Sun Jan 31 16:24:20 PST 2010 simo...@microsoft.com
* Fix Trac #3831: blowup in SpecConstr
It turned out that there were two bugs. First, we were getting an
exponential number of specialisations when we had a deep nest of
join points. See Note [Avoiding exponential blowup]. I fixed this
by dividing sc_count (in ScEnv) by the number of specialisations
when recursing. Crude but effective.
Second, when making specialisations I was looking at the result of
applying specExpr to the RHS of the function, whereas I should have
been looking at the original RHS. See Note [Specialise original
body].
There's a tantalising missed opportunity here, though. In this
example (recorded as a test simplCore/should_compile/T3831), each join
point has *exactly one* call pattern, so we should really just
specialise for that alone, in which case there's zero code-blow-up.
In particular, we don't need the *original* RHS at all. I need to think
more about how to exploit this.
But the blowup is now limited, so compiling terminfo with -O2 works again.
M ./compiler/specialise/SpecConstr.lhs -41 +96
View patch online:
http://darcs.haskell.org/ghc/_darcs/patches/20100201002420-1287e-da7031609e7c01b4ed22ba2aaaa31a434894bb4f.gz
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