[LLVMdev] __sync_add_and_fetch in objc block for global variable on ARM

[Alexey Perevalov]

Hi Tim

----------------------------------------
> Date: Wed, 8 Apr 2015 06:53:44 -0700
> Subject: Re: [LLVMdev] __sync_add_and_fetch in objc block for global variable on ARM
> From: t.p.northover at gmail.com
> To: alexey.perevalov at hotmail.com
> CC: llvmdev at cs.uiuc.edu
>
>> in disas I see dmb ish instruction, but I don't know is it enough.
>
> There should be 2 dmb instructions: one before the ldrex/strex loop
> and one after. But I wouldn't expect dropping one to actually cause a
> problem in the code you posted.
Yes, there are two dmb's
=> 0x00008ed8 <+224>:    dmb    ish
   0x00008edc <+228>:    movw    r1, #10800    ; 0x2a30
   0x00008ee0 <+232>:    movt    r1, #1
   0x00008ee4 <+236>:    str    r0, [sp, #44]    ; 0x2c
   0x00008ee8 <+240>:    str    r1, [sp, #40]    ; 0x28
   0x00008eec <+244>:    ldr    r0, [sp, #40]    ; 0x28
   0x00008ef0 <+248>:    ldrexb    r1, [r0]
   0x00008ef4 <+252>:    ldr    r2, [sp, #44]    ; 0x2c
   0x00008ef8 <+256>:    add    r3, r1, r2
   0x00008efc <+260>:    strexb    r12, r3, [r0]
   0x00008f00 <+264>:    cmp    r12, #0
   0x00008f04 <+268>:    str    r1, [sp, #36]    ; 0x24
---Type <return> to continue, or q <return> to quit---
   0x00008f08 <+272>:    bne    0x8eec <__main_block_invoke+244>
   0x00008f0c <+276>:    ldr    r0, [sp, #36]    ; 0x24
   0x00008f10 <+280>:    add    r0, r0, #1
   0x00008f14 <+284>:    dmb    ish
   0x00008f18 <+288>:    ldr    r1, [sp, #40]    ; 0x28
   0x00008f1c <+292>:    strb    r0, [r1]
   0x00008f20 <+296>:    bl    0x8aa0 <pthread_self>

>
> In what way is "count" corrupted, and how do you observe it? What
> assembly is actually produced for the block?
The assembly for whole block is huge even for minimal test case. I attached source code.
I used cocotron derived framework, but due canaries was alive I don't think it's due runtime.
If you undef REPRODUCE_CASE in example, it will not reproduce, I think it's because of introducing
additional time interval.
The output of sample is following (when it reproduced):
after -1316199408 count addr 0x12a30, value 1
canary1 77, canary2 88
after -1324588016 count addr 0x12a30, value 2
canary1 77, canary2 88
after -1324588016 count addr 0x12a30, value 3
canary1 77, canary2 88
after -1324588016 count addr 0x12a30, value 33
canary1 77, canary2 88
after -1324588016 count addr 0x12a30, value 34

>
>> I understand, my clang is out of date. Moving to new version could be painful )
>> Maybe somebody knows, was that bug fixed?
>
> That area's certainly improved, but I'm not aware of any bugs on that
> scale ("can't dispatch 32 threads to atomically increment a single
> variable and print it") in clang 3.3 so I think something else is
> probably going on.
>
> A self-contained, minimal example we can examine would be useful.
>
> Cheers.
>
> Tim.
 		 	   		  
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