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

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Message-ID: <CAMj1kXEQ4v9e6386ogPdy+s+++9H02DMPnDpTq0WSY2e78ts+Q@mail.gmail.com>
Date:   Thu, 2 Apr 2020 14:05:10 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Peter Smith <Peter.Smith@....com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, nd <nd@....com>,
        Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Stefan Agner <stefan@...er.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: OMAP2+: drop unnecessary adrl

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

for v6 and earlier. Could you comment on whether Clang's integrated
assembler could support anything like this?


Thanks,
Ard.



[0] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?h=arm-kaslr-latest&id=fd440f1131553a5201ce3b94905419bd067b93b3