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

[Todd Fiala]

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
>
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>


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