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[Bug ld/26766] Support standalone PGO build


From: mliska at suse dot cz
Subject: [Bug ld/26766] Support standalone PGO build
Date: Fri, 23 Oct 2020 13:31:39 +0000

https://sourceware.org/bugzilla/show_bug.cgi?id=26766

--- Comment #4 from Martin Liska <mliska at suse dot cz> ---
> Please take a look at users/hjl/pgo-lto branch
> 
> https://gitlab.com/x86-binutils/binutils-gdb/-/commits/users/hjl/pgo-lto
> 
> If check-binutils check-gas check-ld training is good enough, I can drop
> "make install" and "make check" steps.

That's very promising!
I have a couple of comments:

1) I would separate --enable-standalone-pgo-binutils=lto into 2 configuration
options: --enable-lto and --enable-pgo-bootstrap

2) --enable-lto should add -flto=jobserver instead of -flto, but I noticed the
following warning when doing that:

lto-wrapper: warning: jobserver is not available: '--jobserver-auth=' is not
present in 'MAKEFLAGS'

That's likely cased by missing '+' as documented:
You must prepend a ‘+’ to the command recipe in the parent Makefile for this to
work.

Can you please check it?

3) I would use only -fprofile-generate without an additional folder location;
or do you see a .gcda file name clashing?

4) I can confirm that gas is trained well; I'm testing that for a Firefox
LTRANS file (~100MB in size).

Anyway, great progress you made.

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