Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!oliveb!pyramid!leadsv!laic!darin
From: darin@laic.UUCP (Darin Johnson)
Newsgroups: comp.os.vms
Subject: Re: 2 C questions
Message-ID: <203@laic.UUCP>
Date: 8 Apr 88 01:42:52 GMT
References: <12387779883.20.STEINBERGER@KL.SRI.COM> <281@cullsj.UUCP>
Organization: Lockheed AI Center, Menlo Park
Lines: 28

In article <281@cullsj.UUCP>, jayz@cullsj.UUCP (Jay Zorzy) writes:
> From article <12387779883.20.STEINBERGER@KL.SRI.COM>, by STEINBERGER@KL.SRI.COM (Richard Steinberger):
> > C vs. Fortran speed:  Since C variables are automatic (i.e., dynamic) by
> > default and Fortran variables are static, is it fair to conclude that in
> > general, a routine in C having the same number of local variables as
> > a "roughly identical" Fortarn routine will take a bit longer because
> > the OS must allocate (and deallocate) space for the local variables?
> 
> It depends on the size of the variables.  Dynamic variables are allocated
> from the stack, so if you've got huge arrays, VMS will obviously have more
> work to do to allocate them on the stack.

I haven't looked at the compiler output for a long time, BUT...
doesn't the allocation of automatic variables just involve adjusting
the stack (and possibly frame) pointers?  (In fact, I am pretty sure
that this is what happens, but I have been known to be worng :-)

If this is all that happens, then the size and number of automatic
variables makes no difference at all to the overhead, which would just
be one or two instructions.  Since I don't know how FORTRAN allocates
static variables (I hope they aren't put into a demand paged section :-)
I would assume the C method is just as efficient.  It is probably more
efficient memory-wise since variables in rarely called routines never get
allocated, whereas the memory for these variables would always hang around
(of course, they could be paged out).
-- 
Darin Johnson (...ucbvax!sun!sunncal!leadsv!laic!darin)
              (...lll-lcc.arpa!leadsv!laic!darin)
	All aboard the DOOMED express!