Try harder to avoid making a variable with RULES into a loop-breaker

[ghc-hetmet.git] / compiler / simplCore / OccurAnal.lhs

diff --git a/compiler/simplCore/OccurAnal.lhs b/compiler/simplCore/OccurAnal.lhs
index 4dfd3bf..dc20fd2 100644 (file)
--- a/compiler/simplCore/OccurAnal.lhs
+++ b/compiler/simplCore/OccurAnal.lhs
@@ -300,16 +300,18 @@ reOrderCycle bndrs (bind : binds)
                -- 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
+       | 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]
+
+       | is_con_app rhs = 2    -- 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
 
-       | inlineCandidate bndr rhs = 2  -- Likely to be inlined
-
-       | idHasRules bndr = 1
-               -- Avoid things with specialisations; we'd like
-               -- to take advantage of them in the subsequent bindings
+       | inlineCandidate bndr rhs = 1  -- Likely to be inlined
 
        | otherwise = 0
 
@@ -352,6 +354,25 @@ makeLoopBreaker bndrs rhs_usg bndr
     rules_only = bndrs `intersectsUFM` rhs_usg
 \end{code}
 
+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.