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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 --- - [AArch64] unnecessary cmp instruction with subtract and conditional select"
href="https://llvm.org/bugs/show_bug.cgi?id=31175">31175</a>
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<th>Summary</th>
<td>[AArch64] unnecessary cmp instruction with subtract and conditional select
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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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
</td>
</tr>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>; ret = (x-y) > 0 ? x-y : 0
define i32 @max(i32 %x, i32 %y) {
%sub = sub nsw i32 %x, %y
%cmp = icmp sgt i32 %sub, 0
%sel = select i1 %cmp, i32 %sub, i32 0
ret i32 %sel
}
Using AArch64 as the target because that shows the problem, but this is likely
a bug for any target that has a subtract that sets comparison flags:
$ ./llc -o - max.ll -mtriple=aarch64
sub w8, w0, w1 <--- could have used 'subs'
cmp w8, #0 <--- and eliminated this cmp
csel w0, w8, wzr, gt
ret
This IR equivalent shows the better codegen:
define i32 @max_better(i32 %x, i32 %y) {
%sub = sub nsw i32 %x, %y
%cmp = icmp sgt i32 %x, %y
%sel = select i1 %cmp, i32 %sub, i32 0
ret i32 %sel
}
$ ./llc -o - max.ll -mtriple=aarch64
subs w8, w0, w1
csel w0, w8, wzr, gt
ret</pre>
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