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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 --- - 30% performance deficiency vs GCC because of excessive vectorization on -march=slm"
href="https://llvm.org/bugs/show_bug.cgi?id=25108">25108</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>30% performance deficiency vs GCC because of excessive vectorization on -march=slm
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>egor.kochetov@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15033" name="attach_15033" title="c++ reproducer, Makefile, two disassembled functions.">attachment 15033</a> <a href="attachment.cgi?id=15033&action=edit" title="c++ reproducer, Makefile, two disassembled functions.">[details]</a></span>
c++ reproducer, Makefile, two disassembled functions.
Here is the C++ source that gets vectorized by clang and is not vectorized by
gcc, it is the function 'compute' of the attached cpp file.
template <typename T>
void compute (T* v) {
for (int i = 0; i < 10000; ++i) {
// fw pass
for (int i = 0; i < 6; ++i) {
v [i] += v [i + 1];
v [i] *= v [i + 2];
}
//back pass
for (int i = 7; i > 1; --i) {
v [i] += v [i - 1];
v [i] *= v [i - 2];
}
}
}
The vectorized version happens to work slower, when compiled both for float and
for double numbers, on a Silvermont Intel machine. The test run timings are as
follows:
clang:
float time: 0.000043 ms
double time: 0.000059 ms
gcc:
float time: 0.000029 ms
double time: 0.000043 ms
The source was compiled with -Ofast -m64 -g -march=slm -mfpmath=sse -fPIE. 32
bit build shows the same result.
This difference reproduces a real one which is measured in seconds in one of
the programs I deal with.
It seems, that vectorization costs in clang are not set up correctly for
-march=slm. When compiling by clang with -fno-slp-vectorize, the resulting
performance matches gcc.
Compilers under consideration are: gcc 6.0 trunk vs clang 3.8 trunk.
The attachment is the archive with the C++ reproducer source, Makefile and
assemblies for float version got from the two compilers.
To run the reproducer using the Makefile attached, call `make clean run; make
clean run COMPILER=gcc` after fixing compiler paths in the Makefile.</pre>
</div>
</p>
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