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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Redundant movs and an extra spill while using all xmm registers"
href="https://llvm.org/bugs/show_bug.cgi?id=26810">26810</a>
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<th>Summary</th>
<td>Redundant movs and an extra spill while using all xmm registers
</td>
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<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>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Register Allocator
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>egor.kochetov@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15974" name="attach_15974" title="The reproducer (1 source file, assemblies for clang and GCC, auxiliary Makefile)">attachment 15974</a> <a href="attachment.cgi?id=15974&action=edit" title="The reproducer (1 source file, assemblies for clang and GCC, auxiliary Makefile)">[details]</a></span>
The reproducer (1 source file, assemblies for clang and GCC, auxiliary
Makefile)
When compiling the loop below, clang produces the binary that runs ~25% slower
than that of GCC.
constexpr unsigned nIterations = 76800000; //< an arbitrary large number
__m128d v[8];
__m128d m1[nIterations];
__m128d m2[8];
for (unsigned i = 0; i < nIterations; ++i) {
const __m128d M = m1 [i];
__m128d a;
a = _mm_mul_pd(M, m2[0]);
v[0] = _mm_add_pd(v[0], a);
a = _mm_mul_pd(M, m2[1]);
v[1] = _mm_add_pd(v[1], a);
a = _mm_mul_pd(M, m2[2]);
v[2] = _mm_add_pd(v[2], a);
a = _mm_mul_pd(M, m2[3]);
v[3] = _mm_add_pd(v[3], a);
a = _mm_mul_pd(M, m2[4]);
v[4] = _mm_add_pd(v[4], a);
a = _mm_mul_pd(M, m2[5]);
v[5] = _mm_add_pd(v[5], a);
a = _mm_mul_pd(M, m2[6]);
v[6] = _mm_add_pd(v[6], a);
a = _mm_mul_pd(M, m2[7]);
v[7] = _mm_add_pd(v[7], a);
}
In particular, there are weird movs around calculating v[6]:
0x08048c4e <+334>: movapd %xmm5,%xmm6
0x08048c52 <+338>: movapd %xmm4,%xmm5
0x08048c56 <+342>: movapd %xmm3,%xmm4
0x08048c5a <+346>: movapd %xmm2,%xmm3
Also, throughout the loop clang uses 3 spills for storing values of v[1], v[6]
and v[7], while GCC uses only two spills (for v[0] and v[1]).
Attached is the source file, annotated assembly for clang and an assembly for
GCC for the function 'loop', and the (optional) Makefile.
The source needs to be compiled with '-m32' to limit the number of xmm
registers available. -O2 and -Ofast produce the same results (though the
register names are changing).</pre>
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