- GHC's uses a many optimsations and GHC specific techniques (unboxed - values, RULES pragmas, and so on) to make the heavily used Prelude code - as fast as possible. - -
- What's this "lazy" thing. Well, pseq is a seq for a parallel setting. - We really mean "evaluate a, then b". But if the strictness analyser sees that pseq is strict - in b, then b might be evaluated before a, which is all wrong. -- pseq a b = a `seq` lazy b -
-Solution: wrap the 'b' in a call to GHC.Base.lazy. This function is just the identity function, -except that it's put into the built-in environment in MkId.lhs. That is, the MkId.lhs defn over-rides the -inlining and strictness information that comes in from GHC.Base.hi. And that makes lazy look -lazy, and have no inlining. So the strictness analyser gets no traction. -
-In the worker/wrapper phase, after strictness analysis, lazy is "manually" inlined (see WorkWrap.lhs), -so we get all the efficiency back. -
-This supersedes an earlier scheme involving an even grosser hack in which par# and seq# returned an -Int#. Now there is no seq# operator at all. - - -
- There is a lot of magic in PrelBase.lhs -
- among other things, the RULES
- pragmas implementing the fold/build
- optimisation. The code for map is
- a good example for how it all works. In the prelude code for version
- 5.03 it reads as follows:
-
-map :: (a -> b) -> [a] -> [b]
-map _ [] = []
-map f (x:xs) = f x : map f xs
-
--- Note eta expanded
-mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
-{-# INLINE [0] mapFB #-}
-mapFB c f x ys = c (f x) ys
-
-{-# RULES
-"map" [~1] forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs)
-"mapList" [1] forall f. foldr (mapFB (:) f) [] = map f
-"mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f.g)
- #-}
-
- Up to (but not including) phase 1, we use the "map" rule to
- rewrite all saturated applications of map with its
- build/fold form, hoping for fusion to happen. In phase 1 and 0, we
- switch off that rule, inline build, and switch on the
- "mapList" rule, which rewrites the foldr/mapFB thing back
- into plain map.
-
- It's important that these two rules aren't both active at once - (along with build's unfolding) else we'd get an infinite loop - in the rules. Hence the activation control using explicit phase numbers. -
- The "mapFB" rule optimises compositions of map. -
- The mechanism as described above is new in 5.03 since January 2002,
- where the [~N] syntax for phase number
- annotations at rules was introduced. Before that the whole arrangement
- was more complicated, as the corresponding prelude code for version
- 4.08.1 shows:
-
-map :: (a -> b) -> [a] -> [b]
-map = mapList
-
--- Note eta expanded
-mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
-mapFB c f x ys = c (f x) ys
-
-mapList :: (a -> b) -> [a] -> [b]
-mapList _ [] = []
-mapList f (x:xs) = f x : mapList f xs
-
-{-# RULES
-"map" forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs)
-"mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f.g)
-"mapList" forall f. foldr (mapFB (:) f) [] = mapList f
- #-}
-
- This code is structured as it is, because the "map" rule first
- breaks the map open, which exposes it to the various
- foldr/build rules, and if no foldr/build rule matches, the "mapList"
- rule closes it again in a later phase of optimisation - after
- build was inlined. As a consequence, the whole thing depends a bit on
- the timing of the various optimsations (the map might be closed again
- before any of the foldr/build rules fires). To make the timing
- deterministic, build gets a {-# INLINE 2 build
- #-} pragma, which delays build's inlining, and thus,
- the closing of the map. [NB: Phase numbering was forward at that time.]
-
-
- -Last modified: Mon Feb 11 20:00:49 EST 2002 - - - -