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Re: [PATCH, binutils/ARM] Fix feature checks based on arch value and force review of logic for new arch
- From: Thomas Preudhomme <thomas dot preudhomme at foss dot arm dot com>
- To: mickael guene <mickael dot guene at st dot com>
- Cc: Nick Clifton <nickc at redhat dot com>, "binutils at sourceware dot org" <binutils at sourceware dot org>
- Date: Mon, 20 Jun 2016 09:52:25 +0100
- Subject: Re: [PATCH, binutils/ARM] Fix feature checks based on arch value and force review of logic for new arch
- Authentication-results: sourceware.org; auth=none
- References: <1877488 dot cBokx9mdiu at e108577-lin> <19045910 dot SPt8PspXUT at e108577-lin> <5762B18C dot 7070204 at st dot com>
On Thursday 16 June 2016 16:02:52 mickael guene wrote:
> Hi Thomas,
Hi Mickael,
>
> 32-bit BL offset is encoded on 24 bits (with lsb set to zero) and
> so biggest value is THM2_MAX_FWD_BRANCH_OFFSET and not
> THM_MAX_FWD_BRANCH_OFFSET.
> Whereas thumb1 BL offset (two 16 bits instructions, see armv5t) offset
> is encoded on 22 bits (with lsb set to zero) and so biggest value is
> THM_MAX_FWD_BRANCH_OFFSET.
Yes, the important bit is the note in the description of the BL instruction in
the ARM Architecture Reference Manual. ARMv6-M, despite not having Thumb-2
technology, has an extended range for the BL instruction.
>
> So as now using_thumb2() returns 0 for armv6-m architecture it
> generates veneer for branch offset between THM_MAX_FWD_BRANCH_OFFSET
> and THM2_MAX_FWD_BRANCH_OFFSET whereas it was not the case before
> your patch.
Indeed. As you noticed, using_thumb2 is used to control both the availability
of that extended BL and the availability of nop.w. Therefore I think your
solution is the right one. I'll create the patch.
Best regards,
Thomas