On Mon, 10 Sep 2012, Rich Felker wrote:
> As for my problem, I can use r7 as the temp ("move $2,$7 ; syscall")
> for syscalls with 3 or fewer args, but for the 4-arg syscall, $7 is
> occupied by an argument, and I'd need to spill the syscall number to
> the stack to be able to restore it if $25 is not available...
If performance or some other factors require you to avoid spilling the
syscall number to the stack or other readily-accessible (e.g. GP-relative)
memory and the number is not a constant you could load with LI, then you
can always store it in a call-saved register, one of $s0-$s8, that are
guaranteed by the syscall ABI to be preserved across.
Relying on any call-clobbered registers, including $7 to be preserved
across a syscall is risky, to say the least, as this is not guaranteed by
the syscall ABI. I do wonder however why we have these instructions to
save/restore $25 in SAVE_SOME/RESTORE_SOME. This dates back to 2.4 at the
very least.
Ralf, any insights?
Maciej
