[Lldb-commits] Possible UnwindAssembly-x86 Problem & Fix

[Todd Fiala]

:-)


On Thu, Jul 24, 2014 at 5:34 PM, Tong Shen <endlessroad at google.com> wrote:

> Thanks Todd!
>
>
> On Thu, Jul 24, 2014 at 5:33 PM, Todd Fiala <tfiala at google.com> wrote:
>
>> Hey Tong - I'll add that and do all the test runs.  I'll get it checked
>> in unless something breaks.
>>
>> -Todd
>>
>>
>> On Thu, Jul 24, 2014 at 5:32 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>>
>>> Hi Jason,
>>>
>>> I don't have any strong preference here, so let's do it the way you see
>>> fit :-)
>>> Your patch works fine, though there's a right bracket missing at the end
>>> of line:
>>>  regloc.SetAtCFAPlusOffset (-(stack_offset + row->GetCFAOffset());
>>>
>>> Thank you for looking into this!
>>>
>>>
>>> On Thu, Jul 24, 2014 at 5:16 PM, Jason Molenda <jmolenda at apple.com>
>>> wrote:
>>>
>>>> Good catch Tong.  I don't think clang generates this instruction
>>>> pattern (it normally pushq's all the registers in the prologue that it
>>>> wants to preserve) so we've never hit this bug.
>>>>
>>>> I don't have a strong preference about mov_reg_to_local_stack_frame_p()
>>>> returning a positive value in rbp_offset except that we have other sections
>>>> returning an offset from the CFA e.g. UnwindPlan::Row::GetCFAOffset() which
>>>> return a positive offset value that must be subtracted from the CFA to get
>>>> the correct address.
>>>>
>>>> If you would prefer to change mov_reg_to_local_stack_frame_p() to
>>>> return a negative value, I'm not going to argue against it.  But maybe a
>>>> slight change to your patch like this?  It's complicated enough code that I
>>>> added a couple of comments while I was at it.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> > On Jul 24, 2014, at 3:33 PM, Tong Shen <endlessroad at google.com>
>>>> wrote:
>>>> >
>>>> > Hi jmolenda, lldb-commits,
>>>> >
>>>> > While hacking around x86 assembly profiler, I found a problem about
>>>> non-volatile register information.
>>>> >
>>>> > At UnwindAssembly-x86.cpp line 630 (ViewVC link), stack_offset is
>>>> calculated but not used.
>>>> > I think it should be used in line 636:
>>>> >         regloc.SetAtCFAPlusOffset (-row->GetCFAOffset() +
>>>> stack_offset);
>>>> > instead of what's there now:
>>>> >         regloc.SetAtCFAPlusOffset (-row->GetCFAOffset());
>>>> >
>>>> > Also, in line 417 of the same file, when calculating stack_offset,
>>>> why is rbp_offset set to abs(offset)?
>>>> >
>>>> > For testing, I wrote an assembly (test.S in attachment) to test if
>>>> lldb can recover non-volatile registers.
>>>> > You can put a breakpoint after where asm_frame() overrides %rbx, then
>>>> do "f 1" & "register read" to see if %rbx is correctly restored.
>>>> > In my test, unmodified lldb gives wrong %rbx.
>>>> > Attached patch solved this problem, and make lldb recover those
>>>> registers correctly.
>>>> >
>>>> > Am I missing anything here? Is the original behavior intentional?
>>>> Please correct me if I'm wrong :-)
>>>> >
>>>> > Thank you.
>>>> >
>>>> > --
>>>> > Best Regards, Tong Shen
>>>> > <test.S><1.patch>
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Best Regards, Tong Shen
>>>
>>> _______________________________________________
>>> lldb-commits mailing list
>>> lldb-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>>>
>>>
>>
>>
>> --
>> Todd Fiala | Software Engineer |  tfiala at google.com |  650-943-3180
>>
>
>
>
> --
> Best Regards, Tong Shen
>



-- 
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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