[project @ 2000-11-07 15:21:38 by simonmar]

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

diff --git a/ghc/compiler/simplCore/SimplUtils.lhs b/ghc/compiler/simplCore/SimplUtils.lhs
index 05c989c..5c7d33d 100644 (file)
--- a/ghc/compiler/simplCore/SimplUtils.lhs
+++ b/ghc/compiler/simplCore/SimplUtils.lhs
@@ -635,24 +635,29 @@ tryRhsTyLam rhs thing_inside              -- Only does something if there's a let
                -- where x* has an INLINE prag on it.  Now, once x* is inlined,
                -- the occurrences of x' will be just the occurrences originally
                -- pinned on x.
-               --         poly_info = vanillaIdInfo `setOccInfo` idOccInfo var
        in
        returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
 
-    mk_silly_bind var rhs = NonRec var rhs
+    mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
                -- Suppose we start with:
                --
-               --      x = let g = /\a -> \x -> f x x
-               --          in 
-               --          /\ b -> let g* = g b in E
+               --      x = /\ a -> let g = G in E
                --
-               -- Then:        * the binding for g gets floated out
-               --              * but then it MIGHT get inlined into the rhs of g*
-               --              * then the binding for g* is floated out of the /\b
-               --              * so we're back to square one
-               -- We rely on the simplifier not to inline g into the RHS of g*,
-               -- because it's a "lone" occurrence, and there is no benefit in
-               -- inlining.  But it's a slightly delicate property; hence this comment
+               -- Then we'll float to get
+               --
+               --      x = let poly_g = /\ a -> G
+               --          in /\ a -> let g = poly_g a in E
+               --
+               -- But now the occurrence analyser will see just one occurrence
+               -- of poly_g, not inside a lambda, so the simplifier will
+               -- PreInlineUnconditionally poly_g back into g!  Badk to square 1!
+               -- (I used to think that the "don't inline lone occurrences" stuff
+               --  would stop this happening, but since it's the *only* occurrence,
+               --  PreInlineUnconditionally kicks in first!)
+               --
+               -- Solution: put an INLINE note on g's RHS, so that poly_g seems
+               --           to appear many times.  (NB: mkInlineMe eliminates
+               --           such notes on trivial RHSs, so do it manually.)
 \end{code}