Subject: Re: Kernel calls - args in registers
To: Michael Smith <>
From: John S. Dyson <>
List: tech-kern
Date: 11/20/1996 00:32:38
> Travis Hassloch x231 stands accused of saying:
> > 
> > One thing I thought might make a worthwhile gain is to make all
> > intrakernel calls use registers -- and if possible, all kernel calls.
> I think Bruce will have opinions here, but IMHO using registers for
> arg passing isn't much of a win, especially on the x86 where there
> are so few of them.  
I have an opinion also...  I have already recompiled much of the FreeBSD kernel
to use the register passing convention.  Not really worth it.  F.E.
our approx 500-530usec fork perf (on a P5-166) doesn't budge almost
at all with a change in calling convention...  The pgcc compiler or
-fomit-frame-pointer makes much more difference (if it works to compile
your kernel :-) -- sometimes "advanced options" don't work to compile
ours...)  (BTW, even that difference is in the 5% or so range.)
I think that algorithmic improvements in the kernel are where "it is."

It is likely that if we use the register passing conventions in the
kernel -- it'll be because there is a worthwhile improvement.  I haven't
seen a significant one -- YET.  If you see one -- let me know!!!