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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Miscompile explicitly vectorized code"
href="http://llvm.org/bugs/show_bug.cgi?id=22753">22753</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Miscompile explicitly vectorized code
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>3.6
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>martin.kronbichler@it.uu.se
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>clang-3.6 miscompiles the following code at -mavx.
$ clang -v
clang version 3.6.0 (tags/RELEASE_360/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation:
$HOME/sw/bin/../lib/gcc/x86_64-linux-gnu/4.9.1
Found candidate GCC installation:
$HOME/sw/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/3.4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.6.3
Selected GCC installation: $HOME/sw/bin/../lib/gcc/x86_64-linux-gnu/4.9.1
Candidate multilib: .;@m64
Selected multilib: .;@m64
-------------------------------------------------
#include <stdio.h>
typedef double __m256d __attribute__((__vector_size__(32)));
class VectorizedArray
{
public:
VectorizedArray &
operator += (const VectorizedArray &vec)
{
data += vec.data;
return *this;
}
__m256d data;
};
inline
VectorizedArray
operator + (const VectorizedArray &u,
const VectorizedArray &v)
{
VectorizedArray tmp = u;
return tmp+=v;
}
int main()
{
VectorizedArray a, b, c;
for (unsigned int i=0; i<4; ++i)
{
((double*)(&a.data))[i] = 2.;
((double*)(&b.data))[i] = -1.;
}
c = a + b;
for (unsigned int i=0; i<4; ++i)
printf("%lf\n", ((const double*)(&c.data))[i]);
}
-------------------------------------------------
$ clang -mavx test.cc && ./a.out
1.000000
1.000000
0.000000
0.000000
(Should be 4 times 1.000000).
The wrong code is in operator+(VectorizedArray const&, VectorizedArray const&):
vzeroupper
callq _ZN15VectorizedArraypLERKS_
vmovaps (%rax), %ymm0
vmovaps %ymm0, 64(%rsp)
vmovupd 64(%rsp), %xmm0
movq %rbp, %rsp
popq %rbp
vzeroupper
retq
Note how the content of %ymm0 gets assigned into an %xmm0 and thus, the upper
128 bits are lost. The code generation is wrong both at -O0 (shown above) and
-O2 and happens for both __m256 and __m256d. clang 3.5 and previous do fine on
this example.</pre>
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