[project @ 1999-06-28 16:40:18 by simonpj]

Add explanatory comments

diff --git a/ghc/compiler/simplCore/SimplCore.lhs b/ghc/compiler/simplCore/SimplCore.lhs
index 2f4aecf..e3ab3d4 100644 (file)
--- a/ghc/compiler/simplCore/SimplCore.lhs
+++ b/ghc/compiler/simplCore/SimplCore.lhs
@@ -209,6 +209,21 @@ simplifyPgm (imported_rule_ids, rule_lhs_fvs)
 
        -- Glom all binds together in one Rec, in case any
        -- transformations have introduced any new dependencies
+       --
+       -- NB: the global invariant is this:
+       --      *** the top level bindings are never cloned, and are always unique ***
+       --
+       -- We sort them into dependency order, but applying transformation rules may
+       -- make something at the top refer to something at the bottom:
+       --      f = \x -> p (q x)
+       --      h = \y -> 3
+       --      
+       --      RULE:  p (q x) = h x
+       --
+       -- Applying this rule makes f refer to h, although it doesn't appear to in the
+       -- source program.  Our solution is to do this occasional glom-together step,
+       -- just once per overall simplfication step.
+
        let { recd_binds = [Rec (flattenBinds binds)] };
 
        (termination_msg, it_count, counts_out, binds') <- iteration us 1 zeroSimplCount recd_binds;