Xref: utzoo comp.arch:16967 comp.lang.misc:5102
Path: utzoo!attcan!uunet!zephyr.ens.tek.com!tekcrl!tekgvs!toma
From: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Newsgroups: comp.arch,comp.lang.misc
Subject: Re: Compiler Costs
Message-ID: <7764@tekgvs.LABS.TEK.COM>
Date: 6 Jul 90 16:06:14 GMT
References: <1797@apctrc.UUCP>
Reply-To: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Followup-To: comp.arch
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 34

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?

Simple Answer: That depends...

If the program implements an algorithm that is well suited for the compiler
(standard matrix multiplication in Fortran, for example) or is a program that
is a standard benchmark for which *every* compiler writer optimizes, you
might not see any improvement at all. If fact it might run slower if you are
not as clever as the compiler writer!

On the other hand, if the algorithm cannot be efficiently represented in
the language (string manipulation in Fortran, complex numbers in C) or there
are special cpu instructions you can take advantage of that the compiler
doesn't, then you can get major improvements, even order of magnitude.

In general, there is only a small portion of the code that uses most of the
execution time, and by coding those portions in assembler, you can get most
of the assembler performance while still enjoying the convenience of a 
high level language for the bulk of the code.

If you are concerned about code size, there still is a major improvement
to be offered with assembler, typically because you can custom tailor 
subroutine calling conventions and register usage. But then again many
modern compilers let you do the same.

(I hope this won't be the start of some "language war" :-)

Tom Almy
toma@tekgvs.labs.tek.com
Standard Disclaimers Apply