[cfe-dev] Status of IR vs. frontend PGO (fprofile-generate vs fprofile-instr-generate)

[Xinliang David Li via cfe-dev]

Sounds good to me.

David

On Mon, Jun 14, 2021 at 11:25 AM Reid Kleckner <rnk at google.com> wrote:

> Sure, there's no rush to deprecate frontend PGO. In the meantime, would it
> be OK to update the open source docs to recommend IR PGO over frontend PGO,
> without making any statement about deprecation? This is mainly to get any
> new PGO users onto what we think is currently the most well-lit path.
>
> On Mon, Jun 14, 2021 at 10:31 AM Alex Lorenz <aleksei_lorenz at apple.com>
> wrote:
>
>> Bumping up this thread. Based on the initial investigation, I think we
>> can switch to the IR PGO instead of the frontend PGO and so you’ll be able
>> to proceed with this deprecation of the frontend PGO. We would like to
>> request some additional time to do a full investigation and prepare for the
>> transition on our end though, ideally we would need about 3 - 6 months to
>> ensure we are prepared for that. Would you be willing to revisit this again
>> in the future once we’re ready for that?
>>
>> Thanks,
>> Alex
>>
>> On May 20, 2021, at 9:54 AM, Xinliang David Li <xinliangli at gmail.com>
>> wrote:
>>
>>
>>
>> On Thu, May 20, 2021 at 9:47 AM Reid Kleckner <rnk at google.com> wrote:
>>
>>> On Wed, May 19, 2021 at 7:48 PM Alex Lorenz <aleksei_lorenz at apple.com>
>>> wrote:
>>>
>>>> As far the possible deprecation of frontend PGO, will that imply that
>>>> the `-fprofile-instr-generate / use` options will get removed, or will they
>>>> still be supported but will leverage IR PGO instead?
>>>>
>>>
>>> That makes sense to me, but we need to untangle the fact that
>>> `-fprofile-instr-generate -fcoverage-mapping` is currently used for
>>> coverage, so a simple alias isn't quite correct.
>>>
>>> I've always wanted a single, high-level coverage flag, and I always
>>> thought it should be spelled --coverage of -fcoverage, but that seems like
>>> it's already taken by gcov instrumentation. =/ I guess we need to bikeshed
>>> a new spelling.
>>>
>>
>> Right.   -fcoverage-mapping itself does not much so it should
>> probably imply frontend instrumentation.
>>
>> For migration purposes, if -fcoverage-mapping is used together with
>> -fprofile-instr-generate (which becomes IR PGO), the latter will be dropped
>> (or a warning is given). The tricky part is if the user uses the option to
>> specify the profile path, then we have a problem.
>>
>> David
>>
>> David
>>
>>
>>
>>
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