[PATCH] D9009: Value profiling compiler-rt changes

[Betul Buyukkurt via llvm-commits]

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> On Thu, Nov 12, 2015 at 10:12 AM, Betul Buyukkurt <betulb at codeaurora.org
> <mailto:betulb at codeaurora.org> > wrote:
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>> davidxl added inline comments.
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>> ================
>> Comment at: lib/profile/InstrProfiling.c:190
>> @@ +189,3 @@
>> +    }
>> +    I->ValueCounters = (void *)PerSiteCountsHead;
>> +    PerSiteCountsHead = (uint8_t *)VDataPtr;
>> ----------------
>> Release memory of ValueCounters ?
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> This is a function that's running only atexit, I thought of adding it in
> but then thought of it as unnecessary too due to this being one of the
> very last actions of the run time during the lifetime of the program.
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> Ok to leave it as it is, but the assumption may not be true in the future.
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> Consider the case when profile dump is not done at exit (e.g. for a server
> program). APIs are provided for the user to periodically dump data for
> selected program phases they care about. One low overhead way of doing
> this
> is let the program continuously do the profiling without the need to check
> the state. APIs are provided so that user can do counter clearing and
> dumping at the start and end of the phase ...
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> Another thing we need to solve in the future is support raw profile data
> on-line merging -- otherwise the raw profile data can get really big with
> the current appending model. Without this, doing clang bootstrap with PGO
> (4
> stage with clang/llvm sources as training data) will be impossible.
>
> VP data certainly makes it trickier.

The reason we're updating the ValueCounters pointer in the
__llvm_profile_data is solely for code uniformity in accessing the members
of the __llvm_profile_data. Yet it's not as conforming to the way the
NamePtr or the CounterPtr members of the struct are processed. To support
the cases you mention above, one needs to introduce also a lock to be able
to write out the data and prevent the value profiling from proceeding
during the data write out. I think all of this can be avoided by not
overwriting the ValueCounters pointer.

-Betul

> David
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>> http://reviews.llvm.org/D9009
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