[Haskell-cafe] Why?

[Janis Voigtländer]

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/
mailto:jv at iai.uni-bonn.de