linux-kernel - Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

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Message-ID: <20151221130338.GA8644@n2100.arm.linux.org.uk>
Date:	Mon, 21 Dec 2015 13:03:38 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Will Deacon <will.deacon@....com>,
	Andrew Pinski <apinski@...ium.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM64: Fix compiling with GCC 6 and Atomics enabled

On Mon, Dec 21, 2015 at 01:46:22PM +0100, Ard Biesheuvel wrote:
> On 21 December 2015 at 13:38, Will Deacon <will.deacon@....com> wrote:
> > On Fri, Dec 18, 2015 at 08:17:35PM -0800, Andrew Pinski wrote:
> >> The problem here is that GCC 6 and above emits .arch now
> >> for each function so now the global .arch_extension has
> >> no effect.  This fixes the problem by putting
> >> .arch_extension inside ARM64_LSE_ATOMIC_INSN so
> >> it is enabled for each place where LSE is used.
> >
> > Hmm, this is going to affect arch/arm/ much more heavily than arch/arm64.
> > .arch_extension is used for virt, mp and sec over there, and it may be
> > tricky to isolate the actual instruction usage (at least, virt looks
> > lost in kvm/arm.c).
> >
> > Why can't gas have an option to accept all instruction encodings that it
> > knows about, inspite of any .arch directives?
> >
> 
> Modern GAS supports things like -march=armv7-a+mp+sec+virt, so it
> probably makes sense to pass that on the command line when building
> for v7 (or +sec only for v6) if the assembler is found to support it
> at build time.

Modern GCCs (GCC 4 at least) add .arch / .cpu pseudo-instructions in
the assembly output which means using -Wa,-march= and -Wa,-mcpu= to
GCC are now totally useless.

(From the bug reports I've seen, I don't deem GCC 5 to be anything but
a total failure, and so as far as I'm concerned, GCC 5 is not to be
used for ARM, and effectively doesn't exist.  I have no experience or
opinions on GCC 6.  Hence, "GCC 4" still counts as "modern" being the
only recent compilers that produce sane results.)

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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