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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang -O2 miscompiles pointer arithmetics"
href="https://bugs.llvm.org/show_bug.cgi?id=33990">33990</a>
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<th>Summary</th>
<td>clang -O2 miscompiles pointer arithmetics
</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>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</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>rhash.admin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>Created <span class=""><a href="attachment.cgi?id=18870" name="attach_18870" title="the miscompiled file">attachment 18870</a> <a href="attachment.cgi?id=18870&action=edit" title="the miscompiled file">[details]</a></span>
the miscompiled file
Hi,
I've got clang miscompilation bug on 64-bit Ubuntu 17.04 with its default clang
4.0.
Bug description:
`clang -S -O2` miscompiles the following C code:
--- Begin of ud2_main.c ---
unsigned long long foo();
int main()
{
char* c = (char*)0 + foo();
int res = c[0] + c[1];
return res;
}
--- End of ud2_main.c ---
The call `clang -S -O2 ud2_main.c` compiles the function main() to:
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
xorl %eax, %eax
callq foo
ud2
Then ud2 instruction leads to core dump.
To compile and test the program you need the second file with foo():
--- Begin of ud2_foo.c ---
unsigned long long foo()
{
static char* ptr = "\3\5";
return (unsigned long long)(ptr - (char*)0);
}
--- End of ud2_foo.c ---
Here is testing results:
$ clang -O2 ud2_main.c ud2_foo.c -o ud2_clang
$ ./ud2_clang
Illegal instruction (core dumped)
# succesfull tests with gcc and 'clang -O0'
$ gcc -O2 ud2_main.c ud2_foo.c -o ud2_gcc && ./ud2_gcc; echo $?
8
$ clang -O0 ud2_main.c ud2_foo.c -o ud2_clangO0 && ./ud2_clangO0; echo $?
8
The bug occurs with -O1 or -O2 optimizations (doesn't with -O0), and only with
the foo() function declared in a separate file.
The system info:
$ uname -a
Linux ubuntu64 4.10.0-28-generic #32-Ubuntu SMP Fri Jun 30 05:32:18 UTC 2017
x86_64 x86_64 x86_64 GNU/Linux
$ clang -v
clang version 4.0.0-1ubuntu1 (tags/RELEASE_400/rc1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin</pre>
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