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From: atk@boulder.Colorado.EDU (Alan T. Krantz)
Newsgroups: comp.arch,comp.lang.misc
Subject: Re: Compiler Costs
Message-ID: <23268@boulder.Colorado.EDU>
Date: 9 Jul 90 15:30:25 GMT
References: <1797@apctrc.UUCP> <1990Jul6.161158.1297@zoo.toronto.edu> <1990Jul8.230954.18881@ecn.purdue.edu>
Sender: news@boulder.Colorado.EDU
Reply-To: atk@boulder.Colorado.EDU (Alan T. Krantz)
Organization: University of Colorado, Boulder
Lines: 35

In article <1990Jul8.230954.18881@ecn.purdue.edu> hankd@dynamo.ecn.purdue.edu (Hank Dietz) writes:
>In article <1990Jul6.161158.1297@zoo.toronto.edu>:
>>In article <1797@apctrc.UUCP> zrra07@backus (Randall R. Appleton) writes:
>>
>>It can be negative.  Good optimizing compilers can often do a better
>>job on large bodies of code than humans can.  Small code fragments
>>often do better when carefully crafted by humans, although this is
>>a very time-consuming activity.
>
>Basically, I agree.  Given the caveat that good compilers are quite
>rare, I have seen good compilers consistently outperform handwritten
>assembly code -- but humans could usually do local tweaks to compiler
>generated code to speed it up even a little more. 

I'm not so sure I've seen this very often. It seems that on some of the
newer machines (IBM RS/6000) which have very special conditions for
optimizing the pipeline this might be more accurate - but on many machines it
isn't. The trick is when you hand write assembly code you get to
optimize register usage across routine calls much better then most
compilers can (actually, to be honest I haven't seen any compilers do
global register optimization - though this is an area of active
research). Anyways, almost every comment I've seen on compilers vs hand
optimization has been on a routine by routine case - and I still believe
(though I could be wrong) that the optimial results from assembly code
comes from register usage across/through routine calls...

Ho hum - I'm sure someone will set me straight ....


 
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