Constants: add a note and fix minor doc glitches

1 files changed, 16 insertions, 24 deletions

diff --git a/compiler/GHC/Unit/State.hs b/compiler/GHC/Unit/State.hs
index 4a1cd29b25..fe4796d1fc 100644
--- a/compiler/GHC/Unit/State.hs
+++ b/compiler/GHC/Unit/State.hs
@@ -77,7 +77,6 @@ import GHC.Driver.Session
 
 import GHC.Platform
 import GHC.Platform.Ways
-import GHC.Platform.Constants
 
 import GHC.Unit.Database
 import GHC.Unit.Info
@@ -596,29 +595,22 @@ initUnits logger dflags cached_dbs = do
                              (homeUnitInstanceOf_ dflags)
                              (homeUnitInstantiations_ dflags)
 
-  -- try to find platform constants
-  mconstants <- do
-    let
-      try_parse d = do
-          let p = d </> "DerivedConstants.h"
-          doesFileExist p >>= \case
-            True  -> Just <$> parseConstantsHeader p
-            False -> return Nothing
-
-      find_constants []     = return Nothing
-      find_constants (x:xs) = try_parse x >>= \case
-          Nothing -> find_constants xs
-          Just c  -> return (Just c)
-
-    if homeUnitId_ dflags == rtsUnitId
-      then do
-        -- we're building the RTS! Try to find the header in its includes
-        find_constants (includePathsGlobal (includePaths dflags))
-      else
-        -- try to find the platform constants in the RTS unit
-        case lookupUnitId unit_state rtsUnitId of
-          Nothing   -> return Nothing
-          Just info -> find_constants (fmap ST.unpack (unitIncludeDirs info))
+  -- Try to find platform constants
+  --
+  -- See Note [Platform constants] in GHC.Platform
+  mconstants <- if homeUnitId_ dflags == rtsUnitId
+    then do
+      -- we're building the RTS! Lookup DerivedConstants.h in the include paths
+      lookupPlatformConstants (includePathsGlobal (includePaths dflags))
+    else
+      -- lookup the DerivedConstants.h header bundled with the RTS unit. We
+      -- don't fail if we can't find the RTS unit as it can be a valid (but
+      -- uncommon) case, e.g. building a C utility program (not depending on the
+      -- RTS) before building the RTS. In any case, we will fail later on if we
+      -- really need to use the platform constants but they have not been loaded.
+      case lookupUnitId unit_state rtsUnitId of
+        Nothing   -> return Nothing
+        Just info -> lookupPlatformConstants (fmap ST.unpack (unitIncludeDirs info))
 
   return (dbs,unit_state,home_unit,mconstants)