[PATCH] D20993: [lit] Add support for PGO profile and code coverage collection

[Sean Silva via llvm-commits]

On Fri, Jun 3, 2016 at 9:47 PM, Xinliang David Li via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

>
>
> On Fri, Jun 3, 2016 at 9:37 PM, Matthias Braun <matze at braunis.de> wrote:
>
>>
>> On Jun 3, 2016, at 9:34 PM, Xinliang David Li via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>>
>>
>>
>> On Fri, Jun 3, 2016 at 9:32 PM, Matthias Braun <matze at braunis.de> wrote:
>>
>>> MatzeB added a comment.
>>>
>>> In http://reviews.llvm.org/D20993#449116, @vsk wrote:
>>>
>>> > In http://reviews.llvm.org/D20993#449092, @MatzeB wrote:
>>> >
>>> > > This adds a bigger bunch of code to lit, I wonder if it is necessary:
>>> > >
>>> > > - I assume the necessity for the logic inside lit is just the fact
>>> that the profile data overwrites each other after running a command. Is
>>> setting LLVM_PROFILE_FILE with a '%p' placeholder not enough to avoid this?
>>> >
>>> >
>>> > Using PID substitution gets close to solving the problem of having
>>> overwritten profiles, but not all the way. On 32-bit systems PID wraparound
>>> would pose a real problem. In this patch I include the hash of the test
>>> command to minimize loss of profiles.
>>>
>>>
>>> This is a really annoying problem to have... Maybe we should find a
>>> solution within the profile infrastructure itself (can we add another flag
>>> that adds a unique suffix to the filename if it already exists?) so not
>>> every user of the profiling infrastructure has to jump through the same
>>> hoops (I've done a similar dance in the test-suite profile support).
>>>
>>> >
>>>
>>> >
>>>
>>> > > The final profile data merging can always be done outside of
>>> llvm-lit afterwards.
>>>
>>> >
>>>
>>> >
>>>
>>> > On a practical level, I don't think this is possible. Raw profiles are
>>> too large. Turning off the cleanup step and running check-llvm produces
>>> over half a terrabyte of data. There are a few ways to address this without
>>> touching lit:
>>>
>>> >
>>>
>>> > 1. Reduce the size of raw profiles. This would make the compiler
>>> runtime larger and more complex.
>>>
>>> > 2. Run a monitor process that does the merging/cleanup. I don't think
>>> that approach has any advantages compared to modifying lit.
>>>
>>> >
>>>
>>> >   Alternatively, we could do in-place raw profile merging in the
>>> compiler runtime. I don't think that's preferable because we'd have to
>>> introduce a lot of complexity to compiler-rt (e.g portable mandatory file
>>> locking).
>>>
>>>
>>> Ah, so that seems harder to solve without an external driver merging the
>>> profile data. I'm still not a big fan of injecting this stuff into the core
>>> of lit, but we may have no other choice today.
>>>
>>
>> This is handled by the compiler-rt -- it should be totally transparent to
>> the user.
>>
>>
>> That would indeed be a big usability improvement. Does it also free you
>> from running an extra tool afterwards to convert from .profraw to .profdata
>> format?
>>
>
> The merging is only for raw format profiles which will be  super fast.
> The conversion from raw to index format is still needed -- but the overhead
> of that is very low because thousands of raw profiles have already being
> reduced to a single raw file (per-executable).
>
> If we really want to simplify the use model, the compiler can be taught to
> recognize raw profile (and convert it to indexed format on the fly) without
> user knowing yet. I am yet to see a good justification for this support.
>

It also sounds like it would have a scaling problem. It would lead to
O(profile size) work done by each compiler invocation, instead of O(data
needed by the compiler). Since O(profile size) is generally O(number
compiler invocations), this would be quadratic in the project size.

-- Sean Silva


>
> thanks,
>
> David
>
>
>>
>> - Matthias
>>
>
>
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