument: Since this is described in terms of optimizations instead of
instructions, it could be argued that the nnan in %0 is sufficient to allow the
optimization, even though %1 does not have nnan. This would mean adding nnan to
%0 affects every instruction that uses %x.
Here is the relevant code from InstructionSimplify.cpp:
// 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());
}</pre>
</div>
</p>
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