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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - bitfields result in really slow code"
href="http://llvm.org/bugs/show_bug.cgi?id=21039">21039</a>
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<th>Summary</th>
<td>bitfields result in really slow code
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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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>Common Code Generator Code
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>jeroen.dobbelaere@synopsys.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13070" name="attach_13070" title="test_bitfields.c: the testcase">attachment 13070</a> <a href="attachment.cgi?id=13070&action=edit" title="test_bitfields.c: the testcase">[details]</a></span>
test_bitfields.c: the testcase
Clang produces code that treats a bitfield as a part of a very large number,
consisting of all adjacent bitfields.
For structs containing a lot of bitfields, this can result in code loading,
storing and doing arithmetic operations on large numbers.
In the provided test example, we clear one specific bitfield.
On the x86/x86_64 architecture, this result in good code, but on all the other
architectures that I tried, the code results in unnecessary load/store
operations.
The problem seems to be that in the legalization and lowering phase, the
loading/masking/storing is split up in smaller chunks, but corresponding
load/stores are not optimized away. For x86, this is less of an issue as it
already knows how to map arithmetic operations onto memory.
Note: maybe the bigger problem is that clang maps bitfields onto such large
numbers in the first place ?</pre>
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