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From: nmm@cl.cam.ac.uk (Nick Maclaren)
Newsgroups: comp.arch,comp.lang.misc
Subject: Re: Compiler Costs
Message-ID: <1990Jul6.185248.12191@cl.cam.ac.uk>
Date: 6 Jul 90 18:52:48 GMT
References: <1797@apctrc.UUCP>
Reply-To: nmm@cl.cam.ac.uk (Nick Maclaren)
Organization: U of Cambridge Comp Lab, UK
Lines: 29

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?


It is impossible to predict, in general, but the following rules of thumb
may be useful.  Consider the minimal program to take unit time (or space);
the following multiplicative factors are good guidelines:

Competence of assembler programmer    1x  -  5x
Competence of 3 GL programmer         1x  -  3x
Suitability of 3 GL for algorithm     1x  -  10x 
Competence of compiler's code         1x  -  3x
Degree of optimisation                1x  -  10x 

You can therefore expect the assembler to be anything from 900 times
faster to 5 times slower!  This excludes even more drastic factors (in
either direction), caused by gross mistakes or misunderstandings (e.g.
dropping into a floating-point emulator to do 2-D array indexing).

Nick Maclaren
University of Cambridge Computer Laboratory
nmm@cl.cam.ac.uk