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

[Greg Clayton]

If we are single stepping then we set the BVR/BCR regs to say "stop when the PC is not equal to its current value". If we are settings breakpoints, we just write a trap into memory and continue. Jim pointed out we are setting a breakpoint after the atomic instruction and not on it, so I don't know how this would affect things...


> On Dec 11, 2014, at 2:21 PM, Mario Zechner <badlogicgames at gmail.com> wrote:
> 
> 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
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>