Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!LBL.GOV!nagy%warner.hepnet
From: nagy%warner.hepnet@LBL.GOV (Frank J. Nagy, VAX Wizard & Guru)
Newsgroups: comp.os.vms
Subject: RE: 2 C questions
Message-ID: <880409111116.20a031e1@LBL.Gov>
Date: 9 Apr 88 18:11:16 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 42

> 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?
> If the C local variables are made static, does this possible performance
> advantage disappear?
     
On the VAX there is very little performance lost due to the automatic
variables.  Assuming the variables are not initialized (in which case
they would be essentially same as static variables in terms of
performance), then all that is needed is a single instruction:

	SUBL2	#<# of bytes of auto variables>,SP

to bump the stack pointer down to allocate the auto variables.

Nothing need be done to deallocate them since the old value of the
stack pointer is taken from the call frame by the RET instruction.
This automatically "pops" the auto variables, the call frame AND
the argument list (when a CALLS instruction is used).

With VAX C versus VAX FORTRAN the performance "hit" in doing a
routine call is that C (also PASCAL for that matter) uses a CALLS
and must PUSH the argument list onto the stack (in reverse order)
before the CALLS.  (Well, normally the argument is build directly
onto the stack and is not PUSHed on from the static location).
VAX FORTRAN uses CALLG and a static argument list; but here
FORTRAN must overwrite argument values which have changed from
the values in the static list - this is true when a subroutine
passes arguments it was called with to an inner routine.  In these
cases, much of the performance advantage of VAX FORTRAN is lost.

The other side of the coin is that by building the argument list
on the stack at run-time, C and PASCAL automatically support
recursion and ASTs in a natural and easy manner.  Try that in
FORTRAN (I have, I know and that's why I use C these days).

= Frank J. Nagy   "VAX Guru & Wizard"
= Fermilab Research Division EED/Controls
= HEPNET: WARNER::NAGY (43198::NAGY) or FNAL::NAGY (43009::NAGY)
= BitNet: NAGY@FNAL
= USnail: Fermilab POB 500 MS/220 Batavia, IL 60510