Re: [Xen-devel] [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

[Peter Maydell <peter.maydell@xxxxxxxxxx>]

On 27 February 2012 16:27, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> R12 is not accessible from the 16 bit "T1" Thumb encoding of mov
> immediate (which can only target r0..r7).
>
> Since we support only ARMv7+ there are "T2" and "T3" encodings available
> which do allow direct mov of an immediate into R12, but are 32 bit Thumb
> instructions.
>
> Should we use r7 instead to maximise instruction density for Thumb code?

r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
makes it worth avoiding in this context.

-- PMM

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