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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86] Consider not restoring the stack between calls"
href="https://llvm.org/bugs/show_bug.cgi?id=27165">27165</a>
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<th>Summary</th>
<td>[X86] Consider not restoring the stack between calls
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Blocks</th>
<td>26299
</td>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>Consider:
void f(int);
void g() {
f(1);
f(2);
}
Compiled with -target i686-pc-win32 -Os, Clang generates:
00000000 <_g>:
0: 6a 01 push $0x1
2: e8 00 00 00 00 call 7 <_g+0x7>
7: 83 c4 04 add $0x4,%esp
a: 6a 02 push $0x2
c: e8 00 00 00 00 call 11 <_g+0x11>
11: 83 c4 04 add $0x4,%esp
14: c3 ret
Since there is no need to align the stack, we could consider not restoring the
stack pointer between the two calls:
MSVC with /Ox:
00000000: 6A 01 push 1
00000002: E8 00 00 00 00 call _f
00000007: 6A 02 push 2
00000009: E8 00 00 00 00 call _f
0000000E: 83 C4 08 add esp,8
00000011: C3 ret
This saves code size at the expense of stack size.
If we used pops to restore the stack here (see PR26333) that might be cheaper
than a consolidated "addl $8, %esp" though.</pre>
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