X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fprelude%2FPrelRules.lhs;h=165d0088a7fea16d4789f77d0a23f26a083ea8a6;hb=2eb04ca0f8d0ec72b417cddc60672c696b4a3daa;hp=69901d34f69ebf381dde865b8a3c250cfd6511b8;hpb=1ca40c2037a0e973dd73d98cd20313ecdbfeb6fc;p=ghc-hetmet.git

diff --git a/compiler/prelude/PrelRules.lhs b/compiler/prelude/PrelRules.lhs
index 69901d3..165d008 100644
--- a/compiler/prelude/PrelRules.lhs
+++ b/compiler/prelude/PrelRules.lhs
@@ -44,9 +44,7 @@ import Name		( Name, nameOccName )
 import Outputable
 import FastString
 import StaticFlags      ( opt_SimplExcessPrecision )
-
-import Data.Bits as Bits	( Bits(..), shiftL, shiftR )
-	-- shiftL and shiftR were not always methods of Bits
+import Data.Bits as Bits
 import Data.Word	( Word )
 \end{code}
 
@@ -447,6 +445,33 @@ dataToTagRule other = Nothing
 %*									*
 %************************************************************************
 
+Note [Scoping for Builtin rules]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When compiling a (base-package) module that defines one of the
+functions mentioned in the RHS of a built-in rule, there's a danger
+that we'll see
+
+	f = ...(eq String x)....
+
+	....and lower down...
+
+	eqString = ...
+
+Then a rewrite would give
+
+	f = ...(eqString x)...
+	....and lower down...
+	eqString = ...
+
+and lo, eqString is not in scope.  This only really matters when we get to code
+generation.  With -O we do a GlomBinds step that does a new SCC analysis on the whole
+set of bindings, which sorts out the dependency.  Without -O we don't do any rule
+rewriting so again we are fine.
+
+(This whole thing doesn't show up for non-built-in rules because their dependencies
+are explicit.)
+
+
 \begin{code}
 builtinRules :: [CoreRule]
 -- Rules for non-primops that can't be expressed using a RULE pragma