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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 12 May 2020 10:10:02 +0900
Message-ID: <CAK7LNAQJR5JUc37QcYS2P=WOfDQmGyAh3nJ_m8qTQDnT6LOnyA@mail.gmail.com>
Subject: Re: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10
To:     David Laight <David.Laight@aculab.com>
Cc:     Joe Perches <joe@perches.com>, Arnd Bergmann <arnd@arndb.de>,
        Oleksandr Natalenko <oleksandr@redhat.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Sun, May 10, 2020 at 9:47 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Joe Perches
> > Sent: 08 May 2020 16:06
> > On Fri, 2020-05-08 at 13:49 +0200, Arnd Bergmann wrote:
> > > Personally, I'm more interested in improving compile speed of the kernel
> >
> > Any opinion on precompiled header support?
>
> When ever I've been anywhere near it it is always a disaster.
> It may make sense for C++ where there is lots of complicated
> code to parse in .h files. Parsing C headers is usually easier.
>
> One this I have done that significantly speeds up .h file
> processing is to take the long list of '-I directory' parameters
> that are passed to the compiler and copy the first version
> of each file into a separate 'object headers' directory.
> This saves the compiler doing lots of 'failed opens'.
>
> If each fragment makefile lists its 'public' headers make
> can generate dependency rules that do the copies.
>
> FWIW make is much faster if you delete all the builtin and
> suffix rules and rely on explicit rules for each file.


Kbuild disables Make's builtin rules at least.


# Do not use make's built-in rules and variables
# (this increases performance and avoids hard-to-debug behaviour)
MAKEFLAGS += -rR


--
Best Regards
Masahiro Yamada