Support native bignum (including static builds)

compiler/ghc/default.nix

@@ -31,6 +31,9 @@ let self =
 , # If enabled, GHC will be built with the GPL-free but slower integer-simple
   # library instead of the faster but GPLed integer-gmp library.
   enableIntegerSimple ? !(lib.any (lib.meta.platformMatch stdenv.hostPlatform) gmp.meta.platforms), gmp
+, # If enabled, GHC will be built with the GPL-free native backend of the
+  # bignum library that is nearly as fast as GMP
+  enableNativeBignum ? !((lib.any (lib.meta.platformMatch stdenv.hostPlatform) gmp.meta.platforms) || enableIntegerSimple)
 
 , # If enabled, use -fPIC when compiling static libs.
   enableRelocatedStaticLibs ? stdenv.targetPlatform != stdenv.hostPlatform && !stdenv.targetPlatform.isAarch32
@@ -71,14 +74,29 @@ let self =
 , extra-passthru ? {}
 }@args:
 
-assert !enableIntegerSimple -> gmp != null;
+assert !(enableIntegerSimple || enableNativeBignum) -> gmp != null;
+
+# Early check to make sure only one of these is enabled
+assert enableNativeBignum -> !enableIntegerSimple;
+assert enableIntegerSimple -> !enableNativeBignum;
 
 let
   inherit (stdenv) buildPlatform hostPlatform targetPlatform;
   inherit (haskell-nix.haskellLib) isCrossTarget;
 
   inherit (bootPkgs) ghc;
 
+  ghcHasNativeBignum = builtins.compareVersions ghc-version "9.0" >= 0;
+
+  bignumSpec =
+    assert ghcHasNativeBignum -> !enableIntegerSimple;
+    assert !ghcHasNativeBignum -> !enableNativeBignum;
+    if ghcHasNativeBignum then ''
+      BIGNUM_BACKEND = ${if enableNativeBignum then "native" else "gmp"}
+    '' else ''
+      INTEGER_LIBRARY = ${if enableIntegerSimple then "integer-simple" else "integer-gmp"}
+    '';
+
   # TODO check if this possible fix for segfaults works or not.
   targetLibffi =
     # on native platforms targetPlatform.{libffi, gmp} do not exist; thus fall back
@@ -106,7 +124,7 @@ let
     include mk/flavours/\$(BuildFlavour).mk
     endif
     DYNAMIC_GHC_PROGRAMS = ${if enableShared then "YES" else "NO"}
-    INTEGER_LIBRARY = ${if enableIntegerSimple then "integer-simple" else "integer-gmp"}
+  '' + bignumSpec + ''
     EXTRA_HADDOCK_OPTS += --quickjump --hyperlinked-source
   '' + lib.optionalString (targetPlatform != hostPlatform) ''
     CrossCompilePrefix = ${targetPrefix}
@@ -121,6 +139,10 @@ let
   '' + lib.optionalString enableRelocatedStaticLibs ''
     GhcLibHcOpts += -fPIC
     GhcRtsHcOpts += -fPIC
+    GhcRtsCcOpts += -fPIC
+  '' + lib.optionalString (enableRelocatedStaticLibs && targetPlatform.isx86_64 && !targetPlatform.isWindows) ''
+    GhcLibHcOpts += -fexternal-dynamic-refs
+    GhcRtsHcOpts += -fexternal-dynamic-refs
   '' + lib.optionalString enableDWARF ''
     GhcLibHcOpts += -g3
     GhcRtsHcOpts += -g3

overlays/ghc.nix

@@ -1,9 +1,13 @@
 final: prev: with prev;
   # sadly we need to patch GHC a bit.
-   let
-    ghcPkgOverrides = {
-        enableIntegerSimple = false;
-      };
+  let
+    # The new implementation appeared in GHC 9.0
+    hasNativeBignum = name: !lib.hasPrefix "ghc8" name;
+
+    ghcPkgOverrides = name: { enableIntegerSimple = false; } // lib.optionalAttrs (hasNativeBignum name) {
+      enableNativeBignum = false;
+    };
+
     ghcDrvOverrides = drv: {
         hardeningDisable = (drv.hardeningDisable or []) ++ [ "stackprotector" "format" ] ++ lib.optionals prev.stdenv.hostPlatform.isAarch32 [ "pic" "pie" ];
       };
@@ -20,13 +24,13 @@ final: prev: with prev;
        && !lib.hasPrefix "ghc82" name
        && !lib.hasPrefix "ghcjs" name
        && !lib.hasSuffix "Binary" name;
-     overrideCompiler = compiler:
-       ((compiler.override ghcPkgOverrides).overrideAttrs ghcDrvOverrides) // {
-         dwarf = overrideCompiler compiler.dwarf;
+     overrideCompiler = name: compiler:
+       ((compiler.override (ghcPkgOverrides name)).overrideAttrs ghcDrvOverrides) // {
+         dwarf = overrideCompiler name compiler.dwarf;
        };
    in
      lib.recursiveUpdate prev.haskell-nix {
-       compiler = lib.mapAttrs (_name: overrideCompiler)
+       compiler = lib.mapAttrs overrideCompiler
          (lib.filterAttrs (name: _value: needsPatches name) prev.haskell-nix.compiler);
      };
    }