FW: Space usage

[Simon Peyton-Jones]

| > 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 :-)

It does; see my reply below

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Simon Peyton-Jones
| Sent: 17 August 2004 17:25
| To: Malcolm Wallace; glasgow-haskell-users at haskell.org
| Subject: RE: Space usage
| 
| 
| | You probably mean
| |     J. Sparud, "Fixing Some Space Leaks without a Garbage
Collector",
| FPCA'93.
| |     http://citeseer.ist.psu.edu/sparud93fixing.html
| 
| Indeed.
| 
| | as implemented in hbc.  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.
| 
| GHC does this, but it does not always work.  What starts as
| 	let x = fst t in
| 	let h = case x of { True -> 1; False -> 2 } in
| 	...
| may become
| 	let h = case t of { (x,_) -> case x of {True->1;False->2} }
| 	...
| and now there is no selector thunk any more.  So the selector stuff is
| fragile to otherwise-valuable optimisations. And that's what's
happening
| in this case.  A non-optimising compiler is fine.
| 
| Simon

| 
| Regards
| --
| Adrian Hey
| 
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