linux-kernel - Re: New assembler warnings with binutils 2.29

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Message-ID: <20170814085901.evwj3rrxo74wcgak@armageddon.cambridge.arm.com>
Date:   Mon, 14 Aug 2017 09:59:02 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Laura Abbott <labbott@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, michael.collison@....com,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>
Subject: Re: New assembler warnings with binutils 2.29

On Fri, Aug 11, 2017 at 10:26:06AM +0100, Ard Biesheuvel wrote:
> On 11 August 2017 at 10:22, Catalin Marinas <catalin.marinas@....com> wrote:
> > On Thu, Aug 10, 2017 at 01:13:22PM -0700, Laura Abbott wrote:
> >> Fedora rawhide recently upgraded to binutils 2.29 and this seems
> >> to produce new warnings:
> >>
> >> ./arch/arm64/include/asm/assembler.h: Assembler messages:
> >> ./arch/arm64/include/asm/assembler.h:125: Warning: ignoring attempt to redefine built-in register 'lr'
> >>
> >> This is
> >>
> >> /*
> >>  * Register aliases.
> >>  */
> >> lr      .req    x30             // link register
> >
> > Strange, does gas now think 'lr' is a general purpose register (aliased
> > to x30)? It never was and IIRC the toolchain people many years ago
> > refused to add it, hence the alias above in the kernel. I wonder if they
> > added 'fp' as well...
> >
> > We could remove the alias and replace all 'lr' instances with 'x30'
> > throughout the kernel (no too many) or we add some #ifdef around the
> > above based on the binutils version.
> 
> This is annoying. Replacing lr with x30 achieves the opposite of the
> intent of the binutils change. And using #ifdefs is inaccurate,
> because you can't really test the binutils version only the GCC
> version, and those are not tightly coupled.
> 
> Can you .unreq it?

Not really, with an older binutils I get:

arch/arm64/include/asm/assembler.h:125: Error: unknown register alias 'lr'

I personally consider this a binutils bug. After 6+ years (probably not
all public) of building the kernel just fine, all of a sudden certain
strings became reserved in gas. Three options:

a) binutils reverts the change
b) we stop using lr etc. in Linux for good (since you can't tell which
   gas supports them)
c) we replace .req with #define in the kernel

My preference is (a) but we can go for (c) as being more under our
control.

-- 
Catalin