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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - requesting optimization of safe rotate functions"
href="http://llvm.org/bugs/show_bug.cgi?id=17904">17904</a>
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<th>Summary</th>
<td>requesting optimization of safe rotate functions
</td>
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<th>Product</th>
<td>new-bugs
</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
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>regehr@cs.utah.edu
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>This is the obvious rotate idiom in C (this one is from Nettle):
#define ROTL32(n,x) (((x)<<(n)) | ((x)>>(32-(n))))
LLVM does an admirable job of recognizing the code and turning it into a rotate
instruction, when this is available.
The problem is that this code executes undefined behavior when n==0 or n==32.
Most crypto libraries are careful not to rotate by 32, but out of 10 libraries
that I examined, 5 execute undefined behavior when rotating by zero.
We can make the obvious modification to protect against undefined rotate by 0:
#define ROTL32(n,x) ((n)==0?(x):(((x)<<(n)) | ((x)>>(32-(n)))))
Notice that this can be turned into exactly the same object code as the earlier
macro since the rotate-by-0 case is already handled by the rotate instruction.
However, this isn't what we get out of the compiler:
rotl32c:
testl %esi, %esi
je .LBB2_2
movb %sil, %cl
roll %cl, %edi
.LBB2_2:
movl %edi, %eax
ret
I'm in the process or trying to convince crypto library maintainers to fix
their rotate functions and macros, and this will be easier if the fix doesn't
have a performance penalty.
So would it be possible for you folks to teach the compiler to emit the better
code for the safe rotate idiom?</pre>
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