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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 - Missed optimization: extra XOR and FMA"
href="https://bugs.llvm.org/show_bug.cgi?id=43239">43239</a>
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<th>Summary</th>
<td>Missed optimization: extra XOR and FMA
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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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>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>For the code below:
#include <cmath>
double t1(double x, double y)
{
return std::sqrt(x*x + y*y);
}
clang(trunk) with '-O3 -ffast-math' generates:
t1(double, double): # @t1(double, double)
mulsd xmm0, xmm0
mulsd xmm1, xmm1
addsd xmm1, xmm0
xorps xmm0, xmm0
sqrtsd xmm0, xmm1
ret
gcc(trunk) with '-O3 -ffast-math' is better here (doesn't add extra xorps
instr) and generates:
t1(double, double):
mulsd xmm1, xmm1
mulsd xmm0, xmm0
addsd xmm0, xmm1
sqrtsd xmm0, xmm0
ret
Godbolt playground: <a href="https://godbolt.org/z/Cx9EDI">https://godbolt.org/z/Cx9EDI</a>
Also note that with '-O3 -march=native -ffast-math' both compilers (gcc(trunk)
and clang(trunk)) generate equal code:
t1(double, double):
vmulsd xmm1, xmm1, xmm1
vfmadd132sd xmm0, xmm1, xmm0
vsqrtsd xmm0, xmm0, xmm0
ret
Godbolt playground: <a href="https://godbolt.org/z/fHgGxw">https://godbolt.org/z/fHgGxw</a>
But without '-ffast-math' outputs are different: clang doesn't use FMA instr.
Godbolt playground: <a href="https://godbolt.org/z/X46_hD">https://godbolt.org/z/X46_hD</a></pre>
</div>
</p>
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