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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Inline Assembler Comments"
href="https://bugs.llvm.org/show_bug.cgi?id=33770">33770</a>
</td>
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<th>Summary</th>
<td>Inline Assembler Comments
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>4.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>rigger.manuel@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I'm investigating how inline assembly is used in C projects. While doing so, I
found that a number of projects include comments as part of their inline
assembly snippets. The code snippet below is an example:
#include <stdint.h>
uint64_t atomic_swap_long(volatile uint64_t *p, uint64_t v) {
__asm __volatile(
"xchgq %1,%0; # atomic_swap_long"
: "+r" (v), /* 0 */
"+m" (*p)); /* 1 */
return v;
}
When compiling with GCC (gcc -O1 -S) the following code is produced for the
inline assembly snippet on my system:
#APP
# 4 "inline-assembly-comment.c" 1
xchgq (%rdi),%rax; # atomic_swap_long
# 0 "" 2
#NO_APP
With Clang 3.8 and below, an empty line is inserted instead of the comment:
#APP
xchgq %rsi, (%rdi)
#NO_APP
With Clang 3.9 and 4.0, the newline disappeared but the comment is still not
part of the assembly code.
The comment is retained on the LLVM IR level:
%5 = call i64 asm sideeffect "xchgq\09$1,$0; # atomic_swap_long",
"=r,=*m,0,*m,~{dirflag},~{fpsr},~{flags}"(i64* %4, i64 %3, i64* %4)
On the mailing list (see
<a href="http://lists.llvm.org/pipermail/cfe-dev/2017-July/054657.html">http://lists.llvm.org/pipermail/cfe-dev/2017-July/054657.html</a>), I got the
advice to delete the trailing semicolon. This works for Clang 4.0.</pre>
</div>
</p>
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