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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 --- - DAGCombiner missing byte swap idioms" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23863&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=MWNH6MHcHqFMogFflNkReihRJff1MEVVFBdLVjJBEPs&s=CtcQ3fKkZnkIjdx07BGN0dCL2WPeAOab9CF3txgs7XQ&e=">23863</a>
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<th>Summary</th>
<td>DAGCombiner missing byte swap idioms
</td>
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<th>Product</th>
<td>libraries
</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
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>charlesturner7c5@gmail.com
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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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<p>
<div>
<pre>LLVM is missing an opportunity to reduce the following byte-swap routine into
(when available) target-specific byte swap routines:
uint32_t bswap(uint32_t x)
{
x = (x & 0x0000FFFF) << 16 | (x & 0xFFFF0000) >> 16;
x = (x & 0x00FF00FF) << 8 | (x & 0xFF00FF00) >> 8;
return x;
}
Also noticed this one mentioned in lib/Target/README.txt that I'll add here,
unsigned long reverse(unsigned v) {
unsigned t;
t = v ^ ((v << 16) | (v >> 16));
t &= ~0xff0000;
v = (v << 24) | (v >> 8);
return v ^ (t >> 8);
}
DAGCombiner should be taught about the first one at least, since GCC does
recognise that idiom. Whether LLVM should ever recognise the second one is left
to taste.</pre>
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</p>
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