linux-kernel - Re: [PATCH] ARM: OMAP2+: drop unnecessary adrl

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Message-ID: <CAMj1kXEe835GbXU5qgX-QQ5n4SmwQO1nAoAZw5pUVCbR=J8XmQ@mail.gmail.com>
Date:   Thu, 2 Apr 2020 16:36:08 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Stefan Agner <stefan@...er.ch>
Cc:     linux-omap@...r.kernel.org, Tony Lindgren <tony@...mide.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Smith <Peter.Smith@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>, nd <nd@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: OMAP2+: drop unnecessary adrl

On Thu, 2 Apr 2020 at 16:34, Stefan Agner <stefan@...er.ch> wrote:
>
> On 2020-04-02 14:05, Ard Biesheuvel wrote:
> > On Thu, 2 Apr 2020 at 13:50, Peter Smith <Peter.Smith@....com> wrote:
> >>
> >> > I take it this implies that the LLVM linker does not support the
> >> > R_ARM_ALU_PC_Gn relocations? Since otherwise, adrl could simply be
> >> > expanded to a pair of adds with the appropriate relocations, letting
> >> > the linker fix up the immediates (and the ADD vs SUB bits)
> >>
> >> Not at the moment. I have a patch in review to add the G0 variants for these in Arm state at reviews.llvm.org/D75349 . As far as I know LLVM MC does not have support for generating the relocations either. This could be added though. I agree that using the G* relocations with a pair of add/sub instructions would be the ideal solution. The adrl psuedo is essentially that but implemented at assembly time. I think it would be possible to implement in LLVM but at the time (4+ years ago) I wasn't confident in finding someone that would think that adrl support was worth the disruption, for example the current Arm assembly backend can only produce 1 instruction as output and adrl requires two.
> >>
> >> I'd be happy to look at group relocation support in LLD, I haven't got a lot of spare time so progress is likely to be slow though.
> >>
> >
> > For Linux, I have proposed another approach in the past, which is to
> > define a (Linux-local) adr_l macro with unlimited range [0], which
> > basically comes down to place relative movw/movt pairs for v7+, and
> > something along the lines of
> >
> >         ldr <reg>, 222f
> > 111:    add <reg>, <reg>, pc
> >         .subsection 1
> > 222:    .long <sym> - (111b + 8)
> >         .previous
>
> Just to confirm: The instance at hand today seems to be working fine
> without adrl, so I guess we are fine here, do you agree?
>

I agree. Apologies for hijacking the thread :-)

> There are a couple more instances of adrl in arch/arm/crypto/, maybe
> that is where the adr_l macro could come in.
>

There are various places in the arch code that could be cleaned up
along these lines.

But you're right - this is a separate discussion that deserves a
thread of its own. I was just satisfying my own curiosity.