[project @ 2001-08-24 13:06:36 by simonpj]

------------------------------
	Another demand analyser glitch
	------------------------------

This fixes, I hope, (in an unsatisfactory way) another demand-analyser
glitch, that resulted in a runtime "entered absent arg" error.

In DmdAnal.annotateLamIdBndr, we now have:

	-- This gross hack is needed because otherwise we label
	-- a lambda binder with demand 'B'.  But in terms of calling
	-- conventions that's Abs, because we don't pass it.  But
	-- when we do a w/w split we get
	--	fw x = (\x y:B -> ...) x (error "oops")
	-- And then the simplifier things the 'B' is a strict demand
	-- and evaluates the (error "oops").  Sigh

The hack zaps 'B' to 'Abs', just like 'lazify' does.

Need to talk to John about what the principled reasons are.

diff --git a/ghc/compiler/stranal/DmdAnal.lhs b/ghc/compiler/stranal/DmdAnal.lhs
index 2bae197..e2b3ed9 100644 (file)
--- a/ghc/compiler/stranal/DmdAnal.lhs
+++ b/ghc/compiler/stranal/DmdAnal.lhs
@@ -481,9 +481,20 @@ annotateLamIdBndr dmd_ty@(DmdType fv ds res) id
 -- For lambdas we add the demand to the argument demands
 -- Only called for Ids
   = ASSERT( isId id )
-    (DmdType fv' (dmd:ds) res, setIdNewDemandInfo id dmd)
+    (DmdType fv' (hacked_dmd:ds) res, setIdNewDemandInfo id hacked_dmd)
   where
     (fv', dmd) = removeFV fv id res
+    hacked_dmd = case dmd of
+                   Bot   -> Abs
+                   Err   -> Abs
+                   other -> dmd
+       -- This gross hack is needed because otherwise we label
+       -- a lambda binder with demand 'B'.  But in terms of calling
+       -- conventions that's Abs, because we don't pass it.  But
+       -- when we do a w/w split we get
+       --      fw x = (\x y:B -> ...) x (error "oops")
+       -- And then the simplifier things the 'B' is a strict demand
+       -- and evaluates the (error "oops").  Sigh
 
 removeFV fv var res = (fv', dmd)
                where
@@ -603,6 +614,7 @@ defer = lub Abs
 
 lazify :: Demand -> Demand
 -- The 'Defer' demands are just Lazy at function boundaries
+-- Ugly!  Ask John how to improve it.
 lazify (Seq k Defer ds) = Lazy
 lazify (Seq k Now   ds) = Seq k Now (map lazify ds)
 lazify Bot             = Abs   -- Don't pass args that are consumed by bottom/err