Update Hadrian

* support ghc-bignum backend selection in flavours and command-line

* support ghc-bignum "--check" flag (compare results of selected backend
  against results of the native one) in flavours and command-line (e.g.
  pass --bignum=check-gmp" to check the "gmp" backend)

* remove the hack to workaround #15286

* build GMP only when the gmp backend is used

* remove hacks to workaround `text` package flags about integer-*. We
  fix `text` to use ghc-bignum unconditionally in another patch

22 files changed, 137 insertions, 132 deletions

diff --git a/hadrian/doc/make.md b/hadrian/doc/make.md
index b0e19e4721..318b736fa5 100644
--- a/hadrian/doc/make.md
+++ b/hadrian/doc/make.md
@@ -94,17 +94,6 @@ time you fire up a build. This is not possible with the Make build system.
   ```
   See [flavours documentation](https://gitlab.haskell.org/ghc/ghc/blob/master/hadrian/doc/flavours.md) for info on flavours.
 
-- Building with `integer-simple` as the integer library
-
-  ``` sh
-  # Make
-  echo "INTEGER_LIBRARY=integer-simple" >> mk/build.mk
-  make
-
-  # Hadrian
-  build --integer-simple
-  ```
-
 - Freezing the stage 1 GHC compiler
 
   ``` sh
diff --git a/hadrian/doc/user-settings.md b/hadrian/doc/user-settings.md
index 9963bac5ed..ada2b98760 100644
--- a/hadrian/doc/user-settings.md
+++ b/hadrian/doc/user-settings.md
@@ -21,8 +21,10 @@ data Flavour = Flavour {
     args :: Args,
     -- | Build these packages.
     packages :: Stage -> Action [Package],
-    -- | Either 'integerGmp' or 'integerSimple'.
-    integerLibrary :: Action Package,
+    -- | Bignum backend: 'native', 'gmp', 'ffi', etc.
+    bignumBackend :: String,
+    -- | Check bignum backend against native
+    bignumCheck :: Bool,
     -- | Build libraries these ways.
     libraryWays :: Ways,
     -- | Build RTS these ways.
@@ -168,12 +170,12 @@ userPackage = library "user-package"
 You will also need to add `userPackage` to a specific build stage by modifying
 the `packages` setting of the user flavour as otherwise it will not be built.
 
-You can choose which integer library to use when building GHC using the
-`integerLibrary` setting of the build flavour. Possible values are: `integerGmp`
-(default) and `integerSimple`.
+You can choose which Bignum backend to use when buidling GHC using the
+`bignumBackend` setting of the build flavour. Possible values are: `gmp`
+(default), `native` or `ffi`.
 ```haskell
 userFlavour :: Flavour
-userFlavour = defaultFlavour { name = "user", integerLibrary = pure integerSimple }
+userFlavour = defaultFlavour { name = "user", bignumBackend = "native" }
 ```
 
 #### Specifying the final stage to build
diff --git a/hadrian/hadrian.cabal b/hadrian/hadrian.cabal
index 19ba672c09..11f34a26ba 100644
--- a/hadrian/hadrian.cabal
+++ b/hadrian/hadrian.cabal
@@ -102,7 +102,6 @@ executable hadrian
                        , Settings.Builders.Xelatex
                        , Settings.Default
                        , Settings.Flavours.Benchmark
-                       , Settings.Flavours.Common
                        , Settings.Flavours.Development
                        , Settings.Flavours.Llvm
                        , Settings.Flavours.Performance
diff --git a/hadrian/src/CommandLine.hs b/hadrian/src/CommandLine.hs
index 5446fda574..4582a8b258 100644
--- a/hadrian/src/CommandLine.hs
+++ b/hadrian/src/CommandLine.hs
@@ -1,6 +1,6 @@
 module CommandLine (
     optDescrs, cmdLineArgsMap, cmdFlavour, lookupFreeze1, lookupFreeze2,
-    cmdIntegerSimple, cmdProgressInfo, cmdConfigure, cmdCompleteSetting,
+    cmdBignum, cmdBignumCheck, cmdProgressInfo, cmdConfigure, cmdCompleteSetting,
     cmdDocsArgs, lookupBuildRoot, TestArgs(..), TestSpeed(..), defaultTestArgs,
     cmdPrefix
     ) where
@@ -26,7 +26,8 @@ data CommandLineArgs = CommandLineArgs
     , flavour        :: Maybe String
     , freeze1        :: Bool
     , freeze2        :: Bool
-    , integerSimple  :: Bool
+    , bignum         :: Maybe String
+    , bignumCheck    :: Bool
     , progressInfo   :: ProgressInfo
     , buildRoot      :: BuildRoot
     , testArgs       :: TestArgs
@@ -42,7 +43,8 @@ defaultCommandLineArgs = CommandLineArgs
     , flavour        = Nothing
     , freeze1        = False
     , freeze2        = False
-    , integerSimple  = False
+    , bignum         = Nothing
+    , bignumCheck    = False
     , progressInfo   = Brief
     , buildRoot      = BuildRoot "_build"
     , testArgs       = defaultTestArgs
@@ -96,6 +98,13 @@ readConfigure = Right $ \flags -> flags { configure = True }
 readFlavour :: Maybe String -> Either String (CommandLineArgs -> CommandLineArgs)
 readFlavour ms = Right $ \flags -> flags { flavour = lower <$> ms }
 
+readBignum :: Maybe String -> Either String (CommandLineArgs -> CommandLineArgs)
+readBignum Nothing   = Right id
+readBignum (Just ms) = Right $ \flags -> case break (== '-') (lower ms) of
+   (backend,"")          -> flags { bignum = Just backend }
+   ("check",'-':backend) -> flags { bignum = Just backend, bignumCheck = True }
+   _                     -> flags { bignum = Just (lower ms) }
+
 readBuildRoot :: Maybe FilePath -> Either String (CommandLineArgs -> CommandLineArgs)
 readBuildRoot ms =
     maybe (Left "Cannot parse build-root") (Right . set) (go =<< ms)
@@ -109,9 +118,6 @@ readFreeze1, readFreeze2 :: Either String (CommandLineArgs -> CommandLineArgs)
 readFreeze1 = Right $ \flags -> flags { freeze1 = True }
 readFreeze2 = Right $ \flags -> flags { freeze1 = True, freeze2 = True }
 
-readIntegerSimple :: Either String (CommandLineArgs -> CommandLineArgs)
-readIntegerSimple = Right $ \flags -> flags { integerSimple = True }
-
 readProgressInfo :: Maybe String -> Either String (CommandLineArgs -> CommandLineArgs)
 readProgressInfo ms =
     maybe (Left "Cannot parse progress-info") (Right . set) (go =<< lower <$> ms)
@@ -250,8 +256,8 @@ optDescrs =
       "Freeze Stage1 GHC."
     , Option [] ["freeze2"] (NoArg readFreeze2)
       "Freeze Stage2 GHC."
-    , Option [] ["integer-simple"] (NoArg readIntegerSimple)
-      "Build GHC with integer-simple library."
+    , Option [] ["bignum"] (OptArg readBignum "BIGNUM")
+      "Select GHC BigNum backend: native, gmp, ffi."
     , Option [] ["progress-info"] (OptArg readProgressInfo "STYLE")
       "Progress info style (None, Brief, Normal or Unicorn)."
     , Option [] ["docs"] (OptArg readDocsArg "TARGET")
@@ -355,8 +361,11 @@ lookupFreeze1 = freeze1 . lookupExtra defaultCommandLineArgs
 lookupFreeze2 :: Map.HashMap TypeRep Dynamic -> Bool
 lookupFreeze2 = freeze2 . lookupExtra defaultCommandLineArgs
 
-cmdIntegerSimple :: Action Bool
-cmdIntegerSimple = integerSimple <$> cmdLineArgs
+cmdBignum :: Action (Maybe String)
+cmdBignum = bignum <$> cmdLineArgs
+
+cmdBignumCheck :: Action Bool
+cmdBignumCheck = bignumCheck <$> cmdLineArgs
 
 cmdProgressInfo :: Action ProgressInfo
 cmdProgressInfo = progressInfo <$> cmdLineArgs
diff --git a/hadrian/src/Flavour.hs b/hadrian/src/Flavour.hs
index 64ce931e78..56488f0b0a 100644
--- a/hadrian/src/Flavour.hs
+++ b/hadrian/src/Flavour.hs
@@ -25,8 +25,10 @@ data Flavour = Flavour {
     args :: Args,
     -- | Build these packages.
     packages :: Stage -> Action [Package],
-    -- | Either 'integerGmp' or 'integerSimple'.
-    integerLibrary :: Action Package,
+    -- | 'native', 'gmp', 'ffi'.
+    bignumBackend :: String,
+    -- | Check selected backend against native backend
+    bignumCheck :: Bool,
     -- | Build libraries these ways.
     libraryWays :: Ways,
     -- | Build RTS these ways.
diff --git a/hadrian/src/Packages.hs b/hadrian/src/Packages.hs
index e65e8c9709..fa87d8edcd 100644
--- a/hadrian/src/Packages.hs
+++ b/hadrian/src/Packages.hs
@@ -3,7 +3,7 @@ module Packages (
     -- * GHC packages
     array, base, binary, bytestring, cabal, checkApiAnnotations, checkPpr,
     compareSizes, compiler, containers, deepseq, deriveConstants, directory,
-    exceptions, filepath, genapply, genprimopcode, ghc, ghcBoot, ghcBootTh,
+    exceptions, filepath, genapply, genprimopcode, ghc, ghcBignum, ghcBoot, ghcBootTh,
     ghcCompact, ghcHeap, ghci, ghcPkg, ghcPrim, haddock, haskeline,
     hsc2hs, hp2ps, hpc, hpcBin, integerGmp, integerSimple, iserv, iservProxy,
     libffi, libiserv, mtl, parsec, pretty, primitive, process, remoteIserv, rts,
@@ -33,7 +33,7 @@ ghcPackages :: [Package]
 ghcPackages =
     [ array, base, binary, bytestring, cabal, checkPpr, checkApiAnnotations
     , compareSizes, compiler, containers, deepseq, deriveConstants, directory
-    , exceptions, filepath, genapply, genprimopcode, ghc, ghcBoot, ghcBootTh
+    , exceptions, filepath, genapply, genprimopcode, ghc, ghcBignum, ghcBoot, ghcBootTh
     , ghcCompact, ghcHeap, ghci, ghcPkg, ghcPrim, haddock, haskeline, hsc2hs
     , hp2ps, hpc, hpcBin, integerGmp, integerSimple, iserv, libffi, libiserv, mtl
     , parsec, pretty, process, rts, runGhc, stm, templateHaskell
@@ -63,6 +63,7 @@ filepath            = lib  "filepath"
 genapply            = util "genapply"
 genprimopcode       = util "genprimopcode"
 ghc                 = prg  "ghc-bin"         `setPath` "ghc"
+ghcBignum           = lib  "ghc-bignum"
 ghcBoot             = lib  "ghc-boot"
 ghcBootTh           = lib  "ghc-boot-th"
 ghcCompact          = lib  "ghc-compact"
diff --git a/hadrian/src/Rules/Generate.hs b/hadrian/src/Rules/Generate.hs
index 32d29019a7..6222fcd3af 100644
--- a/hadrian/src/Rules/Generate.hs
+++ b/hadrian/src/Rules/Generate.hs
@@ -50,13 +50,13 @@ derivedConstantsFiles =
 compilerDependencies :: Expr [FilePath]
 compilerDependencies = do
     stage   <- getStage
-    isGmp   <- (== integerGmp) <$> getIntegerPackage
+    isGmp   <- (== "gmp") <$> getBignumBackend
     ghcPath <- expr $ buildPath (vanillaContext stage compiler)
-    gmpPath <- expr $ buildPath (vanillaContext stage integerGmp)
+    ghcBignumPath <- expr $ buildPath (vanillaContext stage ghcBignum)
     rtsPath <- expr (rtsBuildPath stage)
     libDir <- expr $ stageLibPath stage
     mconcat [ return $ (libDir -/-) <$> derivedConstantsFiles
-            , notStage0 ? isGmp ? return [gmpPath -/- "include/ghc-gmp.h"]
+            , notStage0 ? isGmp ? return [ghcBignumPath -/- "include/ghc-gmp.h"]
             , notStage0 ? return ((rtsPath -/-) <$> libffiHeaderFiles)
             , return $ fmap (ghcPath -/-)
                   [ "primop-can-fail.hs-incl"
@@ -316,7 +316,7 @@ generateSettings = do
         , ("LLVM opt command", expr $ settingsFileSetting SettingsFileSetting_OptCommand)
         , ("LLVM clang command", expr $ settingsFileSetting SettingsFileSetting_ClangCommand)
 
-        , ("integer library", pkgName <$> getIntegerPackage)
+        , ("BigNum backend", getBignumBackend)
         , ("Use interpreter", expr $ yesNo <$> ghcWithInterpreter)
         , ("Use native code generator", expr $ yesNo <$> ghcWithNativeCodeGen)
         , ("Support SMP", expr $ yesNo <$> targetSupportsSMP)
diff --git a/hadrian/src/Rules/Gmp.hs b/hadrian/src/Rules/Gmp.hs
index 3fe45d251f..6d89a5b0e9 100644
--- a/hadrian/src/Rules/Gmp.hs
+++ b/hadrian/src/Rules/Gmp.hs
@@ -18,9 +18,9 @@ gmpObjects s = do
     then return []
     else do
       -- Indirectly ensure object creation
-      let ctx = vanillaContext s integerGmp
-      integerGmpPath <- buildPath ctx
-      need [integerGmpPath -/- "include/ghc-gmp.h"]
+      let ctx = vanillaContext s ghcBignum
+      ghcBignumPath <- buildPath ctx
+      need [ghcBignumPath -/- "include/ghc-gmp.h"]
 
       gmpPath <- gmpIntreePath s
       map (unifyPath . (gmpPath -/-)) <$>
@@ -49,13 +49,13 @@ gmpRules = do
     let
       -- Path to libraries/integer-gmp/gmp in the source tree
       gmpBase :: FilePath
-      gmpBase = pkgPath integerGmp -/- "gmp"
+      gmpBase = pkgPath ghcBignum -/- "gmp"
 
     -- Build in-tree gmp if necessary
-    -- Produce: integer-gmp/build/include/ghc-gmp.h
+    -- Produce: ghc-bignum/build/include/ghc-gmp.h
     --   In-tree: copy gmp.h from in-tree build
     --   External: copy ghc-gmp.h from base sources
-    root -/- "stage*/libraries/integer-gmp/build/include/ghc-gmp.h" %> \header -> do
+    root -/- "stage*/libraries/ghc-bignum/build/include/ghc-gmp.h" %> \header -> do
         let includeP   = takeDirectory header
             buildP     = takeDirectory includeP
             packageP   = takeDirectory buildP
@@ -80,13 +80,13 @@ gmpRules = do
 
         let
           -- parse a path of the form "//stage*/gmp/xxx" and returns a vanilla
-          -- context from it for integer-gmp package.
+          -- context from it for ghc-bignum package.
           makeGmpPathContext gmpP = do
                let
                    stageP   = takeDirectory gmpP
                    stageS   = takeFileName stageP
                stage <- parsePath parseStage "<stage>" stageS
-               pure (vanillaContext stage integerGmp)
+               pure (vanillaContext stage ghcBignum)
 
           gmpPath = root -/- "stage*/gmp"
 
diff --git a/hadrian/src/Rules/Library.hs b/hadrian/src/Rules/Library.hs
index 066f609a49..805213d6ae 100644
--- a/hadrian/src/Rules/Library.hs
+++ b/hadrian/src/Rules/Library.hs
@@ -12,6 +12,7 @@ import Oracles.ModuleFiles
 import Packages
 import Rules.Gmp
 import Rules.Register
+import Settings
 import Target
 import Utilities
 
@@ -131,11 +132,15 @@ cObjects context = do
 
 -- | Return extra object files needed to build the given library context. The
 -- resulting list is currently non-empty only when the package from the
--- 'Context' is @integer-gmp@.
+-- 'Context' is @ghc-bignum@ built with in-tree GMP backend.
 extraObjects :: Context -> Action [FilePath]
 extraObjects context
-    | package context == integerGmp = gmpObjects (stage context)
-    | otherwise                     = return []
+    | package context == ghcBignum = do
+         interpretInContext context getBignumBackend >>= \case
+            "gmp" -> gmpObjects (stage context)
+            _     -> return []
+
+    | otherwise = return []
 
 -- | Return all the object files to be put into the library we're building for
 -- the given 'Context'.
diff --git a/hadrian/src/Rules/Register.hs b/hadrian/src/Rules/Register.hs
index 51df69f116..6bbae7e0bd 100644
--- a/hadrian/src/Rules/Register.hs
+++ b/hadrian/src/Rules/Register.hs
@@ -39,8 +39,10 @@ configurePackageRules = do
         let pkg = unsafeFindPackageByPath path
         let ctx = Context stage pkg vanilla
         buildP <- buildPath ctx
-        when (pkg == integerGmp) $
-          need [buildP -/- "include/ghc-gmp.h"]
+        when (pkg == ghcBignum) $ do
+          isGmp <- (== "gmp") <$> interpretInContext ctx getBignumBackend
+          when isGmp $
+            need [buildP -/- "include/ghc-gmp.h"]
         needLibrary =<< contextDependencies ctx
         Cabal.configurePackage ctx
 
@@ -129,8 +131,10 @@ buildConf _ context@Context {..} _conf = do
              , path -/- "ghcversion.h" ]
 
     -- we need to generate this file for GMP
-    when (package == integerGmp) $
-        need [path -/- "include/ghc-gmp.h"]
+    when (package == ghcBignum) $ do
+        bignum <- interpretInContext context getBignumBackend
+        when (bignum == "gmp") $
+            need [path -/- "include/ghc-gmp.h"]
 
     -- Copy and register the package.
     Cabal.copyPackage context
diff --git a/hadrian/src/Settings.hs b/hadrian/src/Settings.hs
index 7c6a24af27..373e976b44 100755
--- a/hadrian/src/Settings.hs
+++ b/hadrian/src/Settings.hs
@@ -1,7 +1,7 @@
 module Settings (
     getArgs, getLibraryWays, getRtsWays, flavour, knownPackages,
     findPackageByName, unsafeFindPackageByName, unsafeFindPackageByPath,
-    isLibrary, stagePackages, getIntegerPackage, completeSetting
+    isLibrary, stagePackages, getBignumBackend, getBignumCheck, completeSetting
     ) where
 
 import CommandLine
@@ -35,6 +35,16 @@ getLibraryWays = expr flavour >>= libraryWays
 getRtsWays :: Ways
 getRtsWays = expr flavour >>= rtsWays
 
+getBignumBackend :: Expr String
+getBignumBackend = expr $ cmdBignum >>= \case
+   Nothing -> bignumBackend <$> flavour
+   Just b  -> pure b
+
+getBignumCheck :: Expr Bool
+getBignumCheck = expr $ cmdBignum >>= \case
+   Nothing -> bignumCheck <$> flavour
+   Just _  -> cmdBignumCheck
+
 stagePackages :: Stage -> Action [Package]
 stagePackages stage = do
     f <- flavour
@@ -67,9 +77,6 @@ flavour = do
         [f] -> tweak f
         _   -> error $ "Multiple build flavours named " ++ flavourName
 
-getIntegerPackage :: Expr Package
-getIntegerPackage = expr (integerLibrary =<< flavour)
-
 -- TODO: switch to Set Package as the order of packages should not matter?
 -- Otherwise we have to keep remembering to sort packages from time to time.
 knownPackages :: [Package]
diff --git a/hadrian/src/Settings/Builders/RunTest.hs b/hadrian/src/Settings/Builders/RunTest.hs
index 293465b52b..c10e93aae0 100644
--- a/hadrian/src/Settings/Builders/RunTest.hs
+++ b/hadrian/src/Settings/Builders/RunTest.hs
@@ -87,6 +87,8 @@ runTestBuilderArgs = builder RunTest ? do
     top         <- expr $ topDirectory
     ghcFlags    <- expr runTestGhcFlags
     cmdrootdirs <- expr (testRootDirs <$> userSetting defaultTestArgs)
+    bignumBackend <- getBignumBackend
+    bignumCheck   <- getBignumCheck
     let defaultRootdirs = ("testsuite" -/- "tests") : libTests
         rootdirs | null cmdrootdirs = defaultRootdirs
                  | otherwise        = cmdrootdirs
@@ -121,9 +123,11 @@ runTestBuilderArgs = builder RunTest ? do
             , arg "-e", arg $ asBool "config.have_vanilla="   (hasLibWay vanilla)
             , arg "-e", arg $ asBool "config.have_dynamic="   (hasLibWay dynamic)
             , arg "-e", arg $ asBool "config.have_profiling=" (hasLibWay profiling)
+            , arg "-e", arg $ asBool "config.have_fast_bignum=" (bignumBackend /= "native" && not bignumCheck)
             , arg "-e", arg $ asBool "ghc_with_smp=" withSMP
             , arg "-e", arg $ asBool "ghc_with_llvm=" withLlvm
 
+
             , arg "-e", arg $ "config.ghc_dynamic_by_default=" ++ show hasDynamicByDefault
             , arg "-e", arg $ "config.ghc_dynamic=" ++ show hasDynamic
 
diff --git a/hadrian/src/Settings/Default.hs b/hadrian/src/Settings/Default.hs
index 5c6fb7254d..cb1222103b 100644
--- a/hadrian/src/Settings/Default.hs
+++ b/hadrian/src/Settings/Default.hs
@@ -9,8 +9,8 @@ module Settings.Default (
     SourceArgs (..), sourceArgs, defaultBuilderArgs, defaultPackageArgs,
     defaultArgs,
 
-    -- * Default build flavour
-    defaultFlavour
+    -- * Default build flavour and BigNum backend
+    defaultFlavour, defaultBignumBackend
     ) where
 
 import qualified Hadrian.Builder.Ar
@@ -23,7 +23,6 @@ import Expression
 import Flavour
 import Oracles.Flag
 import Packages
-import Settings
 import Settings.Builders.Alex
 import Settings.Builders.DeriveConstants
 import Settings.Builders.Cabal
@@ -50,6 +49,10 @@ defaultPackages Stage1 = stage1Packages
 defaultPackages Stage2 = stage2Packages
 defaultPackages Stage3 = return []
 
+-- | Default bignum backend.
+defaultBignumBackend :: String
+defaultBignumBackend = "gmp"
+
 -- | Packages built in 'Stage0' by default. You can change this in "UserSettings".
 stage0Packages :: Action [Package]
 stage0Packages = do
@@ -63,6 +66,7 @@ stage0Packages = do
              , genapply
              , genprimopcode
              , ghc
+             , ghcBignum
              , ghcBoot
              , ghcBootTh
              , ghcHeap
@@ -86,7 +90,6 @@ stage0Packages = do
 -- | Packages built in 'Stage1' by default. You can change this in "UserSettings".
 stage1Packages :: Action [Package]
 stage1Packages = do
-    intLib     <- integerLibrary =<< flavour
     libraries0 <- filter isLibrary <$> stage0Packages
     cross      <- flag CrossCompiling
     return $ libraries0 -- Build all Stage0 libraries in Stage1
@@ -99,13 +102,14 @@ stage1Packages = do
              , exceptions
              , filepath
              , ghc
+             , ghcBignum
              , ghcCompact
              , ghcPkg
              , ghcPrim
              , haskeline
              , hp2ps
              , hsc2hs
-             , intLib
+             , integerGmp
              , pretty
              , process
              , rts
@@ -202,7 +206,8 @@ defaultFlavour = Flavour
     { name               = "default"
     , args               = defaultArgs
     , packages           = defaultPackages
-    , integerLibrary     = (\x -> if x then integerSimple else integerGmp) <$> cmdIntegerSimple
+    , bignumBackend      = defaultBignumBackend
+    , bignumCheck        = False
     , libraryWays        = defaultLibraryWays
     , rtsWays            = defaultRtsWays
     , dynamicGhcPrograms = defaultDynamicGhcPrograms
diff --git a/hadrian/src/Settings/Default.hs-boot b/hadrian/src/Settings/Default.hs-boot
index e2996d9c71..a6585384a1 100644
--- a/hadrian/src/Settings/Default.hs-boot
+++ b/hadrian/src/Settings/Default.hs-boot
@@ -1,7 +1,7 @@
 module Settings.Default (
     SourceArgs (..), sourceArgs, defaultBuilderArgs, defaultPackageArgs,
     defaultArgs, defaultLibraryWays, defaultRtsWays,
-    defaultFlavour
+    defaultFlavour, defaultBignumBackend
     ) where
 
 import Flavour
@@ -18,3 +18,4 @@ sourceArgs :: SourceArgs -> Args
 defaultBuilderArgs, defaultPackageArgs, defaultArgs :: Args
 defaultLibraryWays, defaultRtsWays :: Ways
 defaultFlavour :: Flavour
+defaultBignumBackend :: String
diff --git a/hadrian/src/Settings/Flavours/Common.hs b/hadrian/src/Settings/Flavours/Common.hs
deleted file mode 100644
index cba0bcd978..0000000000
--- a/hadrian/src/Settings/Flavours/Common.hs
+++ /dev/null
@@ -1,11 +0,0 @@
-module Settings.Flavours.Common where
-
-import Expression
-
--- See https://gitlab.haskell.org/ghc/ghc/issues/15286 and
--- https://phabricator.haskell.org/D4880
-naturalInBaseFixArgs :: Args
-naturalInBaseFixArgs = mconcat
-  [ input "**/Natural.hs" ? pure ["-fno-omit-interface-pragmas"]
-  , input "**/Num.hs" ? pure ["-fno-ignore-interface-pragmas"]
-  ]
diff --git a/hadrian/src/Settings/Flavours/GhcInGhci.hs b/hadrian/src/Settings/Flavours/GhcInGhci.hs
index 8533172064..950b96f926 100644
--- a/hadrian/src/Settings/Flavours/GhcInGhci.hs
+++ b/hadrian/src/Settings/Flavours/GhcInGhci.hs
@@ -3,7 +3,6 @@ module Settings.Flavours.GhcInGhci (ghcInGhciFlavour) where
 import Expression
 import Flavour
 import {-# SOURCE #-} Settings.Default
-import Settings.Flavours.Common
 
 -- Please update doc/flavours.md when changing this file.
 ghcInGhciFlavour :: Flavour
@@ -21,7 +20,6 @@ ghciArgs :: Args
 ghciArgs = sourceArgs SourceArgs
     { hsDefault  = mconcat $
         [ pure ["-O0", "-H64m"]
-        , naturalInBaseFixArgs
         ]
     , hsLibrary  = mempty
     , hsCompiler = mempty
diff --git a/hadrian/src/Settings/Flavours/Profiled.hs b/hadrian/src/Settings/Flavours/Profiled.hs
index 7f222bf33c..49339f1446 100644
--- a/hadrian/src/Settings/Flavours/Profiled.hs
+++ b/hadrian/src/Settings/Flavours/Profiled.hs
@@ -3,7 +3,6 @@ module Settings.Flavours.Profiled (profiledFlavour) where
 import Expression
 import Flavour
 import {-# SOURCE #-} Settings.Default
-import Settings.Flavours.Common (naturalInBaseFixArgs)
 
 -- Please update doc/flavours.md when changing this file.
 profiledFlavour :: Flavour
@@ -17,7 +16,6 @@ profiledArgs :: Args
 profiledArgs = sourceArgs SourceArgs
     { hsDefault  = mconcat
         [ pure ["-O0", "-H64m"]
-        , naturalInBaseFixArgs
         ]
     , hsLibrary  = notStage0 ? arg "-O"
     , hsCompiler = mconcat [stage0 ? arg "-O2", notStage0 ? arg "-O"]
diff --git a/hadrian/src/Settings/Flavours/Quick.hs b/hadrian/src/Settings/Flavours/Quick.hs
index 16ff99a091..deea0c47dd 100644
--- a/hadrian/src/Settings/Flavours/Quick.hs
+++ b/hadrian/src/Settings/Flavours/Quick.hs
@@ -4,7 +4,6 @@ import Expression
 import Flavour
 import Oracles.Flag
 import {-# SOURCE #-} Settings.Default
-import Settings.Flavours.Common
 
 -- Please update doc/flavours.md when changing this file.
 quickFlavour :: Flavour
@@ -27,7 +26,6 @@ quickArgs :: Args
 quickArgs = sourceArgs SourceArgs
     { hsDefault  = mconcat $
         [ pure ["-O0", "-H64m"]
-        , naturalInBaseFixArgs
         ]
     , hsLibrary  = notStage0 ? arg "-O"
     , hsCompiler =    stage0 ? arg "-O2"
diff --git a/hadrian/src/Settings/Flavours/QuickCross.hs b/hadrian/src/Settings/Flavours/QuickCross.hs
index 3f2776ed50..5e9dc05f08 100644
--- a/hadrian/src/Settings/Flavours/QuickCross.hs
+++ b/hadrian/src/Settings/Flavours/QuickCross.hs
@@ -4,7 +4,6 @@ import Expression
 import Flavour
 import Oracles.Flag
 import {-# SOURCE #-} Settings.Default
-import Settings.Flavours.Common
 
 -- Please update doc/flavours.md when changing this file.
 quickCrossFlavour :: Flavour
@@ -28,7 +27,6 @@ quickCrossArgs :: Args
 quickCrossArgs = sourceArgs SourceArgs
     { hsDefault  = mconcat $
         [ pure ["-O0", "-H64m"]
-        , naturalInBaseFixArgs
         ]
     , hsLibrary  = notStage0 ? mconcat [ arg "-O", arg "-fllvm" ]
     , hsCompiler = stage0 ? arg "-O2"
diff --git a/hadrian/src/Settings/Flavours/Quickest.hs b/hadrian/src/Settings/Flavours/Quickest.hs
index c0fd72764f..58a8c280da 100644
--- a/hadrian/src/Settings/Flavours/Quickest.hs
+++ b/hadrian/src/Settings/Flavours/Quickest.hs
@@ -3,7 +3,6 @@ module Settings.Flavours.Quickest (quickestFlavour) where
 import Expression
 import Flavour
 import {-# SOURCE #-} Settings.Default
-import Settings.Flavours.Common
 
 -- Please update doc/flavours.md when changing this file.
 quickestFlavour :: Flavour
@@ -18,7 +17,6 @@ quickestArgs :: Args
 quickestArgs = sourceArgs SourceArgs
     { hsDefault  = mconcat $
         [ pure ["-O0", "-H64m"]
-        , naturalInBaseFixArgs
         ]
     , hsLibrary  = mempty
     , hsCompiler = stage0 ? arg "-O"
diff --git a/hadrian/src/Settings/Packages.hs b/hadrian/src/Settings/Packages.hs
index 8bb0f13fc5..8c9d7875d4 100644
--- a/hadrian/src/Settings/Packages.hs
+++ b/hadrian/src/Settings/Packages.hs
@@ -13,7 +13,6 @@ packageArgs = do
     stage        <- getStage
     rtsWays      <- getRtsWays
     path         <- getBuildPath
-    intLib       <- getIntegerPackage
     compilerPath <- expr $ buildPath (vanillaContext stage compiler)
     let -- Do not bind the result to a Boolean: this forces the configure rule
         -- immediately and may lead to cyclic dependencies.
@@ -27,16 +26,12 @@ packageArgs = do
     mconcat
         --------------------------------- base ---------------------------------
         [ package base ? mconcat
-          [ builder (Cabal Flags) ? notStage0 ? arg (pkgName intLib)
+          [ builder (Cabal Flags) ? notStage0 ? arg (pkgName ghcBignum)
 
           -- This fixes the 'unknown symbol stat' issue.
           -- See: https://github.com/snowleopard/hadrian/issues/259.
           , builder (Ghc CompileCWithGhc) ? arg "-optc-O2" ]
 
-        ------------------------------ bytestring ------------------------------
-        , package bytestring ?
-          builder (Cabal Flags) ? intLib == integerSimple ? arg "integer-simple"
-
         --------------------------------- cabal --------------------------------
         -- Cabal is a large library and slow to compile. Moreover, we build it
         -- for Stage0 only so we can link ghc-pkg against it, so there is little
@@ -81,10 +76,7 @@ packageArgs = do
             [ ghcWithNativeCodeGen ? arg "ncg"
             , ghcWithInterpreter ? notStage0 ? arg "ghci"
             , cross ? arg "-terminfo"
-            , notStage0 ? intLib == integerGmp ?
-              arg "integer-gmp"
-            , notStage0 ? intLib == integerSimple ?
-              arg "integer-simple" ]
+            ]
 
           , builder (Haddock BuildPackage) ? arg ("--optghc=-I" ++ path) ]
 
@@ -193,8 +185,8 @@ packageArgs = do
         , package hsc2hs ?
           builder (Cabal Flags) ? arg "in-ghc-tree"
 
-        ------------------------------ integerGmp ------------------------------
-        , gmpPackageArgs
+        ------------------------------ ghc-bignum ------------------------------
+        , ghcBignumArgs
 
         ---------------------------------- rts ---------------------------------
         , package rts ? rtsPackageArgs -- RTS deserves a separate function
@@ -203,40 +195,50 @@ packageArgs = do
         , package runGhc ?
           builder Ghc ? input "**/Main.hs" ?
           (\version -> ["-cpp", "-DVERSION=" ++ show version]) <$> getSetting ProjectVersion
+        ]
 
-        --------------------------------- text ---------------------------------
-        -- The package @text@ is rather tricky. It's a boot library, and it
-        -- tries to determine on its own if it should link against @integer-gmp@
-        -- or @integer-simple@. For Stage0, we need to use the integer library
-        -- that the bootstrap compiler has (since @interger@ is not a boot
-        -- library) and therefore we copy it over into the Stage0 package-db.
-        -- Maybe we should stop doing this? And subsequently @text@ for Stage1
-        -- detects the same integer library again, even though we don't build it
-        -- in Stage1, and at that point the configuration is just wrong.
-        , package text ?
-          builder (Cabal Flags) ? notStage0 ? intLib == integerSimple ?
-          pure ["+integer-simple", "-bytestring-builder"] ]
-
-gmpPackageArgs :: Args
-gmpPackageArgs = do
-    package integerGmp ? do
-        -- These are only used for non-in-tree builds.
-        librariesGmp <- getSetting GmpLibDir
-        includesGmp  <- getSetting GmpIncludeDir
-
-        mconcat
-          [ builder (Cabal Setup) ? mconcat
-            [ flag GmpInTree ? arg "--configure-option=--with-intree-gmp"
-            , flag GmpFrameworkPref ?
-              arg "--configure-option=--with-gmp-framework-preferred"
-
-              -- Ensure that the integer-gmp package registration includes
-              -- knowledge of the system gmp's library and include directories.
-            , notM (flag GmpInTree) ? mconcat
-              [ if not (null librariesGmp) then arg ("--extra-lib-dirs=" ++ librariesGmp) else mempty
-              , if not (null includesGmp) then arg ("--extra-include-dirs=" ++ includesGmp) else mempty
-              ]
-            ]
+ghcBignumArgs :: Args
+ghcBignumArgs = package ghcBignum ? do
+    -- These are only used for non-in-tree builds.
+    librariesGmp <- getSetting GmpLibDir
+    includesGmp <- getSetting GmpIncludeDir
+
+    backend <- getBignumBackend
+    check   <- getBignumCheck
+
+    mconcat
+          [ -- select BigNum backend
+            builder (Cabal Flags) ? arg backend
+
+          , -- check the selected backend against native backend
+            builder (Cabal Flags) ? check ? arg "check"
+
+            -- backend specific
+          , case backend of
+               "gmp" -> mconcat
+                   [ builder (Cabal Setup) ? mconcat
+
+                       -- enable GMP backend: configure script will produce
+                       -- `ghc-bignum.buildinfo` and `include/HsIntegerGmp.h`
+                     [ arg "--configure-option=--with-gmp"
+
+                       -- enable in-tree support: don't depend on external "gmp"
+                       -- library
+                     , flag GmpInTree ? arg "--configure-option=--with-intree-gmp"
+
+                       -- prefer framework over library (on Darwin)
+                     , flag GmpFrameworkPref ?
+                       arg "--configure-option=--with-gmp-framework-preferred"
+
+                       -- Ensure that the ghc-bignum package registration includes
+                       -- knowledge of the system gmp's library and include directories.
+                     , notM (flag GmpInTree) ? mconcat
+                       [ if not (null librariesGmp) then arg ("--extra-lib-dirs=" ++ librariesGmp) else mempty
+                       , if not (null includesGmp) then arg ("--extra-include-dirs=" ++ includesGmp) else mempty
+                       ]
+                     ]
+                  ]
+               _ -> mempty
           ]
 
 -- | RTS-specific command line arguments.
diff --git a/hadrian/src/Settings/Warnings.hs b/hadrian/src/Settings/Warnings.hs
index 5999822e71..717443ca19 100644
--- a/hadrian/src/Settings/Warnings.hs
+++ b/hadrian/src/Settings/Warnings.hs
@@ -3,7 +3,6 @@ module Settings.Warnings (defaultGhcWarningsArgs, ghcWarningsArgs) where
 import Expression
 import Oracles.Flag
 import Packages
-import Settings
 
 -- See @mk/warnings.mk@ for warning-related arguments in the Make build system.
 
@@ -17,7 +16,6 @@ defaultGhcWarningsArgs = mconcat
 -- | Package-specific warnings-related arguments, mostly suppressing various warnings.
 ghcWarningsArgs :: Args
 ghcWarningsArgs = do
-    isIntegerSimple <- (== integerSimple) <$> getIntegerPackage
     mconcat
         [ stage0 ? mconcat
         [ libraryPackage       ? pure [ "-fno-warn-deprecated-flags" ]
@@ -47,8 +45,6 @@ ghcWarningsArgs = do
                                       , "-Wno-deprecations" ]
         , package rts          ? pure [ "-Wcpp-undef" ]
         , package terminfo     ? pure [ "-Wno-unused-imports" ]
-        , isIntegerSimple      ?
-          package text         ? pure [ "-Wno-unused-imports" ]
         , package transformers ? pure [ "-Wno-unused-matches"
                                       , "-Wno-unused-imports"
                                       , "-Wno-redundant-constraints"