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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Codegen for checked addition is particularly poor"
href="https://llvm.org/bugs/show_bug.cgi?id=27571">27571</a>
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<th>Summary</th>
<td>Codegen for checked addition is particularly poor
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>3.7
</td>
</tr>
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<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>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: ARM
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>dbau.pp@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following C code results in code with a pile of seemingly unnecessary
instructions:
int checked(int a, int b) {
int res;
if (__builtin_sadd_overflow(a, b, &res)) {
__builtin_abort();
}
return res;
}
With `clang-3.7 -O3 -target arm-unknown-linux-gnueabihf -S
-fomit-frame-pointer` (the last is to just remove irrelevant cruft, it doesn't
change the core codegen) this gives:
checked: @ @checked
.fnstart
@ BB#0:
mov r2, r0
add r0, r2, r1
mov r1, #1
cmp r0, r2
movwvc r1, #0
cmp r1, #0
bxeq lr
mov lr, pc
b abort
The sequence `mov r1, #1` through `cmp r1 #0` seems particularly pointless: I
believe can be replaced by two instructions:
cmp r0, r2
bxvc lr
(I haven't found an arm variant with good codegen---v7 and v5 etc. are all
similar, and v6m is even worse!)
This seems to be particular to 32-bit ARM: x86 (64- and 32-bit) and AArch64
seem far better.</pre>
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