https://bugs.llvm.org/show_bug.cgi?id=50785
Bug ID: 50785
Summary: clang incorrectly produces ARM STM instead of STR in
thumb mode
Product: clang
Version: 12.0
Hardware: PC
OS: Linux
Status: NEW
Severity: release blocker
Priority: P
Component: C
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
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:
https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/jchuff.h#L84
(called by
https://github.com/libjpeg-turbo/libjpeg-turbo/blob/1a1fb615db39880044b789bdb36b351865d9ec4a/simd/arm/aarch32/jchuff-neon.c#L295)
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.
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