[LLVMdev] Recent changes in -gmlt break sample profiling

[Eric Christopher]

On Sun Oct 26 2014 at 5:18:53 PM Xinliang David Li <xinliangli at gmail.com>
wrote:

> On Sun, Oct 26, 2014 at 4:59 PM, Eric Christopher <echristo at gmail.com>
> wrote:
>
>> On Fri, Oct 24, 2014 at 3:27 PM, Diego Novillo <dnovillo at google.com>
>> wrote:
>> >
>> >
>> > On Fri Oct 24 2014 at 6:21:14 PM David Blaikie <dblaikie at gmail.com>
>> wrote:
>> >>
>> >> On Fri, Oct 24, 2014 at 3:16 PM, Diego Novillo <dnovillo at google.com>
>> >> wrote:
>> >>>
>> >>>
>> >>>
>> >>> On Fri Oct 24 2014 at 6:11:21 PM David Blaikie <dblaikie at gmail.com>
>> >>> wrote:
>> >>>>
>> >>>> On Fri, Oct 24, 2014 at 2:48 PM, Diego Novillo <dnovillo at google.com>
>> >>>> wrote:
>> >>>>>
>> >>>>> I'm not sure if this was intended, but it's going to be a problem
>> for
>> >>>>> sample profiles.
>> >>>>>
>> >>>>> When we compile with -gmlt, the profiler expects to find the line
>> >>>>> number for all the function headers so that it can compute relative
>> line
>> >>>>> locations for the profile.
>> >>>>>
>> >>>>> The tool that reads the ELF binary is not finding them, so it writes
>> >>>>> out absolute line numbers, which are impossible to match during the
>> >>>>> profile-use phase.
>> >>>>>
>> >>>>> The problem seems to be that we are missing DW_TAG_subprogram for
>> all
>> >>>>> the functions in the file.
>> >>>>>
>> >>>>> Attached are the dwarf dumps of the same program. One compiled with
>> my
>> >>>>> system's clang 3.4 and the other with today's trunk. In both
>> compiles, I
>> >>>>> used -gline-tables-only.
>> >>>>>
>> >>>>> The trunk version is missing all the subprogram tags for the
>> functions
>> >>>>> in the file. This breaks the sample profiler.
>> >>>>>
>> >>>>> Should I file a bug, or is -gmlt going to be like this from now on?
>> The
>> >>>>> latter would be a problem for us.
>> >>>>
>> >>>>
>> >>>> Open to negotiation, but this change is intentional ( for details,
>> see
>> >>>> the commit: http://llvm.org/viewvc/llvm-project?rev=218129&view=rev
>> ).
>> >>>
>> >>>
>> >>> Well, this breaks us. Very hard. It absolutely blocks us from using
>> >>> SamplePGO with LLVM.
>> >>
>> >>
>> >> Sorry about that - I knew ASan needed gmlt data of a certain form and
>> >> worked carefully to ensure llvm-symbolizer could still symbolize with
>> these
>> >> changes, but wasn't aware of this particular dependence from PGO (just
>> >> assumed it used the line table directly).
>> >
>> >
>> > It does, but it uses relative line numbers. That's why it needs the
>> source
>> > locs for the function headers.
>> >
>> >>
>> >>
>> >>>
>> >>> The alternative would be to make the compiler use absolute line
>> numbers,
>> >>> but in the experience we've collected with GCC, this makes the
>> profiles very
>> >>> brittle wrt source changes.
>> >>
>> >>
>> >> It'd be interesting to see the data - I guess you throw out profiles
>> that
>> >> don't match on a per-function basis? So adding a few lines to one
>> function
>> >> will invalidate the profile for that function, but not all the
>> subsequent
>> >> ones in the file?
>> >
>> >
>> > Right. Dehao started using absolute numbers, but then moved to relative
>> > numbers when he saw that the performance drops were fairly pronounced
>> as the
>> > profile aged. I don't recall the exact drops he noticed.
>> >
>> >>
>> >>
>> >>>
>> >>> I don't have a better idea atm. Would there be any other way to
>> generate
>> >>> relative line numbers?
>> >>
>> >>
>> >> Possibly.
>> >>
>> >>>
>> >>>   Perhaps we could use the first line number we find with samples?
>> >>
>> >>
>> >> Probably - I guess you use the ELF symbol table to compute the address
>> >> range of each function? Given that, you could find the smallest line
>> number
>> >> in that address range in the line table, and make everything else
>> relative
>> >> to that.
>> >
>> >
>> > That could probably work, but I'm not sure how the ELF reading code in
>> the
>> > conversion tool does this calculation. I'll check it out.
>> >
>> >>
>> >>
>> >>>
>> >>> The problem here is that this line will shift, depending on how the
>> >>> profile was obtained.
>> >>
>> >>
>> >> Not sure what you're referring to here ^
>> >
>> >
>> > Say the function starts at line 20. If in one profile run, we get
>> samples at
>> > line 20 and line 23, then we'll compute the relative locations as 0 and
>> 3.
>> > But if the first sample you get is at line 21, then you'll compute the
>> > relative locations as  0 and 2.
>> >
>> > Using the address ranges in the line table may be the way to go. I'll
>> look
>> > at this next week.
>> >
>>
>> I'm nearly certain this is the way to go here.
>>
>
> How does it solve the problem?
>
>
You can get the address of the function from the symbol table and the
starting line from the line table with that address and then use the line
table normally for lookups.

-eric


> David
>
>
>>
>> -eric
>>
> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20141027/87149444/attachment.html>