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<base href="https://llvm.org/bugs/" />
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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 --- - [X86] Clang miscompiles an inline asm output at -O1"
href="https://llvm.org/bugs/show_bug.cgi?id=26802">26802</a>
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<tr>
<th>Summary</th>
<td>[X86] Clang miscompiles an inline asm output at -O1
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>3.7
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Other
</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>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>cameron.mcinally@nyu.edu
</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>At -O1, Clang seems to miscompile the output of this inline assembly:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static inline char *string_cp(char *dest, const char *src) {
int d0, d1, d2;
__asm__ __volatile__("1:\tlodsb\n\t"
"stosb\n\t"
"testb %%al,%%al\n\t"
"jne 1b"
: "=&S" (d0), "=&D" (d1)
: "0" (src), "1" (dest)
: "memory");
return dest;
}
int main(void) {
const char *inpt = "Hi there.";
char *out;
out = (char *)malloc(32*sizeof(char));
memset(out,'\0',32);
printf("inpt = %s, out = %s\n",inpt,out);
out = string_cp(out,inpt);
printf("inpt = %s, out = %s\n",inpt,out);
return 0;
}
The expected output is:
inpt = Hi there., out =
inpt = Hi there., out = Hi there.
Looking at the assembly, we can see that %rax is not restored after the inline
asm loop has clobbered it.
# BB#0: # %entry
movq %rdi, %rax
movl $.L.str, %esi
#APP
.Ltmp2:
lodsb (%rsi), %al
stosb %al, %es:(%rdi)
testb %al, %al
jne .Ltmp2
#NO_APP
# %rax should be restored here. The loop
# clobbers it.
retq
Also note that this does compile correctly at -O0, and that inlining avoids the
problem at -O2.</pre>
</div>
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