On Thu, 19 Dec 2002, H. Peter Anvin wrote:
>
> Unfortunately it means taking an indirect call cost for every invocation...
Ehh.. I just tested the "cost" of this on a PIII (comparing a indirect
call with a direct one), and it's exactly one extra cycle.
ONE CYCLE.
On a P4 the difference was 4 cycles. On my test P95 system I didn't see
any difference at all. And I don't have an athlon handy in my office.
That's the difference between
static void *address = &do_nothing;
asm("call *%0" :"m" (address))
and
asm("call do_nothing");
So it's between 0-4 cycles on machines that take 200 - 1000 cycles for
just the system call overhead.
And for that "overhead", you get a binary that trivially works on all
kernels, _and_ doesn't need extra mmap's etc (which are _easily_ thousands
of cycles).
Linus
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This archive was generated by hypermail 2b29 : Mon Dec 23 2002 - 22:00:25 EST