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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Inline assembly input operand inefficiency"
href="https://bugs.llvm.org/show_bug.cgi?id=36811">36811</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Inline assembly input operand inefficiency
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<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>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nruslan_devel@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I have noticed that whenever "mr" is specified for an assembly input operand
which may accept both memory and register, clang/llvm does not seem to generate
efficient code.
(-O2 is used for all examples)
For example,
1.
unsigned long func(unsigned long x)
{
unsigned long r;
asm ("bsf %1, %0"
: "=r" (r)
: "mr" (x)
: "cc");
return r;
}
generates code which unnecessarily moves %rdi to memory
func: # @func
.cfi_startproc
# %bb.0:
movq %rdi, -8(%rsp)
#APP
bsfq -8(%rsp), %rax
#NO_APP
retq
2. whereas, if we change "mr" to simply "r" (for x)
we get optimal code
func: # @func
.cfi_startproc
# %bb.0:
#APP
bsfq %rdi, %rax
#NO_APP
retq
3. gcc generates optimal code in both cases
func:
.LFB0:
.cfi_startproc
#APP
# 4 "1.c" 1
bsf %rdi, %rax
# 0 "" 2
#NO_APP
ret</pre>
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