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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Invalid code generation / optimization of C++ with -O3"
href="http://llvm.org/bugs/show_bug.cgi?id=19824">19824</a>
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<th>Summary</th>
<td>Invalid code generation / optimization of C++ with -O3
</td>
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<th>Product</th>
<td>clang
</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>MacOS X
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>dane@mapbox.com
</td>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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<div>
<pre>After upgrading to Xcode 5.1.1 / Apple LLVM version 5.1 (clang-503.0.40) (based
on LLVM 3.4svn) I'm seeing a case where clang++ seems to mis-generate code when
the -O3 flag is used. The symptom is failing testcases in my application that
have been passing for years. Certainly there may be a bug in my code, but here
are the reasons I think clang is at fault:
- if I compile with -Os the problem goes away (only happens with -O3)
- if I put a statement like `std::clog << "foo\n";` in the problem area then
clang seems to generate the correct code. This seems to indicate to me that
clang is over-eagerly optimizing away code and effectively leading to it not
being run.
- the problem did not exist with older clang
The application in question is Mapnik and the problem code involves c++11
lambda functions. I can reproduce the problem with latest clang on OS X and
Ubuntu precise by using the llvm 3.4 packages from <a href="http://llvm.org/apt/">http://llvm.org/apt/</a>.
In particular it is this block of code that is (seemingly) disappearing and
never executed at runtime:
<a href="https://github.com/mapnik/mapnik/blob/master/src/grid/process_markers_symbolizer.cpp#L123-L132">https://github.com/mapnik/mapnik/blob/master/src/grid/process_markers_symbolizer.cpp#L123-L132</a>.
If I put `std::clog << "something\n";` inside that `vector_dispatch_type` class
right here
(<a href="https://github.com/mapnik/mapnik/blob/master/include/mapnik/grid/grid_marker_helpers.hpp#L225-L228">https://github.com/mapnik/mapnik/blob/master/include/mapnik/grid/grid_marker_helpers.hpp#L225-L228</a>)
then the code does run at runtime and works properly. This seems to indicate to
me that over-eager optimization is at fault - could it be?
I've read <a href="http://llvm.org/docs/HowToSubmitABug.html#incorrect-code-generation">http://llvm.org/docs/HowToSubmitABug.html#incorrect-code-generation</a>
but its not clear to me how to use bugpoint for further providing a testcase
since I'm new to reporting bugs against clang. I can provide the output of
-save-temps or -emit-llvm easily, just let me know if these would help.</pre>
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