Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754220AbbKXOBX (ORCPT ); Tue, 24 Nov 2015 09:01:23 -0500 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:53460 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752861AbbKXOBG (ORCPT ); Tue, 24 Nov 2015 09:01:06 -0500 Date: Tue, 24 Nov 2015 14:00:47 +0000 From: Russell King - ARM Linux To: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= Cc: Arnd Bergmann , linux-arm-kernel@lists.infradead.org, Stephen Boyd , Nicolas Pitre , Peter Maydell , "linux-arm-msm@vger.kernel.org" , Daniel Lezcano , lkml - Kernel Mailing List , Steven Rostedt , Christopher Covington Subject: Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions Message-ID: <20151124140047.GC8644@n2100.arm.linux.org.uk> References: <1448068997-26631-1-git-send-email-sboyd@codeaurora.org> <20151124001306.GI19156@codeaurora.org> <20151124085349.GQ19156@codeaurora.org> <4485320.CGMJdfZLya@wuerfel> <20151124104225.GA8644@n2100.arm.linux.org.uk> <20151124122308.GB8644@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2233 Lines: 50 On Tue, Nov 24, 2015 at 12:29:06PM +0000, M?ns Rullg?rd wrote: > Russell King - ARM Linux writes: > > > On Tue, Nov 24, 2015 at 12:10:02PM +0000, M?ns Rullg?rd wrote: > >> Russell King - ARM Linux 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. -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/