[RFT/RFC PATCH 3/6] ARM: add macro to perform far branches (b/bl)

[Nicolas Pitre]

On Thu, 12 Mar 2015, Ard Biesheuvel wrote:

> On 12 March 2015 at 21:32, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> > On Thu, 12 Mar 2015, Ard Biesheuvel wrote:
> >
> >> These macros execute PC-relative branches, but with a larger
> >> reach than the 24 bits that are available in the b and bl opcodes.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> >> ---
> >>  arch/arm/include/asm/assembler.h | 29 +++++++++++++++++++++++++++++
> >>  1 file changed, 29 insertions(+)
> >>
> >> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> >> index f67fd3afebdf..bd08c3c1b73f 100644
> >> --- a/arch/arm/include/asm/assembler.h
> >> +++ b/arch/arm/include/asm/assembler.h
> >> @@ -108,6 +108,35 @@
> >>       .endm
> >>  #endif
> >>
> >> +     /*
> >> +      * Macros to emit relative branches that may exceed the range
> >> +      * of the 24-bit immediate of the ordinary b/bl instructions.
> >> +      * NOTE: this doesn't work with locally defined symbols, as they
> >> +      * might lack the ARM/Thumb annotation (even if they are annotated
> >> +      * as functions)
> >
> > I really hope you won't need a far call with local symbols ever!
> >
> 
> Well, if you use pushsection/popsection, then local, numbered labels
> you refer to can be quite far away in the output image, and those will
> not have the thumb bit set.

Indeed.

> >> +      */
> >> +     .macro  b_far, target, tmpreg
> >> +#if defined(CONFIG_CPU_32v7) || defined(CONFIG_CPU_32v7M)
> >> + ARM(        movt    \tmpreg, #:upper16:(\target - (8888f + 8))      )
> >> + ARM(        movw    \tmpreg, #:lower16:(\target - (8888f + 8))      )
> >> + THUMB(      movt    \tmpreg, #:upper16:(\target - (8888f + 4))      )
> >> + THUMB(      movw    \tmpreg, #:lower16:(\target - (8888f + 4))      )
> >> +8888:        add     pc, pc, \tmpreg
> >> +#else
> >> +     ldr     \tmpreg, 8889f
> >> +8888:        add     pc, pc, \tmpreg
> >> +     .align  2
> >> +8889:
> >> + ARM(        .word   \target - (8888b + 8)           )
> >
> > The Thumb relocation value is missing here.
> >
> 
> Yes, this is bogus. But Thumb2 implies v7 or v7m, so it is not
> actually incorrect in this case.

The ".align 2" would be redundant in that case too.

> But I will fix it in the next version

Is it worth optimizing the ARM mode with movw/movt on ARMv7?  If not 
then this could be simplified as only:

             .macro  b_far, target, tmpreg
 THUMB(      movt    \tmpreg, #:upper16:(\target - (8888f + 4))      )
 THUMB(      movw    \tmpreg, #:lower16:(\target - (8888f + 4))      )
 ARM(        ldr     \tmpreg, 8888f+4                                )
 8888:       add     pc, pc, \tmpreg
 ARM(        .word   \target - (8888b + 8)           )
             .endm


Nicolas