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

[Mario Zechner]

Using "slow-stepping" doesn't fix the issue. It appears that the debugger
is setting some flags when hitting the breakpoint and stepping which
prohibit the intrinsic to evaluate the conditionals properly.

On Thu, Dec 11, 2014 at 8:25 PM, Greg Clayton <gclayton at apple.com> wrote:
>
> Does this work if you first type:
>
> (lldb) setting set target.use-fast-stepping false
>
> Then step?
>
>
> > On Dec 11, 2014, at 1:38 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
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
>
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