Change `Backend` type and remove direct dependencieswip/backend-as-record

With this change, `Backend` becomes an abstract type
(there are no more exposed value constructors).
Decisions that were formerly made by asking "is the
current back end equal to (or different from) this named value
constructor?" are now made by interrogating the back end about
its properties, which are functions exported by `GHC.Driver.Backend`.

There is a description of how to migrate code using `Backend` in the
user guide.

Clients using the GHC API can find a backdoor to access the Backend
datatype in GHC.Driver.Backend.Internal.

Bumps haddock submodule.

Fixes #20927

3 files changed, 11 insertions, 13 deletions

diff --git a/compiler/GHC/Driver/Config/Cmm.hs b/compiler/GHC/Driver/Config/Cmm.hs
index 38bab62048..5e7ebf72dd 100644
--- a/compiler/GHC/Driver/Config/Cmm.hs
+++ b/compiler/GHC/Driver/Config/Cmm.hs
@@ -3,7 +3,6 @@ module GHC.Driver.Config.Cmm
   ) where
 
 import GHC.Cmm.Config
-import GHC.Cmm.Switch (backendSupportsSwitch)
 
 import GHC.Driver.Session
 import GHC.Driver.Backend
@@ -21,8 +20,8 @@ initCmmConfig dflags = CmmConfig
   , cmmOptSink             = gopt Opt_CmmSink             dflags
   , cmmGenStackUnwindInstr = debugLevel dflags > 0
   , cmmExternalDynamicRefs = gopt Opt_ExternalDynamicRefs dflags
-  , cmmDoCmmSwitchPlans    = not . backendSupportsSwitch . backend $ dflags
-  , cmmSplitProcPoints     = (backend dflags /= NCG)
+  , cmmDoCmmSwitchPlans    = not . backendHasNativeSwitch . backend $ dflags
+  , cmmSplitProcPoints     = not (backendSupportsUnsplitProcPoints (backend dflags))
                              || not (platformTablesNextToCode platform)
                              || usingInconsistentPicReg
   }
diff --git a/compiler/GHC/Driver/Config/StgToCmm.hs b/compiler/GHC/Driver/Config/StgToCmm.hs
index 7ec80f24d3..b0b28bf158 100644
--- a/compiler/GHC/Driver/Config/StgToCmm.hs
+++ b/compiler/GHC/Driver/Config/StgToCmm.hs
@@ -8,6 +8,7 @@ import GHC.Driver.Backend
 import GHC.Driver.Session
 import GHC.Platform
 import GHC.Platform.Profile
+import GHC.Utils.Error
 import GHC.Unit.Module
 import GHC.Utils.Outputable
 
@@ -60,9 +61,11 @@ initStgToCmmConfig dflags mod = StgToCmmConfig
   } where profile  = targetProfile dflags
           platform = profilePlatform profile
           bk_end  = backend dflags
-          ncg     = bk_end == NCG
-          llvm    = bk_end == LLVM
           b_blob  = if not ncg then Nothing else binBlobThreshold dflags
+          (ncg, llvm) = case backendPrimitiveImplementation bk_end of
+                          GenericPrimitives -> (False, False)
+                          NcgPrimitives -> (True, False)
+                          LlvmPrimitives -> (False, True)
           x86ish  = case platformArch platform of
                       ArchX86    -> True
                       ArchX86_64 -> True
@@ -71,6 +74,6 @@ initStgToCmmConfig dflags mod = StgToCmmConfig
                       ArchPPC      -> True
                       ArchPPC_64 _ -> True
                       _            -> False
-          vec_err = case backend dflags of
-                      LLVM -> Nothing
-                      _    -> Just (unlines ["SIMD vector instructions require the LLVM back-end.", "Please use -fllvm."])
+          vec_err = case backendSimdValidity (backend dflags) of
+                      IsValid -> Nothing
+                      NotValid msg -> Just msg
diff --git a/compiler/GHC/Driver/Config/Tidy.hs b/compiler/GHC/Driver/Config/Tidy.hs
index d7ad76fc87..89bdf31b2c 100644
--- a/compiler/GHC/Driver/Config/Tidy.hs
+++ b/compiler/GHC/Driver/Config/Tidy.hs
@@ -62,12 +62,8 @@ initStaticPtrOpts hsc_env = do
 
       -- If we are compiling for the interpreter we will insert any necessary
       -- SPT entries dynamically, otherwise we add a C stub to do so
-    , opt_gen_cstub = case backend dflags of
-                        Interpreter -> False
-                        _           -> True
-
+    , opt_gen_cstub = backendWritesFiles (backend dflags)
     , opt_mk_string = mk_string
     , opt_static_ptr_info_datacon = static_ptr_info_datacon
     , opt_static_ptr_datacon      = static_ptr_datacon
     }
-