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

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Message-ID: <yw1xk2p7r371.fsf@unicorn.mansr.com>
Date:	Tue, 24 Nov 2015 12:29:06 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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\@vger.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

Russell King - ARM Linux <linux@....linux.org.uk> writes:

> 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.

Oh, you mean the compiler knows about the instructions but the assembler
doesn't or isn't passed the right options.  It's infuriating when that
happens.

-- 
Måns Rullgård
mans@...sr.com
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