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Re: [Haskell-cafe] Why?

Subject: Re: [Haskell-cafe] Why?
From: Janis Voigtländer
Date: Sat, 12 Dec 2009 07:35:46 +0100
Andrew Coppin wrote:
> On the other hand, turn up the optimisation settings on a C compiler
> high enough and the program breaks. Somewhere the compiler elides a
> second call to a function which actually happens to have some
> side-effect that the compiler isn't aware of, and your stream is now one
> byte out of alignment, or whatever. Fiddling with optimisation settings
> in GHC may make your program slow to a crawl rather than go faster, but
> it *never* makes it segfault on startup.

Note, though, that "slow to crawl" may go as far as "not terminate". GHC's optimizer can make a terminating program nonterminating. For some discussion, see:
http://www.haskell.org/haskellwiki/Correctness_of_short_cut_fusion#Discussion

In fact, one can also make up examples where turning on optimization lets GHC transform a "good" program into one halting with an arbitrary error (division by zero, say).
The recipe for (artificial) such examples can be found in footnote 2 of:

http://www.iai.uni-bonn.de/~jv/TUD-FI09-06.pdf

The error "in the wild" that is mentioned on the wiki page occurred in the context of the stream fusion library development.
Best regards,
Janis.

--
Jun.-Prof. Dr. Janis Voigtländer
http://www.iai.uni-bonn.de/~jv/
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