Add static flag -fsimple-list-literals

The new static flag -fsimple-list-literals makes ExplicitList literals
be desugared in the straightforward way, rather than using 'build' as
now.  See SLPJ comments with Note [Desugaring explicit lists].

I don't expect this flag to be used by users (hence no docs). It's just
there to let me try the performance effects of switching on and off.

3 files changed, 26 insertions, 3 deletions

diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index b91380dcbc..6126b6302e 100644
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -47,6 +47,7 @@ import CoreUtils
 import MkCore
 
 import DynFlags
+import StaticFlags
 import CostCentre
 import Id
 import PrelInfo
@@ -609,6 +610,23 @@ allocation in some nofib programs. Specifically
 
 Of course, if rules aren't turned on then there is pretty much no
 point doing this fancy stuff, and it may even be harmful.
+
+=======>  Note by SLPJ Dec 08.
+
+I'm unconvinced that we should *ever* generate a build for an explicit
+list.  See the comments in GHC.Base about the foldr/cons rule, which 
+points out that (foldr k z [a,b,c]) may generate *much* less code than
+(a `k` b `k` c `k` z).
+
+Furthermore generating builds messes up the LHS of RULES. 
+Example: the foldr/single rule in GHC.Base
+   foldr k z [x] = ...
+We do not want to generate a build invocation on the LHS of this RULE!
+
+To test this I've added a (static) flag -fsimple-list-literals, which
+makes all list literals be generated via the simple route.  
+
+
 \begin{code}
 
 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
@@ -616,7 +634,7 @@ dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
 dsExplicitList elt_ty xs = do
     dflags <- getDOptsDs
     xs' <- mapM dsLExpr xs
-    if not (dopt Opt_EnableRewriteRules dflags)
+    if  opt_SimpleListLiterals || not (dopt Opt_EnableRewriteRules dflags)
         then return $ mkListExpr elt_ty xs'
         else mkBuildExpr elt_ty (mkSplitExplicitList (thisPackage dflags) xs')
   where
diff --git a/compiler/main/StaticFlagParser.hs b/compiler/main/StaticFlagParser.hs
index e68a111a84..0ed93563bd 100644
--- a/compiler/main/StaticFlagParser.hs
+++ b/compiler/main/StaticFlagParser.hs
@@ -159,8 +159,7 @@ static_flags = [
          Supported
 
         -- Pass all remaining "-f<blah>" options to hsc
-  , Flag "f"                      (AnySuffixPred (isStaticFlag) addOpt)
-         Supported
+  , Flag "f" (AnySuffixPred isStaticFlag addOpt) Supported
   ]
 
 isStaticFlag :: String -> Bool
@@ -178,6 +177,7 @@ isStaticFlag f =
     "fno-hi-version-check",
     "dno-black-holing",
     "fno-state-hack",
+    "fsimple-list-literals",
     "fno-ds-multi-tyvar",
     "fruntime-types",
     "fno-pre-inlining",
diff --git a/compiler/main/StaticFlags.hs b/compiler/main/StaticFlags.hs
index 2398c20086..99f3f84ccd 100644
--- a/compiler/main/StaticFlags.hs
+++ b/compiler/main/StaticFlags.hs
@@ -43,6 +43,7 @@ module StaticFlags (
 	-- optimisation opts
 	opt_DsMultiTyVar,
 	opt_NoStateHack,
+        opt_SimpleListLiterals,
 	opt_SpecInlineJoinPoints,
 	opt_CprOff,
 	opt_SimplNoPreInlining,
@@ -227,8 +228,12 @@ opt_DsMultiTyVar		= not (lookUp (fsLit "-fno-ds-multi-tyvar"))
 opt_SpecInlineJoinPoints :: Bool
 opt_SpecInlineJoinPoints	= lookUp  (fsLit "-fspec-inline-join-points")
 
+opt_SimpleListLiterals :: Bool
+opt_SimpleListLiterals	        = lookUp  (fsLit "-fsimple-list-literals")
+
 opt_NoStateHack :: Bool
 opt_NoStateHack			= lookUp  (fsLit "-fno-state-hack")
+
 opt_CprOff :: Bool
 opt_CprOff			= lookUp  (fsLit "-fcpr-off")
 	-- Switch off CPR analysis in the new demand analyser