On Wed, Feb 29, 2012 at 02:44:24PM +0000, Ian Campbell wrote:
If you need a specific register, this means that you must set up that register explicitly inside the asm if you want a guarantee that the code will work:
asm volatile ( "movw r12, %[hvc_num]\n\t"
Is gcc (or gas?) smart enough to optimise this away if it turns out that %[hvc_num] == r12?
No, and it won't do, because %[hvc_num] is specified in these operands:
... "hvc #0" :: [hvc_num] "i" (NUMBER) : "r12"
to be an integer, not a register.
How are system calls implemented on the userspace side? I confess I don't know what the ARM syscall ABI looks like -- is it all registers or is some of it on the stack? It sounds like the solution ought to be pretty similar though.
All registers. We have a few which take a pointer to an in memory array, but those are for some old multiplexed syscalls.