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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Joe Perches' <joe@perches.com>, Arnd Bergmann <arnd@arndb.de>,
        "Oleksandr Natalenko" <oleksandr@redhat.com>
CC:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: RE: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10
Thread-Topic: [PATCH] Kconfig: default to CC_OPTIMIZE_FOR_PERFORMANCE_O3 for
 gcc >= 10
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Date:   Sun, 10 May 2020 12:47:13 +0000
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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.

	David

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