Fix Trac #1402: typo in specialiser

This patch fixes a plain bug in the specialiser (rhs_bndrs instead
of rhs_ids) which made GHC crash in obscure cases.

It exposed a case in which we might not do all possible specialisation;
see Note [Specialisation shape].  It's not an important case, but I've
added a warning in DEBUG mode.

Trac #1402.  Test is spec003.hs

diff --git a/compiler/specialise/Specialise.lhs b/compiler/specialise/Specialise.lhs
index 7a0d8bc..fa06742 100644 (file)
--- a/compiler/specialise/Specialise.lhs
+++ b/compiler/specialise/Specialise.lhs
@@ -783,8 +783,8 @@ specDefn :: Subst                   -- Subst to use for RHS
 
 specDefn subst calls (fn, rhs)
        -- The first case is the interesting one
-  |  rhs_tyvars `lengthIs` n_tyvars    -- Rhs of fn's defn has right number of big lambdas
-  && rhs_bndrs  `lengthAtLeast` n_dicts        -- and enough dict args
+  |  rhs_tyvars `lengthIs`     n_tyvars -- Rhs of fn's defn has right number of big lambdas
+  && rhs_ids    `lengthAtLeast` n_dicts        -- and enough dict args
   && notNull calls_for_me              -- And there are some calls to specialise
 
 --   && not (certainlyWillInline (idUnfolding fn))     -- And it's not small
@@ -806,7 +806,9 @@ specDefn subst calls (fn, rhs)
              rhs_uds `plusUDs` plusUDList spec_uds)
 
   | otherwise  -- No calls or RHS doesn't fit our preconceptions
-  = specExpr subst rhs                 `thenSM` \ (rhs', rhs_uds) ->
+  = WARN( notNull calls_for_me, ptext SLIT("Missed specialisation opportunity for") <+> ppr fn )
+         -- Note [Specialisation shape]
+    specExpr subst rhs                 `thenSM` \ (rhs', rhs_uds) ->
     returnSM ((fn, rhs'), [], rhs_uds)
   
   where
@@ -898,10 +900,42 @@ specDefn subst calls (fn, rhs)
 
       where
        my_zipEqual doc xs ys 
-        | not (equalLength xs ys) = pprPanic "my_zipEqual" (ppr xs $$ ppr ys $$ (ppr fn <+> ppr call_ts) $$ ppr rhs)
+#ifdef DEBUG
+        | not (equalLength xs ys) = pprPanic "my_zipEqual" (vcat 
+                                               [ ppr xs, ppr ys
+                                               , ppr fn <+> ppr call_ts
+                                               , ppr (idType fn), ppr theta
+                                               , ppr n_dicts, ppr rhs_dicts 
+                                               , ppr rhs])
+#endif
         | otherwise               = zipEqual doc xs ys
 \end{code}
 
+Note [Specialisation shape]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We only specialise a function if it has visible top-level lambdas
+corresponding to its overloading.  E.g. if
+       f :: forall a. Eq a => ....
+then its body must look like
+       f = /\a. \d. ...
+
+Reason: when specialising the body for a call (f ty dexp), we want to
+substitute dexp for d, and pick up specialised calls in the body of f.
+
+This doesn't always work.  One example I came across was htis:
+       newtype Gen a = MkGen{ unGen :: Int -> a }
+
+       choose :: Eq a => a -> Gen a
+       choose n = MkGen (\r -> n)
+
+       oneof = choose (1::Int)
+
+It's a silly exapmle, but we get
+       choose = /\a. g `cast` co
+where choose doesn't have any dict arguments.  Thus far I have not
+tried to fix this (wait till there's a real example).
+
+
 Note [Inline specialisations]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 We transfer to the specialised function any INLINE stuff from the