[PATCH v2 02/10] ARM: assembler: introduce adr_l, ldr_l and str_l macros

[Linus Walleij]

On Mon, Sep 21, 2020 at 5:41 PM Ard Biesheuvel <ardb at kernel.org> wrote:

> Like arm64, ARM supports position independent code sequences that
> produce symbol references with a greater reach than the ordinary
> adr/ldr instructions. Since on ARM, the adrl pseudo-instruction is
> only supported in ARM mode (and not at all when using Clang), having
> a adr_l macro like we do on arm64 is useful, and increases symmetry
> as well.
>
> Currently, we use open coded instruction sequences involving literals
> and arithmetic operations. Instead, we can use movw/movt pairs on v7
> CPUs, circumventing the D-cache entirely.
>
> E.g., on v7+ CPUs, we can emit a PC-relative reference as follows:
>
>        movw         <reg>, #:lower16:<sym> - (1f + 8)
>        movt         <reg>, #:upper16:<sym> - (1f + 8)
>   1:   add          <reg>, <reg>, pc
>
> For older CPUs, we can emit the literal into a subsection, allowing it
> to be emitted out of line while retaining the ability to perform
> arithmetic on label offsets.
>
> E.g., on pre-v7 CPUs, we can emit a PC-relative reference as follows:
>
>        ldr          <reg>, 2f
>   1:   add          <reg>, <reg>, pc
>        .subsection  1
>   2:   .long        <sym> - (1b + 8)
>        .previous
>
> This is allowed by the assembler because, unlike ordinary sections,
> subsections are combined into a single section in the object file, and
> so the label references are not true cross-section references that are
> visible as relocations. (Subsections have been available in binutils
> since 2004 at least, so they should not cause any issues with older
> toolchains.)
>
> So use the above to implement the macros mov_l, adr_l, ldr_l and str_l,
> all of which will use movw/movt pairs on v7 and later CPUs, and use
> PC-relative literals otherwise.
>
> Reviewed-by: Nicolas Pitre <nico at fluxnic.net>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

That is obviously a very neat tool you have there.
I worry a bit that people reading the code might have to
issue git log/blame on it to get the commit message in
order to understand what is going on.
Reviewed-by: Linus Walleij <linus.walleij at linaro.org>

Yours,
Linus Walleij