[project @ 2001-10-31 15:22:53 by simonpj]

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

diff --git a/ghc/compiler/simplCore/SimplMonad.lhs b/ghc/compiler/simplCore/SimplMonad.lhs
index 2b0d527..adaa6c4 100644 (file)
--- a/ghc/compiler/simplCore/SimplMonad.lhs
+++ b/ghc/compiler/simplCore/SimplMonad.lhs
@@ -790,8 +790,19 @@ seems a bit fragile.
 \begin{code}
 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> Bool
 preInlineUnconditionally env top_lvl bndr
---  | isTopLevel top_lvl     = False
---     Top-level fusion lost if we do this for (e.g. string constants)
+  | isTopLevel top_lvl     = False
+-- If we don't have this test, consider
+--     x = length [1,2,3]
+-- The full laziness pass carefully floats all the cons cells to
+-- top level, and preInlineUnconditionally floats them all back in.
+-- Result is (a) static allocation replaced by dynamic allocation
+--          (b) many simplifier iterations because this tickles
+--              a related problem
+-- 
+-- On the other hand, I have seen cases where top-level fusion is
+-- lost if we don't inline top level thing (e.g. string constants)
+-- We'll have to see
+
   | not active                    = False
   | opt_SimplNoPreInlining = False
   | otherwise = case idOccInfo bndr of
@@ -856,15 +867,30 @@ because they don't have bindings, so we must inline them no matter how
 gentle we are being.
 
 \begin{code}
-activeInline :: SimplEnv -> OutId -> Bool
-activeInline env id
+activeInline :: SimplEnv -> OutId -> OccInfo -> Bool
+activeInline env id occ
   = case getMode env of
-       SimplGently -> isDataConWrapId id
-               -- No inlining at all when doing gentle stuff,
-               -- except (hack alert) for data con wrappers
-               -- We want to inline data con wrappers even in gentle mode
-               -- because rule LHSs match better then
-       SimplPhase n -> isActive n (idInlinePragma id)
+      SimplGently -> isOneOcc occ
+       -- No inlining at all when doing gentle stuff,
+       -- except for things that occur once
+       -- The reason is that too little clean-up happens if you 
+       -- don't inline use-once things.   Also a bit of inlining is *good* for
+       -- full laziness; it can expose constant sub-expressions.
+       -- Example in spectral/mandel/Mandel.hs, where the mandelset 
+       -- function gets a useful let-float if you inline windowToViewport
+
+       -- NB: we used to have a second exception, for data con wrappers.
+       -- On the grounds that we use gentle mode for rule LHSs, and 
+       -- they match better when data con wrappers are inlined.
+       -- But that only really applies to the trivial wrappers (like (:)),
+       -- and they are now constructed as Compulsory unfoldings (in MkId)
+       -- so they'll happen anyway.
+
+      SimplPhase n -> isActive n (idInlinePragma id)
+
+-- Belongs in BasicTypes; this frag occurs in OccurAnal too
+isOneOcc (OneOcc _ _) = True
+isOneOcc other       = False
 
 activeRule :: SimplEnv -> Maybe (Activation -> Bool)
 -- Nothing => No rules at all