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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] Useless register shuffling with tail-call"
href="https://llvm.org/bugs/show_bug.cgi?id=23085">23085</a>
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<th>Summary</th>
<td>[AArch64] Useless register shuffling with tail-call
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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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>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>
<tr>
<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=14124" name="attach_14124" title="testcase">attachment 14124</a> <a href="attachment.cgi?id=14124&action=edit" title="testcase">[details]</a></span>
testcase
For the attached testcase, LLVM produces:
clang --target aarch64 tail-call-allocation.c -O2 -S
foo:
mov w8, w0 /* Poor register choice. */
movz w9, #0x4e20
cmp w8, w9
b.le .LBB0_2
mov w0, w8
ret
.LBB0_2:
mov w0, wzr
add w1, w8, w8, lsl #6
b bar
GCC produces:
foo:
mov w2, 20000
cmp w0, w2
ble .L4
ret
.p2align 3
.L4:
add w1, w0, w0, lsl 6
mov w0, 0
b bar
In the LLVM case, the scheduling of:
mov w0, wzr
add w1, w8, w8, lsl #6
prevents better register allocation as the live range for w0 as an outgoing
argument overlaps with that of w0 as an incoming argument.</pre>
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