import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
import Unique ( Unique )
import UniqFM ( keysUFM, intersectsUFM )
-import Util ( mapAndUnzip, mapAccumL )
+import Util ( mapAndUnzip )
import Outputable
+
+import Data.List
\end{code}
-- where df is the exported dictionary. Then df makes a really
-- bad choice for loop breaker
- | is_con_app rhs = 3 -- Data types help with cases
- -- This used to have a lower score than inlineCandidate, but
- -- it's *really* helpful if dictionaries get inlined fast,
- -- so I'm experimenting with giving higher priority to data-typed things
+ | idHasRules bndr = 3
+ -- Avoid things with specialisations; we'd like
+ -- to take advantage of them in the subsequent bindings
+ -- Also vital to avoid risk of divergence:
+ -- Note [Recursive rules]
| inlineCandidate bndr rhs = 2 -- Likely to be inlined
+ -- Note [Inline candidates]
- | idHasRules bndr = 1
- -- Avoid things with specialisations; we'd like
- -- to take advantage of them in the subsequent bindings
+ | is_con_app rhs = 1 -- Data types help with cases
| otherwise = 0
rules_only = bndrs `intersectsUFM` rhs_usg
\end{code}
+Note [Inline candidates]
+~~~~~~~~~~~~~~~~~~~~~~~~
+At one point I gave is_con_app a higher score than inline-candidate,
+on the grounds that "it's *really* helpful if dictionaries get inlined fast".
+However a nofib run revealed no change if they were swapped so that
+inline-candidate has the higher score. And it's important that it does,
+else you can get a bad worker-wrapper split thus:
+ rec {
+ $wfoo x = ....foo x....
+
+ {-loop brk-} foo x = ...$wfoo x...
+ }
+But we *want* the wrapper to be inlined! If it isn't, the interface
+file sees the unfolding for $wfoo, and sees that foo is strict (and
+hence it gets an auto-generated wrapper. Result: an infinite inlining
+in the importing scope. So be a bit careful if you change this. A
+good example is Tree.repTree in nofib/spectral/minimax. If is_con_app
+has the higher score, then compiling Game.hs goes into an infinite loop.
+
+Note [Recursive rules]
+~~~~~~~~~~~~~~~~~~~~~~
+Consider this group, which is typical of what SpecConstr builds:
+
+ fs a = ....f (C a)....
+ f x = ....f (C a)....
+ {-# RULE f (C a) = fs a #-}
+
+So 'f' and 'fs' are mutually recursive. If we choose 'fs' as the loop breaker,
+all is well; the RULE is applied, and 'fs' becomes self-recursive.
+
+But if we choose 'f' as the loop breaker, we may get an infinite loop:
+ - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify
+ - fs is inlined (say it's small)
+ - now there's another opportunity to apply the RULE
+
+So it's very important to choose the RULE-variable as the loop breaker.
+This showed up when compiling Control.Concurrent.Chan.getChanContents.
+
Note [Closure conversion]
~~~~~~~~~~~~~~~~~~~~~~~~~
We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm.