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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/x64 clang appears not to respect __attribute__((noinline)) with -O1"
href="https://llvm.org/bugs/show_bug.cgi?id=26545">26545</a>
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<th>Summary</th>
<td>x86/x64 clang appears not to respect __attribute__((noinline)) with -O1
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.7
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvmbugs@tomseddon.plus.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15863" name="attach_15863" title="code">attachment 15863</a> <a href="attachment.cgi?id=15863&action=edit" title="code">[details]</a></span>
code
Repro steps:
Compile the attached code with -O1 and generate assembly language.
- From shell: clang -O1 -S main.c (I tested this with Xcode's clang - "Apple
LLVM version 7.0.2 (clang-700.1.81); Target: x86_64-apple-darwin14.5.0; Thread
model: posix")
- See it on gcc explorer: <a href="http://goo.gl/Im4c95">http://goo.gl/Im4c95</a> (link uses x86 clang 3.7.1)
Expected result:
Generated code contains a call to the Test function somewhere, since Test has
the noinline attribute. (This even though -O1 was specified, and the Test
function is an ideal candidate for inlining.)
Actual result:
The compiler appears to have inlined the Test function (or otherwise determined
somehow that the result of the call would be -55).
Notes:
- I couldn't find any clang documentation for noinline so I'm just making
assumptions about what it does (sorry)
- The gcc documentation for noinline suggests this sort of problem is a
possibility - see
<a href="https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-g_t_0040code_007bnoinline_007d-function-attribute-3218">https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-g_t_0040code_007bnoinline_007d-function-attribute-3218</a>
- but the workaround it mentions has no effect in clang
- If you force the compiler to generate the Test function (e.g., by adding
something like ``printf("%p\n",(void *)&Test);''), the call still isn't
generated</pre>
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