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From: rwallace@vax1.tcd.ie
Newsgroups: comp.arch
Subject: Re: Compiler Costs
Message-ID: <6534.26a0d62b@vax1.tcd.ie>
Date: 15 Jul 90 20:46:35 GMT
References: <40052@mips.mips.COM> <628@dg.dg.com> <64045@sgi.sgi.com>
Organization: Computer Laboratory, Trinity College Dublin
Lines: 36

In article <64045@sgi.sgi.com>, karsh@trifolium.esd.sgi.com (Bruce Karsh) writes:
> In article <628@dg.dg.com> publius@dg-pag.webo.dg.com (Publius) writes:
> 
>>Many "large" programs spend most of the CPU time in one or a few relatively
>>"tiny" routines.
> 
> This is an often stated.  Is there any evidence for this?  What does the
> distribution of time look like for, for instance, a compiler?  I'd expect
> that there would be lots of time-sinks.  The lexical analyzer, the symbol table
> hash routine, the symbol table lookup routine, common subexpression finder,
> ... etc, could all be candidate time-sinks.
> 
> How about a text editor?  I could think of a few dozen potential time-sinks
> in a text editor.
> 
> When I've profiled programs, I've often found that the time is spent mostly
> in a few routines, but this isn't always the case.  How often is it true that
> nearly all of the time is spent in only a few routines?  How often are these
 routines tiny?
> 

Usually this is the case. The compiler is an exception because it's an
all-round integer CPU intensive program, but I disagree with you on the text
editor. I was writing a simple text editing facility on an accounting package
in Turbo C on an MS-DOS machine a couple of weeks ago. The editor ended up
spending almost all its time either paging or scrolling. Paging partly involves
setting memory to a particular value, which ended up as a 3-line loop of C
coded, so those 3 lines of code were taking about 30%-40% of the total
execution time. Now the 80x86 is a pretty foul processor for memory writes and
copies but by examining the assembly language output and changing 2 lines of C
code I enabled the compiler to double the speed of the code for that inner
loop. Lo: a 20% speed improvement by changing 2 lines.

Russell Wallace, Trinity College, Dublin
rwallace@vax1.tcd.ie
"To summarize the summary of the summary: people are a problem"