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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Subtract stack pointer instead of push"
href="https://bugs.llvm.org/show_bug.cgi?id=38327">38327</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Subtract stack pointer instead of push
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<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>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Code:
int is_enabled(void);
int tobool(void) {
return !!is_enabled();
}
int tobool2(void) {
if(is_enabled())
return 9;
return 4;
}
Clang -O3:
tobool: # @tobool
push rax
call is_enabled
xor ecx, ecx
test eax, eax
setne cl
mov eax, ecx
pop rcx
ret
tobool2: # @tobool2
push rax
call is_enabled
xor ecx, ecx
test eax, eax
setne cl
lea eax, [rcx + 4*rcx]
add eax, 4
pop rcx
ret
GCC avoid push, it uses sub rsp, 8.
tobool:
sub rsp, 8
call is_enabled
test eax, eax
setne al
add rsp, 8
movzx eax, al
ret
tobool2:
sub rsp, 8
call is_enabled
cmp eax, 1
sbb eax, eax
add rsp, 8
and eax, -5
add eax, 9
ret
Maybe it could be worth (less instructions, register pressure) to follow GCC
decision, I think.</pre>
</div>
</p>
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