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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [PowerPC] icmp ne 0 can be implemented using the carry bit"
href="https://bugs.llvm.org/show_bug.cgi?id=32520">32520</a>
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<th>Summary</th>
<td>[PowerPC] icmp ne 0 can be implemented using the carry bit
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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>All
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<th>Status</th>
<td>NEW
</td>
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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>Backend: PowerPC
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>I noticed this opportunity in <a href="https://reviews.llvm.org/D31483">https://reviews.llvm.org/D31483</a>.
This is from "Optimal Code Sequences" P. 200 of the Compiler Writer's Guide:
ne: not equal to r = v0 != v1;
subf R5,R3,R4
addic R6,R5,-1
subfe R7,R6,R5
define zeroext i1 @ne0(i32 %x) {
%cmp = icmp ne i32 %x, 0
ret i1 %cmp
}
Currently (r299396), the PPC backend seems to choose a cntlzw variant for all
compares against zero:
$ ./llc ne0.ll -o - -mtriple=powerpc64
cntlzw 3, 3
nor 3, 3, 3 <-- extended mnemonic for "not" would be nicer!
rlwinm 3, 3, 27, 31, 31
I don't know anything about recent PPC uarch, so this may not be faster, but it
is smaller:
addic 4, 3, -1
subfe 3, 4, 3</pre>
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