Re: Executing code at compilation time

Ray <bear@sonic.net>
Fri, 19 Mar 2010 10:45:18 -0700

          From comp.compilers

Related articles
[8 earlier articles]
Re: Executing code at compilation time pat@jantar.org (Patryk Zadarnowski) (2010-03-17)
Re: Executing code at compilation time shreyas76@gmail.com (shrey) (2010-03-16)
Re: Executing code at compilation time pronesto@gmail.com (Fernando Magno Quintao Pereira) (2010-03-16)
Re: Executing code at compilation time pat@jantar.org (Patryk Zadarnowski) (2010-03-17)
Re: Executing code at compilation time gah@ugcs.caltech.edu (glen herrmannsfeldt) (2010-03-17)
Re: Executing code at compilation time bear@sonic.net (Ray) (2010-03-19)
Re: Executing code at compilation time bear@sonic.net (Ray) (2010-03-19)
Re: Executing code at compilation time bobduff@shell01.TheWorld.com (Robert A Duff) (2010-03-21)
Re: Executing code at compilation time torbenm@diku.dk (2010-03-22)
Re: Executing code at compilation time bear@sonic.net (Ray) (2010-03-22)
Re: Executing code at compilation time gah@ugcs.caltech.edu (glen herrmannsfeldt) (2010-03-23)
Re: Executing code at compilation time martin@gkc.org.uk (Martin Ward) (2010-03-26)
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From: Ray <bear@sonic.net>
Newsgroups: comp.compilers
Date: Fri, 19 Mar 2010 10:45:18 -0700
Organization: Doubtful
References: 10-03-038 <31F5149B-7EAB-498D-95B4-DE608FDDD3A5@jantar.org> 10-03-050
Keywords: optimize, analysis
Posted-Date: 20 Mar 2010 23:21:05 EDT

For a pretty complete list of closed forms for finite and infinite
sums that compiler writers can use for a reference page, see:


http://en.wikipedia.org/wiki/List_of_mathematical_series


Compilers can use these rules to lift a lot of mathematical operations
out of loops, eliminating the loop completely if that's all that the
loop was doing.


The OP in this thread was asking about a reduction that clearly
results from gcc using an application of a finite- sum rule.


Some folks think such tricks are "not right" in that they change the
"semantics" of the program by changing its algorithm. Others,
including myself, think that a program that gets the same results
faster and with fewer operations than its source code would lead you
to believe is a better program. If I can write an O(n^2) or even
O(2^n) algorithm and get O(1) runtime performance, I'm not going to
lodge a complaint.


Things get hazier in the event of O(infinity) or non-halting programs
- If you can use an infinite-sum rule to produce an executable that
comes to a halt with the result that the source-code operation "would
have" obtained if run to conclusion on some infinitely-fast computer,
is that an optimization or is that cheating?


                                                                Bear



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