On Mon, 2012-02-27 at 21:05 +0000, Peter Maydell wrote: > 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. 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
