Re: Question on patch -fprofile-partial-training

["Martin Liška" <mliska@suse.cz>]

On 5/4/23 14:54, Qing Zhao wrote:
> 
> 
>> On May 4, 2023, at 4:30 AM, Martin Liška <mliska@suse.cz> wrote:
>>
>> On 5/3/23 21:10, Qing Zhao via Gcc-patches wrote:
>>> Hi, Jan,
>>>
>>> You added the following patch into gcc10:
>>>
>>> From 34fbe3f0946f88828765184ed6581bda62cdf49f Mon Sep 17 00:00:00 2001
>>> From: Jan Hubicka <hubicka@ucw.cz>
>>> Date: Thu, 5 Dec 2019 19:12:51 +0100
>>> Subject: [PATCH] cgraphclones.c (localize_profile): New function.
>>>
>>>        * cgraphclones.c (localize_profile): New function.
>>>        (cgraph_node::create_clone): Use it for partial profiles.
>>>        * common.opt (fprofile-partial-training): New flag.
>>>        * doc/invoke.texi (-fprofile-partial-training): Document.
>>>        * ipa-cp.c (update_profiling_info): For partial profiles do not
>>>        set function profile to zero.
>>>        * profile.c (compute_branch_probabilities): With partial profile
>>>        watch if edge count is zero and turn all probabilities to guessed.
>>>        (compute_branch_probabilities): For partial profiles do not apply
>>>        profile when entry count is zero.
>>>        * tree-profile.c (tree_profiling): Only do value_profile_transformations
>>>        when profile is read.
>>>
>>> My question is:
>>
>> Hello.
>>
>> Why would anybody backport such change to unsupported code-stream of GCC 8?
>> Generally speaking, I discourage from doing that.
> 
> Yes, I agree.
> However, many users still use GCC8 right now, and some of them are asking for more performance
> from PGO recently. That’s the reason I am studying this right now. 

I understand there are products that are based on GCC8, but as the branch is officially unsupported, I don't
see a reason to backport a new feature from newer release. It's just asking for troubles. If your clients are
interested in more performance, then they should use a recent supported release.

> 
> From my understanding, -fprofile-partial-training is one important option for PGO performance.

I don't think so, speed benefit would be rather small I guess.

> I’d like
> to see any big technique difficult to prevent it from being back ported to GCC8. 

There might be of course some patch dependencies and I don't see a point why should we waste
time with that.

Cheers,
Martin

> 
> Thanks.
> 
> Qing
> 
>>
>> Martin
>>
>>>
>>> Can this patch be back ported to GCC8 easily? I am wondering any significant
>>> Change between GCC8 and GCC10 that might make the backporting very hard> 
>>> Thanks a lot for your help.
>>>
>>> Qing
>