linux-kernel - Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi

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Date:	Tue, 24 Nov 2015 12:23:08 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Måns Rullgård <mans@...sr.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Stephen Boyd <sboyd@...eaurora.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Peter Maydell <peter.maydell@...aro.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Christopher Covington <cov@...eaurora.org>
Subject: Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi_{u}idiv library
 functions

On Tue, Nov 24, 2015 at 12:10:02PM +0000, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@....linux.org.uk> writes:
> 
> > On Tue, Nov 24, 2015 at 11:38:53AM +0100, Arnd Bergmann wrote:
> >> I suggested using -mcpu=cortex-a15 because there are old gcc versions
> >> that don't know about -march=armv7ve or -march=armv7-a+idiv yet, but
> >> that do understand -mcpu=cortex-a15.
> >
> > That's not all.  The bigger problem is that there are toolchains out
> > there which accept these options, but do _not_ support IDIV in either
> > ARM or Thumb mode.  I'm afraid that makes it impossible to add this
> > feature to the mainline kernel in this form: we need to run a test
> > build to check that -march=armv7ve or what-not really does work
> > through both GCC and GAS.
> 
> If the compiler accepts the option but doesn't actually emit any div
> instructions, is there any real harm?

That's not what I've found.  I've found that asking the assembler
to accept idiv instructions appears to be ignored, which is something
completely different.

Further to this, what it comes down to is the stupid idea that the
compiler should embed .arch / .cpu in the assembly output, which has
the effect of overriding the command line arguments given to it via
-Wa.  So, giving -Wa,-mcpu=cortex-a15 is a total no-op, because the
first thing in the assembly output is:

	.arch armv7-a

which kills off any attempt to set the assembly level ISA from the
command line.

It does appear after all that Ubuntu 14.04 does support sdiv/idiv with
-mcpu=cortex-a15, but there is no -march=armv7ve.

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