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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 --- - -fno-associative-math is not honored"
href="https://llvm.org/bugs/show_bug.cgi?id=27372">27372</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>-fno-associative-math is not honored
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
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<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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>warren_ristow@playstation.sony.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>Re-association enabled via '-ffast-math' is not suppressed with
'-fno-associative-math'. Consider the following test-case:
// -O1 -ffast-math -fno-associative-math
float foo(float a, float x)
{
return ((a + x) - x);
}
Compiling at -O1, results in an Add and a Subtract, as expected. Enabling
fast-math, allows re-association to happen, and the arithmetic ops are folded
away, also as expected. But enabling fast-math while explicitly disabling
associative-math doesn't suppress the re-association (the arithmetic ops are
still folded away). Specifically:
$ clang++ --version
clang version 3.9.0 (trunk 266451)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/warren/llvm/bin
$ clang++ -c -O1 test.cpp
$ objdump -d test.o
test.o: file format elf64-x86-64
...
0000000000000000 <_Z3fooff>:
0: f3 0f 58 c1 addss %xmm1,%xmm0
4: f3 0f 5c c1 subss %xmm1,%xmm0
8: c3 retq
$ clang++ -c -O1 -ffast-math test.cpp
$ objdump -d test.o
...
0000000000000000 <_Z3fooff>:
0: c3 retq
$ clang++ -c -O1 -ffast-math -fno-associative-math test.cpp
$ objdump -d test.o
...
0000000000000000 <_Z3fooff>:
0: c3 retq
$
GCC does honor the '-fno-associative-math' switch (tested with GCC 4.9.2):
$ g++ -c -O1 test.cpp
$ objdump -d test.o
test.o: file format elf64-x86-64
...
0000000000000000 <_Z3fooff>:
0: f3 0f 58 c1 addss %xmm1,%xmm0
4: f3 0f 5c c1 subss %xmm1,%xmm0
8: c3 retq
$ g++ -c -O1 -ffast-math test.cpp
$ objdump -d test.o
...
0000000000000000 <_Z3fooff>:
0: f3 c3 repz retq
$ g++ -c -O1 -ffast-math -fno-associative-math test.cpp
$ objdump -d test.o
...
0000000000000000 <_Z3fooff>:
0: f3 0f 58 c1 addss %xmm1,%xmm0
4: f3 0f 5c c1 subss %xmm1,%xmm0
8: c3 retq
$
This appears to be target-independent behavior (noticed on X86, but tried with
a few other architectures, and observed the same issue).
Also, tested with a handful of Clang versions, and it's a longstanding
behavior.</pre>
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