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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loses aliasing information through union when rewriting into conditional move"
href="https://bugs.llvm.org/show_bug.cgi?id=33278">33278</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Loses aliasing information through union when rewriting into conditional move
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>4.0
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>steinar+llvm@gunderson.no
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Given clang 4.0 rc1:
atum17:~> clang-4.0 --version
clang version 4.0.1-+rc1-1 (tags/RELEASE_401/rc1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
The following minimal example (reduced from string-to-double parsing code
in MySQL) is miscompiled, seemingly because the compiler loses track of
aliasing information when converting the add into a conditional operation
and thinks “da” was never changed:
atum17:~> cat test.cc
#include <stdio.h>
typedef union { double d; unsigned int L[2]; } U;
double func(int k)
{
U da, db;
da.d = 1.453125;
db.d = 1.0;
if (k > 0) {
da.L[1] += k;
} else {
k= -k;
db.L[1] += k;
}
return da.d;
}
int main(void)
{
printf("%f\n", func(0x300000));
}
atum17:~> clang-4.0 -O2 -o test test.cc
atum17:~> ./test
1.453125
atum17:~> clang-4.0 -O2 -fno-strict-aliasing -o test test.cc
atum17:~> ./test
11.625000
As I understand it, accessing through an union like this is not a
strict aliasing violation since C11 and C++11, so the code should
give the same answer in both cases. Clang 3.9 and GCC 7.1.0 gives
the correct answer. The generated x86 code is:
00000000004004f0 <_Z4funci>:
4004f0: 89 f8 mov %edi,%eax
4004f2: f7 d8 neg %eax
4004f4: 0f 4c c7 cmovl %edi,%eax
4004f7: 48 b9 00 00 00 00 00 movabs $0x3ff7400000000000,%rcx
4004fe: 40 f7 3f
400501: 48 89 4c 24 f8 mov %rcx,-0x8(%rsp)
400506: 48 b9 00 00 00 00 00 movabs $0x3ff0000000000000,%rcx
40050d: 00 f0 3f
400510: 48 89 4c 24 f0 mov %rcx,-0x10(%rsp)
400515: 85 ff test %edi,%edi
400517: 48 8d 4c 24 f8 lea -0x8(%rsp),%rcx
40051c: 48 8d 54 24 f0 lea -0x10(%rsp),%rdx
400521: 48 0f 4f d1 cmovg %rcx,%rdx
400525: 48 83 ca 04 or $0x4,%rdx
400529: 01 02 add %eax,(%rdx)
40052b: f2 0f 10 05 b5 00 00 movsd 0xb5(%rip),%xmm0 # 4005e8
<_IO_stdin_used+0x8>
400532: 00
400533: c3 retq
400534: 66 66 66 2e 0f 1f 84 data16 data16 nopw %cs:0x0(%rax,%rax,1)
40053b: 00 00 00 00 00
In other words, it seems to convert the if into something like
(k > 0 ? &da.L[1] : &db.L[0]) += (k > 0 ? k : -k)
but then basically everything is thrown away at the last instruction,
and it loads the value directly from a constant.</pre>
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