Path: utzoo!attcan!uunet!mstan!amull
From: amull@Morgan.COM (Andrew P. Mullhaupt)
Newsgroups: comp.lang.misc
Subject: Re: Compiler Costs
Summary: You can _lose_ at this game pretty easily
Message-ID: <1196@s8.Morgan.COM>
Date: 7 Jul 90 16:13:51 GMT
References: <1797@apctrc.UUCP> <2317@l.cc.purdue.edu>
Organization: Morgan Stanley & Co. NY, NY
Lines: 28

In article <1797@apctrc.UUCP>, zrra07@backus (Randall R. Appleton) writes:
> I have a simple question:  Does anyone know what sort of speed-up
> on gets by going from a good implementation of some algorithm in a
> third generation language (C, Fortran) and a good optimizing compiler
> to hand-coded assembly?
> 
> In other words, if I take my average well written program, compiled
> with a good optimizing compiler, and re-write it in assembler, what sort
> of speedup should I expect to see?

On many machines, like the CDC 6600 and the R/6000, the FORTRAN compilers
are going to get as much or more than you are. You will normally
produce very slow code on a 6600 unless you really understand the
scheduling of the functional units. The FORTRAN compiler for the
R/6000 produces such good code that IBM has decided to implement
ESSL (a major assembly code on the 3090) in FORTRAN on the R/6000.
Their reason is that they can't beat the FORTRAN code.

There are machines like the i860 where hand coded assembler is a lot
faster than most compiled code, but if people still have i860 compilers
ten years from now, you should expect the compiler code to be faster
than hand generated, since the i860 is so complicated. Once you figure
out how to take advantage of the weird hardware, the compiler _always_
takes advantage. The assembler language programmer is not as
consistent.

Later,
Andrew Mullhaupt