GNU Arm Embedded Toolchain

  1. Bug #1738730
  2. Comment #17

Comment 17 for bug 1738730

Revision history for this message
john (jkovach) wrote : Re: Simple OR'ed assignment breaks the code silently

Just tested with the latest release. GCC: (GNU Tools for Arm Embedded Processors 7-2017-q4-major) 7.2.1 20170904 (release) [ARM/embedded-7-branch revision 255204

gcc -mcpu=cortex-m3 -mthumb -O0 -munaligned-access -S test.c -o test.s

test.c:

#pragma pack(1)
struct s { short i; } volatile *p;
void f(void) { p->i = 0; }

output:

ldr r3, .L2
ldr r3, [r3]
ldrb r2, [r3]
movs r2, #0
strb r2, [r3]
ldrb r2, [r3, #1]
movs r2, #0
strb r2, [r3, #1]

There two problems:
1) Byte-wise access. One would expect "-munaligned-access" to help avoid this.
2) The variable is read before it is written for no apparent reason. This is in violation of the volatile semantics, as far as I can see. And this problem goes away when I remove "volatile" from source.