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, 3 insertions, 12 deletions

diff --git a/compiler/GHC/Settings/IO.hs b/compiler/GHC/Settings/IO.hs
index cd1c210ee7..51f101aaad 100644
--- a/compiler/GHC/Settings/IO.hs
+++ b/compiler/GHC/Settings/IO.hs
@@ -48,7 +48,6 @@ initSettings top_dir = do
       libexec :: FilePath -> FilePath
       libexec file = top_dir </> "bin" </> file
       settingsFile = installed "settings"
-      platformConstantsFile = installed "platformConstants"
 
       readFileSafe :: FilePath -> ExceptT SettingsError m String
       readFileSafe path = liftIO (doesFileExist path) >>= \case
@@ -56,16 +55,11 @@ initSettings top_dir = do
         False -> throwE $ SettingsError_MissingData $ "Missing file: " ++ path
 
   settingsStr <- readFileSafe settingsFile
-  platformConstantsStr <- readFileSafe platformConstantsFile
   settingsList <- case maybeReadFuzzy settingsStr of
     Just s -> pure s
     Nothing -> throwE $ SettingsError_BadData $
       "Can't parse " ++ show settingsFile
   let mySettings = Map.fromList settingsList
-  platformConstants <- case maybeReadFuzzy platformConstantsStr of
-    Just s -> pure s
-    Nothing -> throwE $ SettingsError_BadData $
-      "Can't parse " ++ show platformConstantsFile
   -- See Note [Settings file] for a little more about this file. We're
   -- just partially applying those functions and throwing 'Left's; they're
   -- written in a very portable style to keep ghc-boot light.
@@ -91,7 +85,7 @@ initSettings top_dir = do
   cpp_prog <- getToolSetting "Haskell CPP command"
   cpp_args_str <- getSetting "Haskell CPP flags"
 
-  platform <- either pgmError pure $ getTargetPlatform settingsFile mySettings platformConstants
+  platform <- either pgmError pure $ getTargetPlatform settingsFile mySettings
 
   let unreg_cc_args = if platformUnregisterised platform
                       then ["-DNO_REGS", "-DUSE_MINIINTERPRETER"]
@@ -227,17 +221,14 @@ initSettings top_dir = do
       , platformMisc_llvmTarget = llvmTarget
       }
 
-    , sPlatformConstants = platformConstants
-
     , sRawSettings    = settingsList
     }
 
 getTargetPlatform
   :: FilePath     -- ^ Settings filepath (for error messages)
   -> RawSettings  -- ^ Raw settings file contents
-  -> PlatformConstants -- ^ Platform constants
   -> Either String Platform
-getTargetPlatform settingsFile settings constants = do
+getTargetPlatform settingsFile settings = do
   let
     getBooleanSetting = getRawBooleanSetting settingsFile settings
     readSetting :: (Show a, Read a) => String -> Either String a
@@ -265,5 +256,5 @@ getTargetPlatform settingsFile settings constants = do
     , platformIsCrossCompiling = crossCompiling
     , platformLeadingUnderscore = targetLeadingUnderscore
     , platformTablesNextToCode  = tablesNextToCode
-    , platformConstants = constants
+    , platform_constants = Nothing -- will be filled later when loading (or building) the RTS unit
     }