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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed strlen==0 optimization"
href="http://llvm.org/bugs/show_bug.cgi?id=19903">19903</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed strlen==0 optimization
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</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>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dnovillo@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dblaikie@gmail.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The optimizer will generally optimize a strlen==0 comparison, but under certain
circumstances, it will miss the opportunity:
int foo(const char *s) {
if (strlen(s) == 0) {
return 42;
}
return strlen(s);
}
Instead of calling strlen on the else branch, LLVM will float the call to
strlen at the top:
.Ltmp0:
.cfi_def_cfa_offset 16
callq strlen
testq %rax, %rax
movl $42, %ecx
cmovnel %eax, %ecx
movl %ecx, %eax
popq %rdx
retq
Whereas GCC 4.6 will first avoid calling strlen until it really needs to:
.cfi_def_cfa_offset 16
cmpb $0, (%rdi)
movl $42, %eax
jne .L5
addq $8, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.p2align 4,,10
.p2align 3
.L5:
.cfi_restore_state
call strlen
addq $8, %rsp
.cfi_def_cfa_offset 8
.p2align 4,,1
ret
I'm not sure how important this is. I only caught it trying to test something
else.</pre>
</div>
</p>
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