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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 - +g and +X inline asm constraints generate worse code than +r"
href="https://bugs.llvm.org/show_bug.cgi?id=35489">35489</a>
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<th>Summary</th>
<td>+g and +X inline asm constraints generate worse code than +r
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Testcase:
int f(int n) { asm volatile("":"+g"(n)); return n; }
int k = f(4) + 1;
Targeting x86_64, we get this mess:
movl $4, -4(%rsp)
movl $4, %eax
#APP
#NO_APP
movl %eax, -4(%rsp)
addl $1, %eax
movl %eax, k(%rip)
A "+r" constraint, which should be *more* constraining (and thus produce worse
code in general) gives this:
movl $4, %eax
#APP
#NO_APP
addl $1, %eax
movl %eax, k(%rip)
GCC produces the better code for both constraints.</pre>
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