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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 - clang incorrectly produces ARM STM instead of STR in thumb mode"
href="https://bugs.llvm.org/show_bug.cgi?id=50785">50785</a>
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<th>Summary</th>
<td>clang incorrectly produces ARM STM instead of STR in thumb mode
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>12.0
</td>
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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>release blocker
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>matthieu.bouron@gmail.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>When compiling to ARM (32-bit) thumb mode, clang incorrectly produces stm
instead of str in some cases. This is causing issues (sigbus: illegal
alignment) if the destination pointer is not aligned.
This can be reproduced with the following minimal C code:
#include <stdint.h>
uint8_t *f(uint8_t *buf)
{
*(uint32_t*)buf = 0;
return buf + 4;
}
which, when compiled with -O2 -mthumb, produces the following ARM ASM:
f:
movs r1, #0
stm r0!, {r1}
bx lr
whereas GCC (or clang without -mthumb) produces:
f:
movs r3, #0
str r3, [r0], #4
bx lr
This happens on clang 9.x, 10.x, 11.x and 12.x as well as the versions provided
by the Android NDK (r21e, r22b).
This issue was found when debugging a crash occurring in libjpeg-turbo 2.1.0
when compiled to ARM (32-bit) thumb node / Android. The crash happens at:
<a href="https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/jchuff.h#L84">https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/jchuff.h#L84</a>
(called by
<a href="https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/aarch32/jchuff-neon.c#L295">https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/aarch32/jchuff-neon.c#L295</a>)
where:
*((uint32_t *)buffer) = BUILTIN_BSWAP32(put_buffer);
buffer += 4;
gets compiled to rev + stm instead of rev + str, causing a sigbus error
(illegal alignment) because buffer is not necessarily aligned.</pre>
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