Hadrian: Add support for building stage3

This ticket enables the building of a `stage3` compiler by making the
build logic more consistent and predictable in Hadrian.

Two of the main changes are:

1. In order to build anything at stageN we use the package database
present at stageN. Fixing #16069
2. `haddock` and `ghc-tags` are built
as stage1 executables (with the stage1 compiler) rather than as
stage2 compiler. Fixing
[hadrian#661](https://github.com/snowleopard/hadrian/issues/661)

In order to build a stage3 compiler, you have to set the new `finalStage` hadrian option to `Stage3`.

19 files changed, 130 insertions, 101 deletions

diff --git a/hadrian/README.md b/hadrian/README.md
index 16a26d0cbe..dcb5e68f5d 100644
--- a/hadrian/README.md
+++ b/hadrian/README.md
@@ -183,17 +183,24 @@ $ ./configure [--prefix=PATH] && make install
 
 workflow, for now.
 
+#### Building stage3
+
+It is possible to define a flavour which builds a `stage3` compiler, which is a
+compiler built using `stage2`. This is useful for cross-compilation. Detailed
+instructions can be found in the
+[user settings part of the manual](doc/user-settings.md#specifying-the-final-stage-to-build).
+
 Troubleshooting
 ---------------
 
 Here are a few simple suggestions that might help you fix the build:
-  
+
 * Hadrian is occasionally broken by changes in GHC. If this happens, you might want to switch
   to an earlier GHC commit.
-  
+
 * If Hadrian fails with the message `Configuration file hadrian/cfg/system.config is missing`,
   you have probably forgotten to pass the `--configure` flag during the first build.
-  
+
 * If you need help in debugging Hadrian, read the [wiki](https://github.com/snowleopard/hadrian/wiki)
   and Shake's [debugging tutorial](https://shakebuild.com/debugging).
 
diff --git a/hadrian/cfg/system.config.in b/hadrian/cfg/system.config.in
index 344706d630..7e132f7d77 100644
--- a/hadrian/cfg/system.config.in
+++ b/hadrian/cfg/system.config.in
@@ -93,18 +93,22 @@ project-git-commit-id = @ProjectGitCommitId@
 conf-cc-args-stage0         = @CONF_CC_OPTS_STAGE0@
 conf-cc-args-stage1         = @CONF_CC_OPTS_STAGE1@
 conf-cc-args-stage2         = @CONF_CC_OPTS_STAGE2@
+conf-cc-args-stage3         = @CONF_CC_OPTS_STAGE3@
 
 conf-cpp-args-stage0        = @CONF_CPP_OPTS_STAGE0@
 conf-cpp-args-stage1        = @CONF_CPP_OPTS_STAGE1@
 conf-cpp-args-stage2        = @CONF_CPP_OPTS_STAGE2@
+conf-cpp-args-stage3        = @CONF_CPP_OPTS_STAGE3@
 
 conf-gcc-linker-args-stage0 = @CONF_GCC_LINKER_OPTS_STAGE0@
 conf-gcc-linker-args-stage1 = @CONF_GCC_LINKER_OPTS_STAGE1@
 conf-gcc-linker-args-stage2 = @CONF_GCC_LINKER_OPTS_STAGE2@
+conf-gcc-linker-args-stage3 = @CONF_GCC_LINKER_OPTS_STAGE3@
 
 conf-ld-linker-args-stage0  = @CONF_LD_LINKER_OPTS_STAGE0@
 conf-ld-linker-args-stage1  = @CONF_LD_LINKER_OPTS_STAGE1@
 conf-ld-linker-args-stage2  = @CONF_LD_LINKER_OPTS_STAGE2@
+conf-ld-linker-args-stage3  = @CONF_LD_LINKER_OPTS_STAGE3@
 
 # Include and library directories:
 #=================================
diff --git a/hadrian/doc/cross-compile.md b/hadrian/doc/cross-compile.md
index 724d0e17bd..6bd4fe8ef8 100644
--- a/hadrian/doc/cross-compile.md
+++ b/hadrian/doc/cross-compile.md
@@ -16,7 +16,6 @@ After all the dependencies are in place:
 - `git submodule update --init`
 - `./configure --target=arm-linux-gnueabihf`
 - `cd hadrian`
-- Modify `src/UserSettings.hs`, set `stage1Only` to `True`.
 - Build the compiler by e.g. `./build.sh --flavour=quickest --integer-simple -V -j`
 
 After that, you should have built `inplace/bin/ghc-stage1` cross compiler. We will go to the next section to validate this.
diff --git a/hadrian/doc/user-settings.md b/hadrian/doc/user-settings.md
index 0c6d4ba3cd..6c6aab7a9f 100644
--- a/hadrian/doc/user-settings.md
+++ b/hadrian/doc/user-settings.md
@@ -138,6 +138,22 @@ You can choose which integer library to use when builing GHC using the
 userFlavour :: Flavour
 userFlavour = defaultFlavour { name = "user", integerLibrary = integerSimple }
 ```
+
+### Specifying the final stage to build
+
+The `finalStage` variable can be set to indicate after which stage we should
+stop the compilation pipeline. By default it is set to `Stage2` which indicates
+that we will build everything which uses the `Stage1` `ghc` and then stop.
+
+```
+finalStage :: Stage
+finalStage = Stage2
+```
+
+Using this mechanism we can also build a `Stage3` compiler by setting
+`finalStage = Stage3` or just a `Stage1` compiler by setting
+`finalStage = Stage1`.
+
 ## Build ways
 
 Packages can be built in a number of ways, such as `vanilla`, `profiling` (with
@@ -202,10 +218,6 @@ verboseCommand = return True
 
 ## Miscellaneous
 
-By setting `stage1Only = True` you can disable building Stage2 GHC and Stage2
-utilities such as `haddock`. Note that all Stage0 and Stage1 libraries will
-still be built.
-
 To change the default behaviour of Hadrian with respect to building split
 objects, override the `splitObjects` setting of the `Flavour` record:
 ```haskell
diff --git a/hadrian/src/Base.hs b/hadrian/src/Base.hs
index 68862ed144..cb0dba06ea 100644
--- a/hadrian/src/Base.hs
+++ b/hadrian/src/Base.hs
@@ -83,9 +83,9 @@ generatedPath :: Action FilePath
 generatedPath = buildRoot <&> (-/- generatedDir)
 
 -- | Path to the package database for a given build stage, relative to the build
--- root. Note that @StageN@, where @N > 1@, uses the 'Stage1' package database.
+-- root.
 relativePackageDbPath :: Stage -> FilePath
-relativePackageDbPath stage = stageString (min stage Stage1) -/- "lib/package.conf.d"
+relativePackageDbPath stage = stageString stage -/- "lib/package.conf.d"
 
 -- | Path to the package database used in a given 'Stage', including
 --   the build root.
diff --git a/hadrian/src/Builder.hs b/hadrian/src/Builder.hs
index 85c2c4c280..a3a3f547fd 100644
--- a/hadrian/src/Builder.hs
+++ b/hadrian/src/Builder.hs
@@ -152,8 +152,8 @@ builderProvenance = \case
     Ghc _ Stage0     -> Nothing
     Ghc _ stage      -> context (pred stage) ghc
     GhcPkg _ Stage0  -> Nothing
-    GhcPkg _ _       -> context Stage0 ghcPkg
-    Haddock _        -> context Stage2 haddock
+    GhcPkg _ s       -> context (pred s) ghcPkg
+    Haddock _        -> context Stage1 haddock
     Hpc              -> context Stage1 hpcBin
     Hp2Ps            -> context Stage0 hp2ps
     Hsc2Hs _         -> context Stage0 hsc2hs
diff --git a/hadrian/src/Context.hs b/hadrian/src/Context.hs
index 672b3a6004..8036eb00bc 100644
--- a/hadrian/src/Context.hs
+++ b/hadrian/src/Context.hs
@@ -103,9 +103,9 @@ pkgGhciLibraryFile context@Context {..} = do
 -- | Path to the configuration file of a given 'Context'.
 pkgConfFile :: Context -> Action FilePath
 pkgConfFile Context {..} = do
-    root <- buildRoot
     pid  <- pkgIdentifier package
-    return $ root -/- relativePackageDbPath stage -/- pid <.> "conf"
+    dbPath <- packageDbPath stage
+    return $ dbPath -/- pid <.> "conf"
 
 -- | Given a 'Context' and a 'FilePath' to a source file, compute the 'FilePath'
 -- to its object file. For example:
diff --git a/hadrian/src/Packages.hs b/hadrian/src/Packages.hs
index 24f56908a0..d0fe41765a 100644
--- a/hadrian/src/Packages.hs
+++ b/hadrian/src/Packages.hs
@@ -183,21 +183,23 @@ autogenPath context@Context {..}
   where
     autogen dir = contextPath context <&> (-/- dir -/- "autogen")
 
--- | RTS is considered a Stage1 package.
-rtsContext :: Context
-rtsContext = vanillaContext Stage1 rts
+-- | RTS is considered a Stage1 package. This determines RTS build directory.
+rtsContext :: Stage -> Context
+rtsContext stage = vanillaContext stage rts
 
 -- | Path to the RTS build directory.
-rtsBuildPath :: Action FilePath
-rtsBuildPath = buildPath rtsContext
+rtsBuildPath :: Stage -> Action FilePath
+rtsBuildPath stage = buildPath (rtsContext stage)
 
--- | The 'libffi' library is considered a 'Stage1' package.
-libffiContext :: Context
-libffiContext = vanillaContext Stage1 libffi
+-- | Build directory for libffi
+-- This probably doesn't need to be stage dependent but it is for
+-- consistency for now.
+libffiContext :: Stage -> Context
+libffiContext stage = vanillaContext stage libffi
 
 -- | Build directory for in-tree 'libffi' library.
-libffiBuildPath :: Action FilePath
-libffiBuildPath = buildPath libffiContext
+libffiBuildPath :: Stage -> Action FilePath
+libffiBuildPath stage = buildPath (libffiContext stage)
 
 -- | Name of the 'libffi' library.
 libffiLibraryName :: Action FilePath
diff --git a/hadrian/src/Rules.hs b/hadrian/src/Rules.hs
index e51dae4e07..f634f22828 100644
--- a/hadrian/src/Rules.hs
+++ b/hadrian/src/Rules.hs
@@ -30,18 +30,22 @@ allStages :: [Stage]
 allStages = [minBound .. maxBound]
 
 -- | This rule calls 'need' on all top-level build targets that Hadrian builds
--- by default, respecting the 'stage1Only' flag.
+-- by default, respecting the 'finalStage' flag.
 topLevelTargets :: Rules ()
 topLevelTargets = action $ do
     verbosity <- getVerbosity
-    when (verbosity >= Loud) $ do
-        (libraries, programs) <- partition isLibrary <$> stagePackages Stage1
-        libNames <- mapM (name Stage1) libraries
-        pgmNames <- mapM (name Stage1) programs
+    forM_ [ Stage1 ..] $ \stage -> do
+      when (verbosity >= Loud) $ do
+        (libraries, programs) <- partition isLibrary <$> stagePackages stage
+        libNames <- mapM (name stage) libraries
+        pgmNames <- mapM (name stage) programs
+        let stageHeader t ps =
+              "| Building " ++ show stage ++ " "
+                            ++ t ++ ": " ++ intercalate ", " ps
         putNormal . unlines $
-            [ "| Building Stage1 libraries: " ++ intercalate ", " libNames
-            , "| Building Stage1 programs : " ++ intercalate ", " pgmNames ]
-    let buildStages = [Stage0, Stage1] ++ [Stage2 | not stage1Only]
+            [ stageHeader "libraries" libNames
+            , stageHeader "programs" pgmNames ]
+    let buildStages = [ s | s <- [Stage0 ..], s < finalStage ]
     targets <- concatForM buildStages $ \stage -> do
         packages <- stagePackages stage
         mapM (path stage) packages
@@ -101,7 +105,7 @@ packageRules = do
     Rules.Program.buildProgramRules readPackageDb
     Rules.Register.configurePackageRules
 
-    forM_ [Stage0, Stage1] (Rules.Register.registerPackageRules writePackageDb)
+    forM_ [Stage0 ..] (Rules.Register.registerPackageRules writePackageDb)
 
     -- TODO: Can we get rid of this enumeration of contexts? Since we iterate
     --       over it to generate all 4 types of rules below, all the time, we
diff --git a/hadrian/src/Rules/BinaryDist.hs b/hadrian/src/Rules/BinaryDist.hs
index 9cf442c3f5..a589c7af98 100644
--- a/hadrian/src/Rules/BinaryDist.hs
+++ b/hadrian/src/Rules/BinaryDist.hs
@@ -118,9 +118,9 @@ bindistRules = do
         need ["docs"]
         copyDirectory (root -/- "docs") bindistFilesDir
 
-        -- We copy the binary (<build root>/stage2/bin/haddock) to
+        -- We copy the binary (<build root>/stage1/bin/haddock) to
         -- the bindist's bindir (<build root>/bindist/ghc-.../bin/).
-        haddockPath <- programPath (vanillaContext Stage2 haddock)
+        haddockPath <- programPath (vanillaContext Stage1 haddock)
         copyFile haddockPath (bindistFilesDir -/- "bin" -/- "haddock")
 
         -- We then 'need' all the files necessary to configure and install
diff --git a/hadrian/src/Rules/Generate.hs b/hadrian/src/Rules/Generate.hs
index 7c5989908a..e03575347b 100644
--- a/hadrian/src/Rules/Generate.hs
+++ b/hadrian/src/Rules/Generate.hs
@@ -58,7 +58,7 @@ compilerDependencies = do
     isGmp   <- (== integerGmp) <$> getIntegerPackage
     ghcPath <- expr $ buildPath (vanillaContext stage compiler)
     gmpPath <- expr gmpBuildPath
-    rtsPath <- expr rtsBuildPath
+    rtsPath <- expr (rtsBuildPath stage)
     mconcat [ return [root -/- platformH stage]
             , return ((root -/-) <$> includesDependencies)
             , return ((root -/-) <$> derivedConstantsDependencies)
@@ -84,7 +84,8 @@ compilerDependencies = do
 generatedDependencies :: Expr [FilePath]
 generatedDependencies = do
     root    <- getBuildRoot
-    rtsPath <- expr rtsBuildPath
+    stage   <- getStage
+    rtsPath <- expr (rtsBuildPath stage)
     mconcat [ package compiler ? compilerDependencies
             , package ghcPrim  ? ghcPrimDependencies
             , package rts      ? return (fmap (rtsPath -/-) libffiDependencies
@@ -188,7 +189,7 @@ generateRules = do
     -- TODO: simplify, get rid of fake rts context
     root -/- generatedDir ++ "//*" %> \file -> do
         withTempDir $ \dir -> build $
-            target rtsContext DeriveConstants [] [file, dir]
+            target (rtsContext Stage1) DeriveConstants [] [file, dir]
   where
     file <~ gen = file %> \out -> generate out emptyTarget gen
 
diff --git a/hadrian/src/Rules/Libffi.hs b/hadrian/src/Rules/Libffi.hs
index 58ac1efbdc..465065e7ce 100644
--- a/hadrian/src/Rules/Libffi.hs
+++ b/hadrian/src/Rules/Libffi.hs
@@ -13,11 +13,11 @@ libffiDependencies = ["ffi.h", "ffitarget.h"]
 libffiLibrary :: FilePath
 libffiLibrary = "inst/lib/libffi.a"
 
-rtsLibffiLibrary :: Way -> Action FilePath
-rtsLibffiLibrary way = do
+rtsLibffiLibrary :: Stage -> Way -> Action FilePath
+rtsLibffiLibrary stage way = do
     name    <- libffiLibraryName
     suf     <- libsuf way
-    rtsPath <- rtsBuildPath
+    rtsPath <- rtsBuildPath stage
     return $ rtsPath -/- "lib" ++ name ++ suf
 
 fixLibffiMakefile :: FilePath -> String -> String
@@ -27,33 +27,35 @@ fixLibffiMakefile top =
     . replace "@INSTALL@" ("$(subst ../install-sh," ++ top ++ "/install-sh,@INSTALL@)")
 
 -- TODO: check code duplication w.r.t. ConfCcArgs
-configureEnvironment :: Action [CmdOption]
-configureEnvironment = do
-    cFlags  <- interpretInContext libffiContext $ mconcat
+configureEnvironment :: Stage -> Action [CmdOption]
+configureEnvironment stage = do
+    cFlags  <- interpretInContext (libffiContext stage) $ mconcat
                [ cArgs
                , getStagedSettingList ConfCcArgs ]
-    ldFlags <- interpretInContext libffiContext ldArgs
-    sequence [ builderEnvironment "CC" $ Cc CompileC Stage1
-             , builderEnvironment "CXX" $ Cc CompileC Stage1
-             , builderEnvironment "LD" (Ld Stage1)
-             , builderEnvironment "AR" (Ar Unpack Stage1)
+    ldFlags <- interpretInContext (libffiContext stage) ldArgs
+    sequence [ builderEnvironment "CC" $ Cc CompileC stage
+             , builderEnvironment "CXX" $ Cc CompileC stage
+             , builderEnvironment "LD" (Ld stage)
+             , builderEnvironment "AR" (Ar Unpack stage)
              , builderEnvironment "NM" Nm
              , builderEnvironment "RANLIB" Ranlib
              , return . AddEnv  "CFLAGS" $ unwords  cFlags ++ " -w"
              , return . AddEnv "LDFLAGS" $ unwords ldFlags ++ " -w" ]
 
 libffiRules :: Rules ()
-libffiRules = do
-    root <- buildRootRules
-    fmap ((root <//> "rts/build") -/-) libffiDependencies &%> \_ -> do
-        libffiPath <- libffiBuildPath
-        need [libffiPath -/- libffiLibrary]
+libffiRules =
+    forM_ [Stage1 ..] $ \stage -> do
+      root <- buildRootRules
+      fmap ((root -/- stageString stage -/- "rts/build") -/-) libffiDependencies
+          &%> \_ -> do
+            libffiPath <- libffiBuildPath stage
+            need [libffiPath -/- libffiLibrary]
 
     -- we set a higher priority because this overlaps
     -- with the static lib rule from Rules.Library.libraryRules.
-    priority 2.0 $ root <//> libffiLibrary %> \_ -> do
+      priority 2.0 $ root -/- stageString stage <//> libffiLibrary %> \_ -> do
         useSystemFfi <- flag UseSystemFfi
-        rtsPath      <- rtsBuildPath
+        rtsPath      <- rtsBuildPath stage
         if useSystemFfi
         then do
             ffiIncludeDir <- setting FfiIncludeDir
@@ -62,22 +64,23 @@ libffiRules = do
                 copyFile (ffiIncludeDir -/- file) (rtsPath -/- file)
             putSuccess "| Successfully copied system FFI library header files"
         else do
-            libffiPath <- libffiBuildPath
-            build $ target libffiContext (Make libffiPath) [] []
+            libffiPath <- libffiBuildPath stage
+            build $ target (libffiContext stage) (Make libffiPath) [] []
 
             hs <- getDirectoryFiles "" [libffiPath -/- "inst/include/*"]
             forM_ hs $ \header ->
                 copyFile header (rtsPath -/- takeFileName header)
 
-            ways <- interpretInContext libffiContext (getLibraryWays <> getRtsWays)
+            ways <- interpretInContext (libffiContext stage)
+                                       (getLibraryWays <> getRtsWays)
             forM_ (nubOrd ways) $ \way -> do
-                rtsLib <- rtsLibffiLibrary way
+                rtsLib <- rtsLibffiLibrary stage way
                 copyFileUntracked (libffiPath -/- libffiLibrary) rtsLib
 
             putSuccess "| Successfully built custom library 'libffi'"
 
-    root <//> "libffi/build/Makefile.in" %> \mkIn -> do
-        libffiPath <- libffiBuildPath
+      root -/- stageString stage -/- "libffi/build/Makefile.in" %> \mkIn -> do
+        libffiPath <- libffiBuildPath stage
         removeDirectory libffiPath
         tarball <- unifyPath . fromSingleton "Exactly one LibFFI tarball is expected"
                <$> getDirectoryFiles "" ["libffi-tarballs/libffi*.tar.gz"]
@@ -90,20 +93,21 @@ libffiRules = do
         removeDirectory (root -/- libname)
         -- TODO: Simplify.
         actionFinally (do
-            build $ target libffiContext (Tar Extract) [tarball] [root]
-            moveDirectory (root -/- libname) libffiPath) $
-                removeFiles root [libname <//> "*"]
+          build $ target (libffiContext stage) (Tar Extract)
+                                                  [tarball]
+                                                  [root -/- stageString stage]
+          moveDirectory (root -/- stageString stage -/- libname) libffiPath)  $
+            removeFiles (root -/- stageString stage) [libname <//> "*"]
 
         top <- topDirectory
         fixFile mkIn (fixLibffiMakefile top)
 
-    -- TODO: Get rid of hard-coded @libffi@.
-    root <//> "libffi/build/Makefile" %> \mk -> do
+      -- TODO: Get rid of hard-coded @libffi@.
+      root -/- stageString stage -/- "libffi/build/Makefile" %> \mk -> do
         need [mk <.> "in"]
-        libffiPath <- libffiBuildPath
-        forM_ ["config.guess", "config.sub"] $ \file ->
+        libffiPath <- libffiBuildPath stage
+        forM_ ["config.guess", "config.sub"] $ \file -> do
             copyFile file (libffiPath -/- file)
-
-        env <- configureEnvironment
+        env <- configureEnvironment stage
         buildWithCmdOptions env $
-            target libffiContext (Configure libffiPath) [mk <.> "in"] [mk]
+          target (libffiContext stage) (Configure libffiPath) [mk <.> "in"] [mk]
diff --git a/hadrian/src/Rules/Program.hs b/hadrian/src/Rules/Program.hs
index 32e74967d1..9d8b6d0a0c 100644
--- a/hadrian/src/Rules/Program.hs
+++ b/hadrian/src/Rules/Program.hs
@@ -88,7 +88,7 @@ buildBinary rs bin context@Context {..} = do
     needLibrary =<< contextDependencies context
     when (stage > Stage0) $ do
         ways <- interpretInContext context (getLibraryWays <> getRtsWays)
-        needLibrary [ rtsContext { way = w } | w <- ways ]
+        needLibrary [ (rtsContext stage) { way = w } | w <- ways ]
     cSrcs  <- interpretInContext context (getContextData cSrcs)
     cObjs  <- mapM (objectPath context) cSrcs
     hsObjs <- hsObjects context
diff --git a/hadrian/src/Settings/Builders/Common.hs b/hadrian/src/Settings/Builders/Common.hs
index 6846c4bc8d..cfe0911292 100644
--- a/hadrian/src/Settings/Builders/Common.hs
+++ b/hadrian/src/Settings/Builders/Common.hs
@@ -61,9 +61,8 @@ packageDatabaseArgs = do
     stage <- getStage
     dbPath <- expr (packageDbPath stage)
     expr (need [dbPath -/- packageDbStamp])
-    root <- getBuildRoot
     prefix <- ifM (builder Ghc) (return "-package-db ") (return "--package-db=")
-    arg $ prefix ++ root -/- relativePackageDbPath stage
+    arg $ prefix ++ dbPath
 
 bootPackageDatabaseArgs :: Args
 bootPackageDatabaseArgs = do
diff --git a/hadrian/src/Settings/Builders/Configure.hs b/hadrian/src/Settings/Builders/Configure.hs
index 068591dfbb..214aed6c27 100644
--- a/hadrian/src/Settings/Builders/Configure.hs
+++ b/hadrian/src/Settings/Builders/Configure.hs
@@ -7,7 +7,8 @@ import Settings.Builders.Common
 configureBuilderArgs :: Args
 configureBuilderArgs = do
     gmpPath    <- expr gmpBuildPath
-    libffiPath <- expr libffiBuildPath
+    stage      <- getStage
+    libffiPath <- expr (libffiBuildPath stage)
     mconcat [ builder (Configure gmpPath) ? do
                 hostPlatform  <- getSetting HostPlatform
                 buildPlatform <- getSetting BuildPlatform
diff --git a/hadrian/src/Settings/Builders/Make.hs b/hadrian/src/Settings/Builders/Make.hs
index 102ba54845..0433888279 100644
--- a/hadrian/src/Settings/Builders/Make.hs
+++ b/hadrian/src/Settings/Builders/Make.hs
@@ -10,11 +10,12 @@ makeBuilderArgs :: Args
 makeBuilderArgs = do
     threads    <- shakeThreads <$> expr getShakeOptions
     gmpPath    <- expr gmpBuildPath
-    libffiPath <- expr libffiBuildPath
+    libffiPaths <- forM [Stage1 ..] $ \s -> expr (libffiBuildPath s)
     let t = show $ max 4 (threads - 2) -- Don't use all Shake's threads
-    mconcat
-        [ builder (Make gmpPath   ) ? pure ["MAKEFLAGS=-j" ++ t]
-        , builder (Make libffiPath) ? pure ["MAKEFLAGS=-j" ++ t, "install"] ]
+    mconcat $
+        (builder (Make gmpPath   ) ? pure ["MAKEFLAGS=-j" ++ t]) :
+        [ builder (Make libffiPath) ? pure ["MAKEFLAGS=-j" ++ t, "install"]
+        | libffiPath <- libffiPaths ]
 
 validateBuilderArgs :: Args
 validateBuilderArgs = builder (Make "testsuite/tests") ? do
diff --git a/hadrian/src/Settings/Default.hs b/hadrian/src/Settings/Default.hs
index b0e269ce1b..2cadc4fc79 100644
--- a/hadrian/src/Settings/Default.hs
+++ b/hadrian/src/Settings/Default.hs
@@ -108,7 +108,9 @@ stage1Packages = do
              , stm
              , time
              , unlit
-             , xhtml                         ]
+             , xhtml
+             , ghcTags                       ]
+          ++ [ haddock | not cross           ]
           ++ [ hpcBin   | not cross          ]
           ++ [ iserv    | not win, not cross ]
           ++ [ libiserv | not win, not cross ]
@@ -119,10 +121,7 @@ stage1Packages = do
 
 -- | Packages built in 'Stage2' by default. You can change this in "UserSettings".
 stage2Packages :: Action [Package]
-stage2Packages = do
-    cross <- flag CrossCompiling
-    return $ [ ghcTags             ]
-          ++ [ haddock | not cross ]
+stage2Packages = stage1Packages
 
 -- | Packages that are built only for the testsuite.
 testsuitePackages :: Action [Package]
diff --git a/hadrian/src/UserSettings.hs b/hadrian/src/UserSettings.hs
index e029dadda7..8e0b4fc769 100644
--- a/hadrian/src/UserSettings.hs
+++ b/hadrian/src/UserSettings.hs
@@ -4,7 +4,7 @@
 -- accidentally commit them.
 module UserSettings (
     userFlavours, userPackages, userDefaultFlavour,
-    verboseCommand, buildProgressColour, successColour, stage1Only
+    verboseCommand, buildProgressColour, successColour, finalStage
     ) where
 
 import Flavour
@@ -50,10 +50,9 @@ buildProgressColour = mkBuildProgressColour (Dull Magenta)
 successColour :: SuccessColour
 successColour = mkSuccessColour (Dull Green)
 
--- TODO: Set this flag from the command line.
--- | Set this flag to 'True' to disable building Stage2 GHC and Stage2 utilities
--- such as @haddock@. All Stage0 and Stage1 libraries will still be built.
--- Also see Note [No stage2 packages when CrossCompiling or Stage1Only] in the
--- top-level @ghc.mk@.
-stage1Only :: Bool
-stage1Only = False
+-- | Stop after building the StageN compiler.
+-- For example, setting the 'finalStage' to 'Stage1' will just build the
+-- 'Stage1' compiler. Setting it to 'Stage3' will build the 'Stage3'
+-- compiler. Setting it to 'Stage0' will mean nothing gets built at all.
+finalStage :: Stage
+finalStage = Stage2
diff --git a/hadrian/src/Utilities.hs b/hadrian/src/Utilities.hs
index d31f6cc2d0..2cc7a6e368 100644
--- a/hadrian/src/Utilities.hs
+++ b/hadrian/src/Utilities.hs
@@ -30,24 +30,21 @@ askWithResources rs target = H.askWithResources rs target getArgs
 
 -- TODO: Cache the computation.
 -- | Given a 'Context' this 'Action' looks up the package dependencies and wraps
--- the results in appropriate contexts. The only subtlety here is that we never
--- depend on packages built in 'Stage2' or later, therefore the stage of the
--- resulting dependencies is bounded from above at 'Stage1'. To compute package
--- dependencies we transitively scan Cabal files using 'pkgDependencies' defined
--- in "Hadrian.Haskell.Cabal".
+-- the results in appropriate contexts.
+-- To compute package dependencies we transitively scan Cabal files using
+-- 'pkgDependencies' defined in "Hadrian.Haskell.Cabal".
 contextDependencies :: Context -> Action [Context]
 contextDependencies Context {..} = do
     depPkgs <- go [package]
-    return [ Context depStage pkg way | pkg <- depPkgs, pkg /= package ]
+    return [ Context stage pkg way | pkg <- depPkgs, pkg /= package ]
   where
-    depStage = min stage Stage1
     go pkgs  = do
         deps <- concatMapM step pkgs
         let newPkgs = nubOrd $ sort (deps ++ pkgs)
         if pkgs == newPkgs then return pkgs else go newPkgs
     step pkg = do
         deps   <- pkgDependencies pkg
-        active <- sort <$> stagePackages depStage
+        active <- sort <$> stagePackages stage
         return $ intersectOrd (compare . pkgName) active deps
 
 cabalDependencies :: Context -> Action [String]