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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] Failure to canonicalize compare results in reloading of constants"
href="https://llvm.org/bugs/show_bug.cgi?id=23084">23084</a>
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<th>Summary</th>
<td>[AArch64] Failure to canonicalize compare results in reloading of constants
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>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>Backend: AArch64
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>james.greenhalgh@arm.com
</td>
</tr>
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<th>CC</th>
<td>james.molloy@arm.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=14123" name="attach_14123" title="Testcase">attachment 14123</a> <a href="attachment.cgi?id=14123&action=edit" title="Testcase">[details]</a></span>
Testcase
In the attached testcase, after tail-call inlining, two comparisons of x
against 20,000 are generated. The compiler does not pick a canonical way to do
this comparison, and ends up checking (x > 20000) for the first comparison and
(x < 20001) for the second. This results in a useless load of the constant
20001.
clang --target aarch64 canonicalize-compare.c -O2 -S
foo:
movz w8, #0x4e20 /* !!!! */
cmp w0, w8
b.gt .LBB0_3
movz w8, #0x4e21 /* !!!! */
.LBB0_2:
add w9, w0, w0, lsl #6
neg w0, w9
cmp w0, w8
b.lt .LBB0_2 /* Could be B.LE and reuse the first constant */
.LBB0_3:
ret
GCC generates:
foo:
mov w1, 20000
cmp w0, w1
bgt .L2
.L3:
add w0, w0, w0, lsl 6
neg w0, w0
cmp w0, w1
ble .L3
.L2:
ret
Which looks neater.</pre>
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