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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 --- - Missed optimization: stack frame pushing/popping is not optimized away for trivial functions"
href="https://llvm.org/bugs/show_bug.cgi?id=26160">26160</a>
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<th>Summary</th>
<td>Missed optimization: stack frame pushing/popping is not optimized away for trivial functions
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.6
</td>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>MacOS X
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mjbshaw@hotmail.com
</td>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>clang unnecessarily pushes and pops the stack frame for trivial functions at
all -O levels:
$ cat t.c
void foo(void) {}
$ clang -O2 -c t.c
$ otool -tV t.o
t.o:
(__TEXT,__text) section
_foo:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp, %rbp
0000000000000004 popq %rbp
0000000000000005 retq
gcc is able to optimize this function to a single retq instruction at
optimization levels of -O1 and above.
----
This is not just seen with empty functions. Other simple functions that have no
need for a full stack frame end up pushing and popping it anyway:
$ cat t.c
int bar(int x) { return x; }
$ clang -O2 -c t.c
$ otool -tV t.o
t.o:
(__TEXT,__text) section
_bar:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp, %rbp
0000000000000004 movl %edi, %eax
0000000000000006 popq %rbp
0000000000000007 retq
gcc is able to optimize this function to just movl %edi, %eax and retq.
It would be ideal if clang could optimize away unnecessary fiddling with %rbp
and $rsp.</pre>
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