Read constants header instead of global platformConstants

With this patch we switch from reading the globally installed
platformConstants file to reading the DerivedConstants.h header file
that is bundled in the RTS unit.  When we build the RTS unit itself, we
get it from its includes directories.

The new parser is more efficient and strict than the Read instance for
PlatformConstants and we get about 2.2MB less allocations in every
cases. However it only really shows in tests that don't allocate much,
hence the following metric decreases.

Metric Decrease:
    Naperian
    T10421
    T10547
    T12150
    T12234
    T12425
    T13035
    T18304
    T18923
    T5837
    T6048
    T18140

1 files changed, 14 insertions, 9 deletions

diff --git a/compiler/GHC.hs b/compiler/GHC.hs
index 134580c653..59f49453ed 100644
--- a/compiler/GHC.hs
+++ b/compiler/GHC.hs
@@ -639,11 +639,13 @@ checkBrokenTablesNextToCode' logger dflags
 setSessionDynFlags :: GhcMonad m => DynFlags -> m ()
 setSessionDynFlags dflags0 = do
   logger <- getLogger
-  dflags <- checkNewDynFlags logger dflags0
+  dflags1 <- checkNewDynFlags logger dflags0
   hsc_env <- getSession
   let old_unit_env    = hsc_unit_env hsc_env
   let cached_unit_dbs = ue_unit_dbs old_unit_env
-  (dbs,unit_state,home_unit) <- liftIO $ initUnits logger dflags cached_unit_dbs
+  (dbs,unit_state,home_unit,mconstants) <- liftIO $ initUnits logger dflags1 cached_unit_dbs
+
+  dflags <- liftIO $ updatePlatformConstants dflags1 mconstants
 
   -- Interpreter
   interp <- if gopt Opt_ExternalInterpreter dflags
@@ -711,27 +713,30 @@ setProgramDynFlags dflags = setProgramDynFlags_ True dflags
 setProgramDynFlags_ :: GhcMonad m => Bool -> DynFlags -> m Bool
 setProgramDynFlags_ invalidate_needed dflags = do
   logger <- getLogger
-  dflags' <- checkNewDynFlags logger dflags
+  dflags0 <- checkNewDynFlags logger dflags
   dflags_prev <- getProgramDynFlags
-  let changed = packageFlagsChanged dflags_prev dflags'
+  let changed = packageFlagsChanged dflags_prev dflags0
   if changed
     then do
         old_unit_env <- hsc_unit_env <$> getSession
         let cached_unit_dbs = ue_unit_dbs old_unit_env
-        (dbs,unit_state,home_unit) <- liftIO $ initUnits logger dflags' cached_unit_dbs
+        (dbs,unit_state,home_unit,mconstants) <- liftIO $ initUnits logger dflags0 cached_unit_dbs
+
+        dflags1 <- liftIO $ updatePlatformConstants dflags0 mconstants
+
         let unit_env = UnitEnv
-              { ue_platform  = targetPlatform dflags'
-              , ue_namever   = ghcNameVersion dflags'
+              { ue_platform  = targetPlatform dflags1
+              , ue_namever   = ghcNameVersion dflags1
               , ue_home_unit = Just home_unit
               , ue_hpt       = ue_hpt old_unit_env
               , ue_eps       = ue_eps old_unit_env
               , ue_units     = unit_state
               , ue_unit_dbs  = Just dbs
               }
-        modifySession $ \h -> h{ hsc_dflags   = dflags'
+        modifySession $ \h -> h{ hsc_dflags   = dflags1
                                , hsc_unit_env = unit_env
                                }
-    else modifySession $ \h -> h{ hsc_dflags = dflags' }
+    else modifySession $ \h -> h{ hsc_dflags = dflags0 }
   when invalidate_needed $ invalidateModSummaryCache
   return changed