linux-kernel - Re: [PATCH 1/2] kbuild: remove top-level built-in.a

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Message-Id: <1548231946.lwdxf373y2.astroid@bobo.none>
Date:   Wed, 23 Jan 2019 18:26:35 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH 1/2] kbuild: remove top-level built-in.a

Masahiro Yamada's on January 15, 2019 7:01 pm:
> Hi Nicholas,
> 
> 
> On Tue, Jan 15, 2019 at 5:07 PM Nicholas Piggin <npiggin@...il.com> wrote:
>>
>> Masahiro Yamada's on January 14, 2019 1:27 pm:
>> > The symbol table in the final archive is unneeded because it is passed
>> > to the linker after the --whole-archive option. Every object file in
>> > the archive is included in the link anyway.
>> >
>> > Pass thin archives from subdirectories directly to the linker, and
>> > remove the final archiving step.
>>
>> This seems like a good improvement. As far as I remember, it was slower
>> to do the final link without the index built. Maybe I was testing it
>> in a revision before moving those files into --whole-archive? If there
>> is no slowdown, then I have no objection.
> 
> 
> Thanks for the review.
> 
> 
> I measured the performance of scripts/link-vmlinux.sh with 'perf stat'.
> (20 times for each)
> 
> 
> 
> Before this patch:
> 
>  Performance counter stats for 'make vmlinux' (20 runs):
> 
>        5.266796537 seconds time elapsed
>           ( +-  0.12% )
> 
> 
> 
> After this patch:
> 
>  Performance counter stats for 'make vmlinux' (20 runs):
> 
>        4.945576395 seconds time elapsed
> 
> 
> 
> 
> So, the performance is slightly better.
> 
> Maybe, does omitting the 'ar' save some time?

Wow yeah that's a good speedup and simplification to boot. Thanks for 
checking it.

> Anyway, this commit is good.

Yes, it's a good improvement.

Thanks,
Nick