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Re: GSoC topic: Implement hot cold splitting at GIMPLE IR level

From: Martin Liška <mliska at suse dot cz>
To: Aditya K <hiraditya at msn dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
Cc: Jan Hubicka <hubicka at ucw dot cz>
Date: Tue, 3 Mar 2020 09:47:43 +0100
Subject: Re: GSoC topic: Implement hot cold splitting at GIMPLE IR level
References: <BYAPR08MB4232069B0A9D5D1E0E4B2A69B6E40@BYAPR08MB4232.namprd08.prod.outlook.com>

Hello.

Thank you for idea. I would like to provide some comments about what GCC can currently
do and I'm curious we need something extra on top of what we do.
Right now, GCC can do hot/cold partitioning based on functions and basic blocks. With
a PGO profile, the optimization is quite aggressive and can save quite some code
being placed into a cold partitioning and being optimized for size. Without a profile,
we do a static profile guess (predict.c), where we also propagate information about cold
blocks (determine_unlikely_bbs). Later in RTL, we utilize the information and make
the real reordering (bb-reorder.c).

Martin

References:
- GSoC topic: Implement hot cold splitting at GIMPLE IR level
  - From: Aditya K

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