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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