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

[Mario Zechner]

Using the OS atomic cmp/swp works (step over/into does not go into infinite
loop):

#include <libkern/OSAtomic.h>

int main(int argc, char * argv[]) {
    int c = 0;
    while(1) {
        if(OSAtomicCompareAndSwap32(0, 1, &c)) {
            break;
        }
    }
    printf("%d\n", c);
}

This means we have a fix for our use case, but i assume that others may use
the intrinsic and be surprised at LLDB's behaviour.

On Thu, Dec 11, 2014 at 10:38 AM, Mario Zechner <badlogicgames at gmail.com>
wrote:
>
> 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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