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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Sub-optimal placement of loop exit blocks"
href="https://bugs.llvm.org/show_bug.cgi?id=51185">51185</a>
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<tr>
<th>Summary</th>
<td>Sub-optimal placement of loop exit blocks
</td>
</tr>
<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>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>momchil.velikov@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=25057" name="attach_25057" title="test case">attachment 25057</a> <a href="attachment.cgi?id=25057&action=edit" title="test case">[details]</a></span>
test case
In the attached test case there's is a (fully unrolled) loop roughly of the
following form
for ... {
if (cond) {
stuff();
break;
}
more_stuff();
}
when compiled for AArch64 with `llc store.ll`, the output looks like:
// iteration 0
cmp w11, w12
b.ne .LBB0_2
mov x11, xzr
b .LBB0_14
.LBB0_2:
...
// iteration 1
cmp w17, w12
b.ne .LBB0_6
add w10, w10, #1
mov w11, #1
b .LBB0_14
.LBB0_6:
...
// iteration 2
cmp w15, w12
b.ne .LBB0_10
add w10, w10, #2
mov w11, #2
b .LBB0_14
.LBB0_10:
...
// iteration 3
cmp w16, w12
b.ne .LBB0_15
add w10, w10, #3
mov w11, #3
// fallthrough
.LBB0_14:
// iterations 0, 1, 2, 3 early exit
...
i.e. the exit blocks are interspersed with "proper" body blocks and the
execution constantly jumps over them. In contrast, GCC's output looks like:
// iteration 0
...
cmp w6, w0
beq .L19
...
// iteration 1
...
cmp w2, w6
beq .L20
...
// iteration 2
...
cmp w6, w4
beq .L22
...
// iteration 3
...
cmp w2, w6
beq .L23
i.e. exit blocks an entirely stashed away and loop iterations are executed
essentially like straight line code.</pre>
</div>
</p>
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