Adrian Hey wrote:
On Tuesday 17 Aug 2004 5:11 pm, Malcolm Wallace wrote:
It is also possible to use Wadler's
garbage-collector fix for this space leak, as implemented in nhc98.
P Wadler, "Fixing a Space Leak with a Garbage Collector", SP&E Sept
1987.
When the GC discovers a selector function applied to an evaluated
argument, it "evaluates" the selector on-the-fly by just swizzling
pointers. It needs some co-operation from the compiler to make
selector functions look obvious, but that isn't too difficult.
So ghc doesn't do this (or something better)? I'm surprised
because it seems like a really basic and simple feature to me. I
implemented a toy FPL a few years ago and even my gc incorporated
this optimisation. It's a bit strange that this should have been
overlooked considering in all other respects ghc is far more
sophisticated than my efforts were :-)
As Simon pointed out, it's not so easy to do this with an
optimizing compiler that may inline selector functions.
The "right" way to do this is to have specialized code
that runs during GC for each thunk. This was investigated
in Christina von Dorrien's licentiate thesis "Stingy evaluation".
-- Lennart
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