linux-kernel - Re: [PATCH v2 1/2] scripts: Add a recorduidiv program

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Date:	Mon, 30 Nov 2015 15:32:09 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Michal Marek <mmarek@...e.com>
Cc:	Stephen Boyd <sboyd@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	linux-kbuild@...r.kernel.org, Nicolas Pitre <nico@...xnic.net>,
	Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Måns Rullgård <mans@...sr.com>
Subject: Re: [PATCH v2 1/2] scripts: Add a recorduidiv program

On Mon, Nov 30, 2015 at 04:11:16PM +0100, Michal Marek wrote:
> On 2015-11-26 00:47, Russell King - ARM Linux wrote:
> > On Wed, Nov 25, 2015 at 01:51:03PM -0800, Stephen Boyd wrote:
> >> The ARM compiler inserts calls to __aeabi_uidiv() and
> >> __aeabi_idiv() when it needs to perform division on signed and
> >> unsigned integers. If a processor has support for the udiv and
> >> sdiv division instructions the calls to these support routines
> >> can be replaced with those instructions. Therefore, record the
> >> location of calls to these library functions into two sections
> >> (one for udiv and one for sdiv) similar to how we trace calls to
> >> mcount. When the kernel boots up it will check to see if the
> >> processor supports the instructions and then patch the call sites
> >> with the instruction.
> > 
> > Do we have any resolution on these programs which modify the object
> > files in-place, rather than breaking any hard-links which may be
> > present (eg, as a result of using ccache in hard-link mode) ?
> 
> Good point, but I do not think anybody is using CCACHE_HARDLINK with the
> kernel.

That's wrong then, because I've been using it for a very long time with
my nightly builds. :)  Therefore, there is somebody!

> As the manpage says, it is going to confuse make, so the time
> saved by ccache would be offset by make trying to recompile all *.c
> files each time.

>From what I've noticed, it makes a big difference when running nightly
builds.  My nightly builds use O= and always build into an empty target
tree, so there are no old objects back-dated to confuse make.

Even if there were, make would spot that the object is older than the
source, and try to re-make the target again, at which point ccache
would re-hardlink the object after looking up the hashed preprocessed
source.

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