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

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Message-ID: <yw1xfuzvqyts.fsf@unicorn.mansr.com>
Date:	Tue, 24 Nov 2015 14:03:27 +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 12:29:06PM +0000, Måns Rullgård wrote:
>> 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.
>
> No, that isn't what I meant.

Then what do you mean?  The compiler either emits the instructions or it
doesn't.  If it doesn't, what the assembler accepts is irrelevant.

-- 
Måns Rullgård
mans@...sr.com
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