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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Inline assembly incompatibility with gcc for array parameters"
href="https://bugs.llvm.org/show_bug.cgi?id=42996">42996</a>
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<th>Summary</th>
<td>Inline assembly incompatibility with gcc for array parameters
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>8.0
</td>
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<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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<tr>
<th>Reporter</th>
<td>js@alien8.de
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Consider the following memcpy example that is adapted from the GCC
documentation [1]:
#include <stddef.h>
void* memcpy(void* dst, const void* src, size_t n)
{
void *orig_dst = dst;
asm ("rep movsb"
: "+S" (src), "+D" (dst), "+c" (n),
"=m" (*(char (*)[]) dst)
: "m" (*(const char (*)[]) src));
return orig_dst;
}
Array types is used to denote array parameters of unknown length. This works
fine on GCC, but results in the following error on clang:
string.c:10:17: error: dereference of pointer to incomplete type 'const char
[]'
: "m" (*(const char (*)[]) src));
The fix seems to be to cast to `(char (*)[n])` instead and this also works on
GCC. It's just unfortunate that the example from the gcc documentation is
broken on clang.
[1] See repne scasb here: <a href="https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html">https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html</a></pre>
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