Re: Question on patch -fprofile-partial-training

[Qing Zhao <qing.zhao@oracle.com>]

> 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. 

From my understanding, -fprofile-partial-training is one important option for PGO performance. I’d like
to see any big technique difficult to prevent it from being back ported to GCC8. 

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