Path: utzoo!mnetor!uunet!husc6!mailrus!ames!pasteur!ucbvax!INDYVAX.BITNET!IMHW400
From: IMHW400@INDYVAX.BITNET
Newsgroups: comp.os.vms
Subject: Re:  2 C questions
Message-ID: <8804111028.AA23176@ucbvax.Berkeley.EDU>
Date: 8 Apr 88 19:14:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 22

Regarding relative speed of C and FORTRAN code:  no, you can't assume that
dynamic allocation takes significantly more time.  Most VAX languages
"allocate" dynamic local storage by assuming that the stack has enough space
left and defining their addresses relative to the stack pointer.  The runtime
overhead involved is a single MOVAx instruction with registers for source
and destination, plus indirection overhead.  Depending on the degree of
overlap during instruction decoding for the particular machine, the indirection
overhead could in fact be zero.  So, the best-case cost for stack allocation
is one very brief instruction per CALL.  So the overhead may hold academic
interest, but no real practical interest.

On the other hand, IF your machine uses a significant amount of time for
each indirection, AND the FORTRAN compiler is using absolute rather than
PC-relative addressing (probably not!) then the overhead in C might become
significant.  But if it is really causing you grief, you probably have refined
your code into too many itty-bitty subroutines.
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Mark H. Wood    IMHW400@INDYVAX.BITNET   (317)274-0749 III U   U PPPP  U   U III
Indiana University - Purdue University at Indianapolis  I  U   U P   P U   U  I
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