On Mon, 2012-02-27 at 21:05 +0000, Peter Maydell wrote:

On 27 February 2012 16:27, Ian Campbell [off-list ref] wrote:

quoted

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.

I think it does.

It actually sounds as if using r12 is fine here, the impact on code
density should be pretty small -- there aren't really all that many call
sites which involve hypercalls.

By way of an example I measured an x86 kernel which should be using more
hypercalls due to pv paging etc and found that 0.014% of the lines in
"objdump -d" contained a call to the hypercall_page. (I know not all
lines of objdump -d output are instructions but it's a reasonable approx
IMHO).

So I think using 3 16 bit instructions slots instead of 2 won't make
much impact in practice.

Thanks,
Ian.