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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to optimize integer to float conversions that can never fault under -ftrapping-math"
href="https://bugs.llvm.org/show_bug.cgi?id=51921">51921</a>
</td>
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<th>Summary</th>
<td>Failure to optimize integer to float conversions that can never fault under -ftrapping-math
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>bool f(float x)
{
return x == 0;
}
With -O3 -ftrapping-math, this gives:
f(float): # @f(float)
xor eax, eax ; eax = 0
cvtsi2ss xmm1, eax ; xmm1 = float(eax -> 0)
ucomiss xmm0, xmm1
setnp cl
sete al
and al, cl
ret
(though this is not unique to x86 and I can reproduce it on platforms like
powerpc64le)
Conversion of 0 to a float can never be inexact, so it seems to me as though
the conversion is unnecessary here. This should also be possible for all
constants that exactly convert to a float/double/long double in equivalent
situations (though from my reading of the IEEE-754 standard it seems the
conversion should be kept under `-ftrapping-math` if the conversion is
inexact).
As such, it should be possible to optimize this to `return x == 0.f;`. This is
done by GCC, but not by LLVM.
Godbolt comparison here: <a href="https://godbolt.org/z/6YPMEGfex">https://godbolt.org/z/6YPMEGfex</a>
(if someone knows how to do alive2 checks for stuff that involves trapping
floating point operations, I would be glad to know)</pre>
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