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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 - WebRTC miscompile after ARMCodeGenPrepare was enabled"
href="https://bugs.llvm.org/show_bug.cgi?id=39060">39060</a>
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<th>Summary</th>
<td>WebRTC miscompile after ARMCodeGenPrepare was enabled
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, sam.parker@arm.com
</td>
</tr></table>
<p>
<div>
<pre>Consider the following:
typedef unsigned short uint16_t;
uint16_t a = 0xFFFF;
uint16_t b = 0;
void f();
void g();
void test() {
if (a != static_cast<uint16_t>(b-1)) {
f(); // Shouldn't reach this point.
} else {
g();
}
}
With clang+llvm at r341931
$ clang -S -o - --target=arm-linux-androideabi -Oz /tmp/a.cc -mthumb
-march=armv7-a -mfloat-abi=softfp -mtune=generic-armv7-a -mfpu=neon
...
ldrh r0, [r0]
ldrh r1, [r1]
subs r0, #1 <--- subtract
uxth r0, r0 <--- truncate to 16-bit
cmp r1, r0 <--- compare
it eq
beq _Z1gv
b _Z1fv
...
At r341932:
...
ldrh r0, [r0]
ldrh r1, [r1]
subs r0, #1
cmp r1, r0
it eq
beq _Z1gv
b _Z1fv
...
The UXTH instruction has gone missing. The comparison will fail because it's
comparing 0xFFFFFFFF (32-bit -1) against 0xFFFF.
Chromium ran into this when running WebRTC tests on Android.</pre>
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