add -fsimpleopt-before-flatten

[ghc-hetmet.git] / compiler / stranal / WorkWrap.lhs

diff --git a/compiler/stranal/WorkWrap.lhs b/compiler/stranal/WorkWrap.lhs
index d329b5a..5cf5e92 100644 (file)
--- a/compiler/stranal/WorkWrap.lhs
+++ b/compiler/stranal/WorkWrap.lhs
@@ -141,9 +141,8 @@ the business.
 
 The only reason this is monadised is for the unique supply.
 
-Note [Don't w/w inline things (a)]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
+Note [Don't w/w INLINE things]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It's very important to refrain from w/w-ing an INLINE function (ie one
 with an InlineRule) because the wrapper will then overwrite the
 InlineRule unfolding.
@@ -160,18 +159,45 @@ Notice that we refrain from w/w'ing an INLINE function even if it is
 in a recursive group.  It might not be the loop breaker.  (We could
 test for loop-breaker-hood, but I'm not sure that ever matters.)
 
-Note [Don't w/w inline things (b)]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-In general, we refrain from w/w-ing *small* functions, because they'll
-inline anyway.  But we must take care: it may look small now, but get
-to be big later after other inling has happened.  So we take the
-precaution of adding an INLINE pragma to any such functions.
+Note [Don't w/w INLINABLE things]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If we have
+  {-# INLINABLE f #-}
+  f x y = ....
+then in principle we might get a more efficient loop by w/w'ing f.
+But that would make a new unfolding which would overwrite the old
+one.  So we leave INLINABLE things alone too.
+
+This is a slight infelicity really, because it means that adding
+an INLINABLE pragma could make a program a bit less efficient,
+because you lose the worker/wrapper stuff.  But I don't see a way 
+to avoid that.
+
+Note [Don't w/w inline small non-loop-breaker things]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In general, we refrain from w/w-ing *small* functions, which are not
+loop breakers, because they'll inline anyway.  But we must take care:
+it may look small now, but get to be big later after other inlining
+has happened.  So we take the precaution of adding an INLINE pragma to
+any such functions.
 
 I made this change when I observed a big function at the end of
-compilation with a useful strictness signature but no w-w.  When 
-I measured it on nofib, it didn't make much difference; just a few
-percent improved allocation on one benchmark (bspt/Euclid.space).  
-But nothing got worse.
+compilation with a useful strictness signature but no w-w.  (It was
+small during demand analysis, we refrained from w/w, and then got big
+when something was inlined in its rhs.) When I measured it on nofib,
+it didn't make much difference; just a few percent improved allocation
+on one benchmark (bspt/Euclid.space).  But nothing got worse.
+
+There is an infelicity though.  We may get something like
+      f = g val
+==>
+      g x = case gw x of r -> I# r
+
+      f {- InlineStable, Template = g val -}
+      f = case gw x of r -> I# r
+
+The code for f duplicates that for g, without any real benefit. It
+won't really be executed, because calls to f will go via the inlining.
 
 Note [Wrapper activation]
 ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -259,19 +285,22 @@ tryWW is_rec fn_id rhs
 ---------------------
 checkSize :: Id -> CoreExpr
          -> UniqSM [(Id,CoreExpr)] -> UniqSM [(Id,CoreExpr)]
- -- See Note [Don't w/w inline things (a) and (b)]
 checkSize fn_id rhs thing_inside
-  | isStableUnfolding unfolding           -- For DFuns and INLINE things, leave their
-  = return [ (fn_id, rhs) ]       -- unfolding unchanged; but still attach 
-                                  -- strictness info to the Id 
+  | isStableUnfolding (realIdUnfolding fn_id)
+  = return [ (fn_id, rhs) ]
+      -- See Note [Don't w/w INLINABLE things]
+      -- and Note [Don't w/w INLINABLABLE things]
+      -- NB: use realIdUnfolding because we want to see the unfolding
+      --     even if it's a loop breaker!
 
-  | certainlyWillInline unfolding
+  | certainlyWillInline (idUnfolding fn_id)
   = return [ (fn_id `setIdUnfolding` inline_rule, rhs) ]
-               -- Note [Don't w/w inline things (b)]
+       -- Note [Don't w/w inline small non-loop-breaker things]
+       -- NB: use idUnfolding because we don't want to apply
+       --     this criterion to a loop breaker!
 
   | otherwise = thing_inside
   where
-    unfolding   = idUnfolding fn_id
     inline_rule = mkInlineUnfolding Nothing rhs
 
 ---------------------
@@ -396,6 +425,11 @@ then the splitting will go deeper too.
 --         in case x of 
 --              I# y -> let x = I# y in x }
 -- See comments above. Is it not beautifully short?
+-- Moreover, it works just as well when there are
+-- several binders, and if the binders are lifted
+-- E.g.     x = e
+--     -->  x = let x = e in
+--              case x of (a,b) -> let x = (a,b)  in x
 
 splitThunk :: Var -> Expr Var -> UniqSM [(Var, Expr Var)]
 splitThunk fn_id rhs = do