[lldb-dev] Can't step over __sync_bool_compare_and_swap on ARM

[Mario Zechner]

Hi,

we are running into a problem with the __sync_bool_compare_and_swap
intrinsic on ARM. Trying to step over the call to
__sync_bool_compare_and_swap will put LLDB into a sort of infinite loop.

This is reproduceable in the latest XCode (6.1.1 (6A2008a)). Create an iOS
project with the following main function:

int main(int argc, char * argv[]) {
    int c = 0;
    while(1) {
        if(__sync_bool_compare_and_swap(&c, 0, 1)) {
            break;
        }
    }
    printf("%d\n", c);
}

The intrinsic compiles down to:

   0xb7010:  dmb    ish
   0xb7014:  movs   r0, #0x1
   0xb7016:  movs   r1, #0x0
   0xb7018:  add    r2, sp, #0x14
   0xb701a:  str    r0, [sp, #0x10]
   0xb701c:  str    r1, [sp, #0xc]
   0xb701e:  str    r2, [sp, #0x8]
-> 0xb7020:  ldr    r0, [sp, #0x8]
   0xb7022:  ldrex  r1, [r0]
   0xb7026:  ldr    r2, [sp, #0xc]
   0xb7028:  cmp    r1, r2
   0xb702a:  str    r1, [sp, #0x4]
   0xb702c:  bne    0xb703a                   ; main + 62 at main.m:15
   0xb702e:  ldr    r1, [sp, #0x10]
   0xb7030:  ldr    r2, [sp, #0x8]
   0xb7032:  strex  r0, r1, [r2]
   0xb7036:  cmp    r0, #0x0
   0xb7038:  bne    0xb7020                   ; main + 36 at main.m:15
   0xb703a:  dmb    ish

When stepping over, LLDB will first set a breakpoint on 0xb702c:  bne
 0xb703a. Next it executes a single step, moving PC to   0xb702e:  ldr
 r1, [sp, #0x10] as the condition is not meet.

LLDB then sets a breakpoint on the next branch instruction at 0xb7038:  bne
   0xb7020. It single steps the instruction, the condition is meet, and we
end up at 0xb7020 again.

The code never breaks out of this loop, LLDB will continue to set the
breakpoints indefinitely.

Any idea how to fix this?

Thanks,
Mario
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