path: root/compiler/GHC/CmmToAsm.hs

-- -----------------------------------------------------------------------------
--
-- (c) The University of Glasgow 1993-2004
--
--
-- -----------------------------------------------------------------------------

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE UnboxedTuples #-}

{-# OPTIONS_GHC -Wno-incomplete-record-updates #-}

-- | Native code generator
--
-- The native-code generator has machine-independent and
-- machine-dependent modules.
--
-- This module ("GHC.CmmToAsm") is the top-level machine-independent
-- module.  Before entering machine-dependent land, we do some
-- machine-independent optimisations (defined below) on the
-- 'CmmStmts's.
--
-- We convert to the machine-specific 'Instr' datatype with
-- 'cmmCodeGen', assuming an infinite supply of registers.  We then use
-- a machine-independent register allocator ('regAlloc') to rejoin
-- reality.  Obviously, 'regAlloc' has machine-specific helper
-- functions (see about "RegAllocInfo" below).
--
-- Finally, we order the basic blocks of the function so as to minimise
-- the number of jumps between blocks, by utilising fallthrough wherever
-- possible.
--
-- The machine-dependent bits break down as follows:
--
--   * ["MachRegs"]  Everything about the target platform's machine
--     registers (and immediate operands, and addresses, which tend to
--     intermingle/interact with registers).
--
--   * ["MachInstrs"]  Includes the 'Instr' datatype (possibly should
--     have a module of its own), plus a miscellany of other things
--     (e.g., 'targetDoubleSize', 'smStablePtrTable', ...)
--
--   * ["MachCodeGen"]  is where 'Cmm' stuff turns into
--     machine instructions.
--
--   * ["PprMach"] 'pprInstr' turns an 'Instr' into text (well, really
--     a 'SDoc').
--
--   * ["RegAllocInfo"] In the register allocator, we manipulate
--     'MRegsState's, which are 'BitSet's, one bit per machine register.
--     When we want to say something about a specific machine register
--     (e.g., ``it gets clobbered by this instruction''), we set/unset
--     its bit.  Obviously, we do this 'BitSet' thing for efficiency
--     reasons.
--
--     The 'RegAllocInfo' module collects together the machine-specific
--     info needed to do register allocation.
--
--    * ["RegisterAlloc"] The (machine-independent) register allocator.
-- -}
--
module GHC.CmmToAsm
   ( nativeCodeGen

   -- * Test-only exports: see trac #12744
   -- used by testGraphNoSpills, which needs to access
   -- the register allocator intermediate data structures
   -- cmmNativeGen emits
   , cmmNativeGen
   , NcgImpl(..)
   , initNCGConfig
   )
where

#include "HsVersions.h"

import GHC.Prelude

import qualified GHC.CmmToAsm.X86   as X86
import qualified GHC.CmmToAsm.PPC   as PPC
import qualified GHC.CmmToAsm.SPARC as SPARC

import GHC.CmmToAsm.Reg.Liveness
import qualified GHC.CmmToAsm.Reg.Linear                as Linear

import qualified GHC.Data.Graph.Color                   as Color
import qualified GHC.CmmToAsm.Reg.Graph                 as Color
import qualified GHC.CmmToAsm.Reg.Graph.Stats           as Color
import qualified GHC.CmmToAsm.Reg.Graph.TrivColorable   as Color

import GHC.Utils.Asm
import GHC.CmmToAsm.Reg.Target
import GHC.Platform
import GHC.CmmToAsm.BlockLayout as BlockLayout
import GHC.Settings.Config
import GHC.CmmToAsm.Instr
import GHC.CmmToAsm.PIC
import GHC.Platform.Reg
import GHC.Platform.Reg.Class (RegClass)
import GHC.CmmToAsm.Monad
import GHC.CmmToAsm.CFG
import GHC.CmmToAsm.Dwarf
import GHC.CmmToAsm.Config
import GHC.CmmToAsm.Types
import GHC.Cmm.DebugBlock

import GHC.Cmm.BlockId
import GHC.StgToCmm.CgUtils ( fixStgRegisters )
import GHC.Cmm
import GHC.Cmm.Utils
import GHC.Cmm.Dataflow.Collections
import GHC.Cmm.Dataflow.Label
import GHC.Cmm.Dataflow.Block
import GHC.Cmm.Opt           ( cmmMachOpFold )
import GHC.Cmm.Ppr
import GHC.Cmm.CLabel

import GHC.Types.Unique.FM
import GHC.Types.Unique.Supply
import GHC.Driver.Session
import GHC.Driver.Ppr
import GHC.Utils.Misc
import GHC.Utils.Logger

import qualified GHC.Utils.Ppr as Pretty
import GHC.Utils.BufHandle
import GHC.Utils.Outputable as Outputable
import GHC.Utils.Panic
import GHC.Utils.Error
import GHC.Utils.Exception (evaluate)
import GHC.Utils.Constants (debugIsOn)

import GHC.Data.FastString
import GHC.Types.Unique.Set
import GHC.Unit
import GHC.Data.Stream (Stream)
import qualified GHC.Data.Stream as Stream

import Data.List (sortBy, groupBy)
import Data.Maybe
import Data.Ord         ( comparing )
import Control.Monad
import System.IO

--------------------
nativeCodeGen :: forall a . Logger -> DynFlags -> Module -> ModLocation -> Handle -> UniqSupply
              -> Stream IO RawCmmGroup a
              -> IO a
nativeCodeGen logger dflags this_mod modLoc h us cmms
 = let config   = initNCGConfig dflags this_mod
       platform = ncgPlatform config
       nCG' :: ( OutputableP Platform statics, Outputable jumpDest, Instruction instr)
            => NcgImpl statics instr jumpDest -> IO a
       nCG' ncgImpl = nativeCodeGen' logger dflags config modLoc ncgImpl h us cmms
   in case platformArch platform of
      ArchX86       -> nCG' (X86.ncgX86     config)
      ArchX86_64    -> nCG' (X86.ncgX86_64  config)
      ArchPPC       -> nCG' (PPC.ncgPPC     config)
      ArchPPC_64 _  -> nCG' (PPC.ncgPPC     config)
      ArchSPARC     -> nCG' (SPARC.ncgSPARC config)
      ArchSPARC64   -> panic "nativeCodeGen: No NCG for SPARC64"
      ArchS390X     -> panic "nativeCodeGen: No NCG for S390X"
      ArchARM {}    -> panic "nativeCodeGen: No NCG for ARM"
      ArchAArch64   -> panic "nativeCodeGen: No NCG for AArch64"
      ArchAlpha     -> panic "nativeCodeGen: No NCG for Alpha"
      ArchMipseb    -> panic "nativeCodeGen: No NCG for mipseb"
      ArchMipsel    -> panic "nativeCodeGen: No NCG for mipsel"
      ArchRISCV64   -> panic "nativeCodeGen: No NCG for RISCV64"
      ArchUnknown   -> panic "nativeCodeGen: No NCG for unknown arch"
      ArchJavaScript-> panic "nativeCodeGen: No NCG for JavaScript"


-- | Data accumulated during code generation. Mostly about statistics,
-- but also collects debug data for DWARF generation.
data NativeGenAcc statics instr
  = NGS { ngs_imports     :: ![[CLabel]]
        , ngs_natives     :: ![[NatCmmDecl statics instr]]
             -- ^ Native code generated, for statistics. This might
             -- hold a lot of data, so it is important to clear this
             -- field as early as possible if it isn't actually
             -- required.
        , ngs_colorStats  :: ![[Color.RegAllocStats statics instr]]
        , ngs_linearStats :: ![[Linear.RegAllocStats]]
        , ngs_labels      :: ![Label]
        , ngs_debug       :: ![DebugBlock]
        , ngs_dwarfFiles  :: !DwarfFiles
        , ngs_unwinds     :: !(LabelMap [UnwindPoint])
             -- ^ see Note [Unwinding information in the NCG]
             -- and Note [What is this unwinding business?] in "GHC.Cmm.DebugBlock".
        }

{-
Note [Unwinding information in the NCG]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Unwind information is a type of metadata which allows a debugging tool
to reconstruct the values of machine registers at the time a procedure was
entered. For the most part, the production of unwind information is handled by
the Cmm stage, where it is represented by CmmUnwind nodes.

Unfortunately, the Cmm stage doesn't know everything necessary to produce
accurate unwinding information. For instance, the x86-64 calling convention
requires that the stack pointer be aligned to 16 bytes, which in turn means that
GHC must sometimes add padding to $sp prior to performing a foreign call. When
this happens unwind information must be updated accordingly.
For this reason, we make the NCG backends responsible for producing
unwinding tables (with the extractUnwindPoints function in NcgImpl).

We accumulate the produced unwind tables over CmmGroups in the ngs_unwinds
field of NativeGenAcc. This is a label map which contains an entry for each
procedure, containing a list of unwinding points (e.g. a label and an associated
unwinding table).

See also Note [What is this unwinding business?] in "GHC.Cmm.DebugBlock".
-}

nativeCodeGen' :: (OutputableP Platform statics, Outputable jumpDest, Instruction instr)
               => Logger
               -> DynFlags
               -> NCGConfig
               -> ModLocation
               -> NcgImpl statics instr jumpDest
               -> Handle
               -> UniqSupply
               -> Stream IO RawCmmGroup a
               -> IO a
nativeCodeGen' logger dflags config modLoc ncgImpl h us cmms
 = do
        -- BufHandle is a performance hack.  We could hide it inside
        -- Pretty if it weren't for the fact that we do lots of little
        -- printDocs here (in order to do codegen in constant space).
        bufh <- newBufHandle h
        let ngs0 = NGS [] [] [] [] [] [] emptyUFM mapEmpty
        (ngs, us', a) <- cmmNativeGenStream logger dflags config modLoc ncgImpl bufh us
                                         cmms ngs0
        _ <- finishNativeGen logger dflags config modLoc bufh us' ngs
        return a

finishNativeGen :: Instruction instr
                => Logger
                -> DynFlags
                -> NCGConfig
                -> ModLocation
                -> BufHandle
                -> UniqSupply
                -> NativeGenAcc statics instr
                -> IO UniqSupply
finishNativeGen logger dflags config modLoc bufh@(BufHandle _ _ h) us ngs
 = withTimingSilent logger dflags (text "NCG") (`seq` ()) $ do
        -- Write debug data and finish
        us' <- if not (ncgDwarfEnabled config)
                  then return us
                  else do
                     (dwarf, us') <- dwarfGen config modLoc us (ngs_debug ngs)
                     emitNativeCode logger dflags config bufh dwarf
                     return us'
        bFlush bufh

        -- dump global NCG stats for graph coloring allocator
        let stats = concat (ngs_colorStats ngs)
        unless (null stats) $ do

          -- build the global register conflict graph
          let graphGlobal
                  = foldl' Color.union Color.initGraph
                  $ [ Color.raGraph stat
                          | stat@Color.RegAllocStatsStart{} <- stats]

          dump_stats (Color.pprStats stats graphGlobal)

          let platform = ncgPlatform config
          dumpIfSet_dyn logger dflags
                  Opt_D_dump_asm_conflicts "Register conflict graph"
                  FormatText
                  $ Color.dotGraph
                          (targetRegDotColor platform)
                          (Color.trivColorable platform
                                  (targetVirtualRegSqueeze platform)
                                  (targetRealRegSqueeze platform))
                  $ graphGlobal


        -- dump global NCG stats for linear allocator
        let linearStats = concat (ngs_linearStats ngs)
        unless (null linearStats) $
          dump_stats (Linear.pprStats (concat (ngs_natives ngs)) linearStats)

        -- write out the imports
        let ctx = ncgAsmContext config
        printSDocLn ctx Pretty.LeftMode h
                $ makeImportsDoc config (concat (ngs_imports ngs))
        return us'
  where
    dump_stats = putDumpMsg logger dflags (mkDumpStyle alwaysQualify)
                   Opt_D_dump_asm_stats "NCG stats"
                   FormatText

cmmNativeGenStream :: forall statics jumpDest instr a . (OutputableP Platform statics, Outputable jumpDest, Instruction instr)
              => Logger
              -> DynFlags
              -> NCGConfig
              -> ModLocation
              -> NcgImpl statics instr jumpDest
              -> BufHandle
              -> UniqSupply
              -> Stream.Stream IO RawCmmGroup a
              -> NativeGenAcc statics instr
              -> IO (NativeGenAcc statics instr, UniqSupply, a)

cmmNativeGenStream logger dflags config modLoc ncgImpl h us cmm_stream ngs
 = loop us (Stream.runStream cmm_stream) ngs
  where
    ncglabel = text "NCG"
    loop :: UniqSupply
              -> Stream.StreamS IO RawCmmGroup a
              -> NativeGenAcc statics instr
              -> IO (NativeGenAcc statics instr, UniqSupply, a)
    loop us s ngs =
      case s of
        Stream.Done a ->
          return (ngs { ngs_imports = reverse $ ngs_imports ngs
                      , ngs_natives = reverse $ ngs_natives ngs
                      , ngs_colorStats = reverse $ ngs_colorStats ngs
                      , ngs_linearStats = reverse $ ngs_linearStats ngs
                      },
                  us,
                  a)
        Stream.Effect m -> m >>= \cmm_stream' -> loop us cmm_stream' ngs
        Stream.Yield cmms cmm_stream' -> do
          (us', ngs'') <-
            withTimingSilent logger
                dflags
                ncglabel (\(a, b) -> a `seq` b `seq` ()) $ do
              -- Generate debug information
              let !ndbgs | ncgDwarfEnabled config = cmmDebugGen modLoc cmms
                         | otherwise              = []
                  dbgMap = debugToMap ndbgs

              -- Generate native code
              (ngs',us') <- cmmNativeGens logger dflags config modLoc ncgImpl h
                                          dbgMap us cmms ngs 0

              -- Link native code information into debug blocks
              -- See Note [What is this unwinding business?] in "GHC.Cmm.DebugBlock".
              let !ldbgs = cmmDebugLink (ngs_labels ngs') (ngs_unwinds ngs') ndbgs
                  platform = targetPlatform dflags
              unless (null ldbgs) $
                dumpIfSet_dyn logger dflags Opt_D_dump_debug "Debug Infos" FormatText
                  (vcat $ map (pdoc platform) ldbgs)

              -- Accumulate debug information for emission in finishNativeGen.
              let ngs'' = ngs' { ngs_debug = ngs_debug ngs' ++ ldbgs, ngs_labels = [] }
              return (us', ngs'')

          loop us' cmm_stream' ngs''


-- | Do native code generation on all these cmms.
--
cmmNativeGens :: forall statics instr jumpDest.
                 (OutputableP Platform statics, Outputable jumpDest, Instruction instr)
              => Logger
              -> DynFlags
              -> NCGConfig
              -> ModLocation
              -> NcgImpl statics instr jumpDest
              -> BufHandle
              -> LabelMap DebugBlock
              -> UniqSupply
              -> [RawCmmDecl]
              -> NativeGenAcc statics instr
              -> Int
              -> IO (NativeGenAcc statics instr, UniqSupply)

cmmNativeGens logger dflags config modLoc ncgImpl h dbgMap = go
  where
    go :: UniqSupply -> [RawCmmDecl]
       -> NativeGenAcc statics instr -> Int
       -> IO (NativeGenAcc statics instr, UniqSupply)

    go us [] ngs !_ =
        return (ngs, us)

    go us (cmm : cmms) ngs count = do
        let fileIds = ngs_dwarfFiles ngs
        (us', fileIds', native, imports, colorStats, linearStats, unwinds)
          <- {-# SCC "cmmNativeGen" #-}
             cmmNativeGen logger dflags modLoc ncgImpl us fileIds dbgMap
                          cmm count

        -- Generate .file directives for every new file that has been
        -- used. Note that it is important that we generate these in
        -- ascending order, as Clang's 3.6 assembler complains.
        let newFileIds = sortBy (comparing snd) $
                         nonDetEltsUFM $ fileIds' `minusUFM` fileIds
            -- See Note [Unique Determinism and code generation]
            pprDecl (f,n) = text "\t.file " <> ppr n <+>
                            pprFilePathString (unpackFS f)

        emitNativeCode logger dflags config h $ vcat $
          map pprDecl newFileIds ++
          map (pprNatCmmDecl ncgImpl) native

        -- force evaluation all this stuff to avoid space leaks
        let platform = targetPlatform dflags
        {-# SCC "seqString" #-} evaluate $ seqList (showSDoc dflags $ vcat $ map (pdoc platform) imports) ()

        let !labels' = if ncgDwarfEnabled config
                       then cmmDebugLabels isMetaInstr native else []
            !natives' = if dopt Opt_D_dump_asm_stats dflags
                        then native : ngs_natives ngs else []

            mCon = maybe id (:)
            ngs' = ngs{ ngs_imports     = imports : ngs_imports ngs
                      , ngs_natives     = natives'
                      , ngs_colorStats  = colorStats `mCon` ngs_colorStats ngs
                      , ngs_linearStats = linearStats `mCon` ngs_linearStats ngs
                      , ngs_labels      = ngs_labels ngs ++ labels'
                      , ngs_dwarfFiles  = fileIds'
                      , ngs_unwinds     = ngs_unwinds ngs `mapUnion` unwinds
                      }
        go us' cmms ngs' (count + 1)


emitNativeCode :: Logger -> DynFlags -> NCGConfig -> BufHandle -> SDoc -> IO ()
emitNativeCode logger dflags config h sdoc = do

        let ctx = ncgAsmContext config
        {-# SCC "pprNativeCode" #-} bufLeftRenderSDoc ctx h sdoc

        -- dump native code
        dumpIfSet_dyn logger dflags
                Opt_D_dump_asm "Asm code" FormatASM
                sdoc

-- | Complete native code generation phase for a single top-level chunk of Cmm.
--      Dumping the output of each stage along the way.
--      Global conflict graph and NGC stats
cmmNativeGen
    :: forall statics instr jumpDest. (Instruction instr, OutputableP Platform statics, Outputable jumpDest)
    => Logger
    -> DynFlags
    -> ModLocation
    -> NcgImpl statics instr jumpDest
        -> UniqSupply
        -> DwarfFiles
        -> LabelMap DebugBlock
        -> RawCmmDecl                                   -- ^ the cmm to generate code for
        -> Int                                          -- ^ sequence number of this top thing
        -> IO   ( UniqSupply
                , DwarfFiles
                , [NatCmmDecl statics instr]                -- native code
                , [CLabel]                                  -- things imported by this cmm
                , Maybe [Color.RegAllocStats statics instr] -- stats for the coloring register allocator
                , Maybe [Linear.RegAllocStats]              -- stats for the linear register allocators
                , LabelMap [UnwindPoint]                    -- unwinding information for blocks
                )

cmmNativeGen logger dflags modLoc ncgImpl us fileIds dbgMap cmm count
 = do
        let config   = ncgConfig ncgImpl
        let platform = ncgPlatform config
        let weights  = ncgCfgWeights config

        let proc_name = case cmm of
                (CmmProc _ entry_label _ _) -> pdoc platform entry_label
                _                           -> text "DataChunk"

        -- rewrite assignments to global regs
        let fixed_cmm =
                {-# SCC "fixStgRegisters" #-}
                fixStgRegisters platform cmm

        -- cmm to cmm optimisations
        let (opt_cmm, imports) =
                {-# SCC "cmmToCmm" #-}
                cmmToCmm config fixed_cmm

        dumpIfSet_dyn logger dflags
                Opt_D_dump_opt_cmm "Optimised Cmm" FormatCMM
                (pprCmmGroup platform [opt_cmm])

        let cmmCfg = {-# SCC "getCFG" #-}
                     getCfgProc platform weights opt_cmm

        -- generate native code from cmm
        let ((native, lastMinuteImports, fileIds', nativeCfgWeights), usGen) =
                {-# SCC "genMachCode" #-}
                initUs us $ genMachCode config modLoc
                                        (cmmTopCodeGen ncgImpl)
                                        fileIds dbgMap opt_cmm cmmCfg

        dumpIfSet_dyn logger dflags
                Opt_D_dump_asm_native "Native code" FormatASM
                (vcat $ map (pprNatCmmDecl ncgImpl) native)

        maybeDumpCfg logger dflags (Just nativeCfgWeights) "CFG Weights - Native" proc_name

        -- tag instructions with register liveness information
        -- also drops dead code. We don't keep the cfg in sync on
        -- some backends, so don't use it there.
        let livenessCfg = if backendMaintainsCfg platform
                                then Just nativeCfgWeights
                                else Nothing
        let (withLiveness, usLive) =
                {-# SCC "regLiveness" #-}
                initUs usGen
                        $ mapM (cmmTopLiveness livenessCfg platform) native

        dumpIfSet_dyn logger dflags
                Opt_D_dump_asm_liveness "Liveness annotations added"
                FormatCMM
                (vcat $ map (pprLiveCmmDecl platform) withLiveness)

        -- allocate registers
        (alloced, usAlloc, ppr_raStatsColor, ppr_raStatsLinear, raStats, stack_updt_blks) <-
         if ( gopt Opt_RegsGraph dflags
           || gopt Opt_RegsIterative dflags )
          then do
                -- the regs usable for allocation
                let (alloc_regs :: UniqFM RegClass (UniqSet RealReg))
                        = foldr (\r -> plusUFM_C unionUniqSets
                                        $ unitUFM (targetClassOfRealReg platform r) (unitUniqSet r))
                                emptyUFM
                        $ allocatableRegs ncgImpl

                -- do the graph coloring register allocation
                let ((alloced, maybe_more_stack, regAllocStats), usAlloc)
                        = {-# SCC "RegAlloc-color" #-}
                          initUs usLive
                          $ Color.regAlloc
                                config
                                alloc_regs
                                (mkUniqSet [0 .. maxSpillSlots ncgImpl])
                                (maxSpillSlots ncgImpl)
                                withLiveness
                                livenessCfg

                let ((alloced', stack_updt_blks), usAlloc')
                        = initUs usAlloc $
                                case maybe_more_stack of
                                Nothing     -> return (alloced, [])
                                Just amount -> do
                                    (alloced',stack_updt_blks) <- unzip <$>
                                                (mapM ((ncgAllocMoreStack ncgImpl) amount) alloced)
                                    return (alloced', concat stack_updt_blks )


                -- dump out what happened during register allocation
                dumpIfSet_dyn logger dflags
                        Opt_D_dump_asm_regalloc "Registers allocated"
                        FormatCMM
                        (vcat $ map (pprNatCmmDecl ncgImpl) alloced)

                dumpIfSet_dyn logger dflags
                        Opt_D_dump_asm_regalloc_stages "Build/spill stages"
                        FormatText
                        (vcat   $ map (\(stage, stats)
                                        -> text "# --------------------------"
                                        $$ text "#  cmm " <> int count <> text " Stage " <> int stage
                                        $$ ppr (fmap (pprInstr platform) stats))
                                $ zip [0..] regAllocStats)

                let mPprStats =
                        if dopt Opt_D_dump_asm_stats dflags
                         then Just regAllocStats else Nothing

                -- force evaluation of the Maybe to avoid space leak
                mPprStats `seq` return ()

                return  ( alloced', usAlloc'
                        , mPprStats
                        , Nothing
                        , [], stack_updt_blks)

          else do
                -- do linear register allocation
                let reg_alloc proc = do
                       (alloced, maybe_more_stack, ra_stats) <-
                               Linear.regAlloc config proc
                       case maybe_more_stack of
                         Nothing -> return ( alloced, ra_stats, [] )
                         Just amount -> do
                           (alloced',stack_updt_blks) <-
                               ncgAllocMoreStack ncgImpl amount alloced
                           return (alloced', ra_stats, stack_updt_blks )

                let ((alloced, regAllocStats, stack_updt_blks), usAlloc)
                        = {-# SCC "RegAlloc-linear" #-}
                          initUs usLive
                          $ liftM unzip3
                          $ mapM reg_alloc withLiveness

                dumpIfSet_dyn logger dflags
                        Opt_D_dump_asm_regalloc "Registers allocated"
                        FormatCMM
                        (vcat $ map (pprNatCmmDecl ncgImpl) alloced)

                let mPprStats =
                        if dopt Opt_D_dump_asm_stats dflags
                         then Just (catMaybes regAllocStats) else Nothing

                -- force evaluation of the Maybe to avoid space leak
                mPprStats `seq` return ()

                return  ( alloced, usAlloc
                        , Nothing
                        , mPprStats, (catMaybes regAllocStats)
                        , concat stack_updt_blks )

        -- Fixupblocks the register allocator inserted (from, regMoves, to)
        let cfgRegAllocUpdates :: [(BlockId,BlockId,BlockId)]
            cfgRegAllocUpdates = (concatMap Linear.ra_fixupList raStats)

        let cfgWithFixupBlks =
                (\cfg -> addNodesBetween weights cfg cfgRegAllocUpdates) <$> livenessCfg

        -- Insert stack update blocks
        let postRegCFG =
                pure (foldl' (\m (from,to) -> addImmediateSuccessor weights from to m ))
                     <*> cfgWithFixupBlks
                     <*> pure stack_updt_blks

        ---- generate jump tables
        let tabled      =
                {-# SCC "generateJumpTables" #-}
                generateJumpTables ncgImpl alloced

        when (not $ null nativeCfgWeights) $ dumpIfSet_dyn logger dflags
                Opt_D_dump_cfg_weights "CFG Update information"
                FormatText
                ( text "stack:" <+> ppr stack_updt_blks $$
                  text "linearAlloc:" <+> ppr cfgRegAllocUpdates )

        ---- shortcut branches
        let (shorted, postShortCFG)     =
                {-# SCC "shortcutBranches" #-}
                shortcutBranches dflags ncgImpl tabled postRegCFG

        let optimizedCFG :: Maybe CFG
            optimizedCFG =
                optimizeCFG (gopt Opt_CmmStaticPred dflags) weights cmm <$!> postShortCFG

        maybeDumpCfg logger dflags optimizedCFG "CFG Weights - Final" proc_name

        --TODO: Partially check validity of the cfg.
        let getBlks (CmmProc _info _lbl _live (ListGraph blocks)) = blocks
            getBlks _ = []

        when ( backendMaintainsCfg platform &&
                (gopt Opt_DoAsmLinting dflags || debugIsOn )) $ do
                let blocks = concatMap getBlks shorted
                let labels = setFromList $ fmap blockId blocks :: LabelSet
                let cfg = fromJust optimizedCFG
                return $! seq (sanityCheckCfg cfg labels $
                                text "cfg not in lockstep") ()

        ---- sequence blocks
        let sequenced :: [NatCmmDecl statics instr]
            sequenced =
                checkLayout shorted $
                {-# SCC "sequenceBlocks" #-}
                map (BlockLayout.sequenceTop
                        ncgImpl optimizedCFG)
                    shorted

        let branchOpt :: [NatCmmDecl statics instr]
            branchOpt =
                {-# SCC "invertCondBranches" #-}
                map invert sequenced
              where
                invertConds :: LabelMap RawCmmStatics -> [NatBasicBlock instr]
                            -> [NatBasicBlock instr]
                invertConds = invertCondBranches ncgImpl optimizedCFG
                invert top@CmmData {} = top
                invert (CmmProc info lbl live (ListGraph blocks)) =
                    CmmProc info lbl live (ListGraph $ invertConds info blocks)

        ---- expansion of SPARC synthetic instrs
        let expanded =
                {-# SCC "sparc_expand" #-}
                ncgExpandTop ncgImpl branchOpt
                --ncgExpandTop ncgImpl sequenced

        dumpIfSet_dyn logger dflags
                Opt_D_dump_asm_expanded "Synthetic instructions expanded"
                FormatCMM
                (vcat $ map (pprNatCmmDecl ncgImpl) expanded)

        -- generate unwinding information from cmm
        let unwinds :: BlockMap [UnwindPoint]
            unwinds =
                {-# SCC "unwindingInfo" #-}
                foldl' addUnwind mapEmpty expanded
              where
                addUnwind acc proc =
                    acc `mapUnion` computeUnwinding dflags ncgImpl proc

        return  ( usAlloc
                , fileIds'
                , expanded
                , lastMinuteImports ++ imports
                , ppr_raStatsColor
                , ppr_raStatsLinear
                , unwinds )

maybeDumpCfg :: Logger -> DynFlags -> Maybe CFG -> String -> SDoc -> IO ()
maybeDumpCfg _logger _dflags Nothing _ _ = return ()
maybeDumpCfg logger dflags (Just cfg) msg proc_name
        | null cfg = return ()
        | otherwise
        = dumpIfSet_dyn logger
                dflags Opt_D_dump_cfg_weights msg
                FormatText
                (proc_name <> char ':' $$ pprEdgeWeights cfg)

-- | Make sure all blocks we want the layout algorithm to place have been placed.
checkLayout :: [NatCmmDecl statics instr] -> [NatCmmDecl statics instr]
            -> [NatCmmDecl statics instr]
checkLayout procsUnsequenced procsSequenced =
        assertPpr (setNull diff) (ppr "Block sequencing dropped blocks:" <> ppr diff)
        procsSequenced
  where
        blocks1 = foldl' (setUnion) setEmpty $
                        map getBlockIds procsUnsequenced :: LabelSet
        blocks2 = foldl' (setUnion) setEmpty $
                        map getBlockIds procsSequenced
        diff = setDifference blocks1 blocks2

        getBlockIds (CmmData _ _) = setEmpty
        getBlockIds (CmmProc _ _ _ (ListGraph blocks)) =
                setFromList $ map blockId blocks

-- | Compute unwinding tables for the blocks of a procedure
computeUnwinding :: Instruction instr
                 => DynFlags -> NcgImpl statics instr jumpDest
                 -> NatCmmDecl statics instr
                    -- ^ the native code generated for the procedure
                 -> LabelMap [UnwindPoint]
                    -- ^ unwinding tables for all points of all blocks of the
                    -- procedure
computeUnwinding dflags _ _
  | debugLevel dflags == 0         = mapEmpty
computeUnwinding _ _ (CmmData _ _) = mapEmpty
computeUnwinding _ ncgImpl (CmmProc _ _ _ (ListGraph blks)) =
    -- In general we would need to push unwinding information down the
    -- block-level call-graph to ensure that we fully account for all
    -- relevant register writes within a procedure.
    --
    -- However, the only unwinding information that we care about in GHC is for
    -- Sp. The fact that GHC.Cmm.LayoutStack already ensures that we have unwind
    -- information at the beginning of every block means that there is no need
    -- to perform this sort of push-down.
    mapFromList [ (blk_lbl, extractUnwindPoints ncgImpl instrs)
                | BasicBlock blk_lbl instrs <- blks ]

-- | Build a doc for all the imports.
--
makeImportsDoc :: NCGConfig -> [CLabel] -> SDoc
makeImportsDoc config imports
 = dyld_stubs imports
            $$
            -- On recent versions of Darwin, the linker supports
            -- dead-stripping of code and data on a per-symbol basis.
            -- There's a hack to make this work in PprMach.pprNatCmmDecl.
            (if platformHasSubsectionsViaSymbols platform
             then text ".subsections_via_symbols"
             else Outputable.empty)
            $$
                -- On recent GNU ELF systems one can mark an object file
                -- as not requiring an executable stack. If all objects
                -- linked into a program have this note then the program
                -- will not use an executable stack, which is good for
                -- security. GHC generated code does not need an executable
                -- stack so add the note in:
            (if platformHasGnuNonexecStack platform
             then text ".section .note.GNU-stack,\"\"," <> sectionType platform "progbits"
             else Outputable.empty)
            $$
                -- And just because every other compiler does, let's stick in
                -- an identifier directive: .ident "GHC x.y.z"
            (if platformHasIdentDirective platform
             then let compilerIdent = text "GHC" <+> text cProjectVersion
                   in text ".ident" <+> doubleQuotes compilerIdent
             else Outputable.empty)

 where
        platform = ncgPlatform config

        -- Generate "symbol stubs" for all external symbols that might
        -- come from a dynamic library.
        dyld_stubs :: [CLabel] -> SDoc
{-      dyld_stubs imps = vcat $ map pprDyldSymbolStub $
                                    map head $ group $ sort imps-}
        -- (Hack) sometimes two Labels pretty-print the same, but have
        -- different uniques; so we compare their text versions...
        dyld_stubs imps
                | needImportedSymbols config
                = vcat $
                        (pprGotDeclaration config :) $
                        map ( pprImportedSymbol config . fst . head) $
                        groupBy (\(_,a) (_,b) -> a == b) $
                        sortBy (\(_,a) (_,b) -> compare a b) $
                        map doPpr $
                        imps
                | otherwise
                = Outputable.empty

        doPpr lbl = (lbl, renderWithContext
                              (ncgAsmContext config)
                              (pprCLabel platform AsmStyle lbl))

-- -----------------------------------------------------------------------------
-- Generate jump tables

-- Analyzes all native code and generates data sections for all jump
-- table instructions.
generateJumpTables
        :: NcgImpl statics instr jumpDest
        -> [NatCmmDecl statics instr] -> [NatCmmDecl statics instr]
generateJumpTables ncgImpl xs = concatMap f xs
    where f p@(CmmProc _ _ _ (ListGraph xs)) = p : concatMap g xs
          f p = [p]
          g (BasicBlock _ xs) = catMaybes (map (generateJumpTableForInstr ncgImpl) xs)

-- -----------------------------------------------------------------------------
-- Shortcut branches

shortcutBranches
        :: forall statics instr jumpDest. (Outputable jumpDest) => DynFlags
        -> NcgImpl statics instr jumpDest
        -> [NatCmmDecl statics instr]
        -> Maybe CFG
        -> ([NatCmmDecl statics instr],Maybe CFG)

shortcutBranches dflags ncgImpl tops weights
  | gopt Opt_AsmShortcutting dflags
  = ( map (apply_mapping ncgImpl mapping) tops'
    , shortcutWeightMap mappingBid <$!> weights )
  | otherwise
  = (tops, weights)
  where
    (tops', mappings) = mapAndUnzip (build_mapping ncgImpl) tops
    mapping = mapUnions mappings :: LabelMap jumpDest
    mappingBid = fmap (getJumpDestBlockId ncgImpl) mapping

build_mapping :: forall instr t d statics jumpDest.
                 NcgImpl statics instr jumpDest
              -> GenCmmDecl d (LabelMap t) (ListGraph instr)
              -> (GenCmmDecl d (LabelMap t) (ListGraph instr)
                 ,LabelMap jumpDest)
build_mapping _ top@(CmmData _ _) = (top, mapEmpty)
build_mapping _ (CmmProc info lbl live (ListGraph []))
  = (CmmProc info lbl live (ListGraph []), mapEmpty)
build_mapping ncgImpl (CmmProc info lbl live (ListGraph (head:blocks)))
  = (CmmProc info lbl live (ListGraph (head:others)), mapping)
        -- drop the shorted blocks, but don't ever drop the first one,
        -- because it is pointed to by a global label.
  where
    -- find all the blocks that just consist of a jump that can be
    -- shorted.
    -- Don't completely eliminate loops here -- that can leave a dangling jump!
    shortcut_blocks :: [(BlockId, jumpDest)]
    (_, shortcut_blocks, others) =
        foldl' split (setEmpty :: LabelSet, [], []) blocks
    split (s, shortcut_blocks, others) b@(BasicBlock id [insn])
        | Just jd <- canShortcut ncgImpl insn
        , Just dest <- getJumpDestBlockId ncgImpl jd
        , not (has_info id)
        , (setMember dest s) || dest == id -- loop checks
        = (s, shortcut_blocks, b : others)
    split (s, shortcut_blocks, others) (BasicBlock id [insn])
        | Just dest <- canShortcut ncgImpl insn
        , not (has_info id)
        = (setInsert id s, (id,dest) : shortcut_blocks, others)
    split (s, shortcut_blocks, others) other = (s, shortcut_blocks, other : others)

    -- do not eliminate blocks that have an info table
    has_info l = mapMember l info

    -- build a mapping from BlockId to JumpDest for shorting branches
    mapping = mapFromList shortcut_blocks

apply_mapping :: NcgImpl statics instr jumpDest
              -> LabelMap jumpDest
              -> GenCmmDecl statics h (ListGraph instr)
              -> GenCmmDecl statics h (ListGraph instr)
apply_mapping ncgImpl ufm (CmmData sec statics)
  = CmmData sec (shortcutStatics ncgImpl (\bid -> mapLookup bid ufm) statics)
apply_mapping ncgImpl ufm (CmmProc info lbl live (ListGraph blocks))
  = CmmProc info lbl live (ListGraph $ map short_bb blocks)
  where
    short_bb (BasicBlock id insns) = BasicBlock id $! map short_insn insns
    short_insn i = shortcutJump ncgImpl (\bid -> mapLookup bid ufm) i
                 -- shortcutJump should apply the mapping repeatedly,
                 -- just in case we can short multiple branches.

-- -----------------------------------------------------------------------------
-- Instruction selection

-- Native code instruction selection for a chunk of stix code.  For
-- this part of the computation, we switch from the UniqSM monad to
-- the NatM monad.  The latter carries not only a Unique, but also an
-- Int denoting the current C stack pointer offset in the generated
-- code; this is needed for creating correct spill offsets on
-- architectures which don't offer, or for which it would be
-- prohibitively expensive to employ, a frame pointer register.  Viz,
-- x86.

-- The offset is measured in bytes, and indicates the difference
-- between the current (simulated) C stack-ptr and the value it was at
-- the beginning of the block.  For stacks which grow down, this value
-- should be either zero or negative.

-- Along with the stack pointer offset, we also carry along a LabelMap of
-- DebugBlocks, which we read to generate .location directives.
--
-- Switching between the two monads whilst carrying along the same
-- Unique supply breaks abstraction.  Is that bad?

genMachCode
        :: NCGConfig
        -> ModLocation
        -> (RawCmmDecl -> NatM [NatCmmDecl statics instr])
        -> DwarfFiles
        -> LabelMap DebugBlock
        -> RawCmmDecl
        -> CFG
        -> UniqSM
                ( [NatCmmDecl statics instr]
                , [CLabel]
                , DwarfFiles
                , CFG
                )

genMachCode config modLoc cmmTopCodeGen fileIds dbgMap cmm_top cmm_cfg
  = do  { initial_us <- getUniqueSupplyM
        ; let initial_st           = mkNatM_State initial_us 0 config
                                                  modLoc fileIds dbgMap cmm_cfg
              (new_tops, final_st) = initNat initial_st (cmmTopCodeGen cmm_top)
              final_delta          = natm_delta final_st
              final_imports        = natm_imports final_st
              final_cfg            = natm_cfg final_st
        ; if   final_delta == 0
          then return (new_tops, final_imports
                      , natm_fileid final_st, final_cfg)
          else pprPanic "genMachCode: nonzero final delta" (int final_delta)
    }

-- -----------------------------------------------------------------------------
-- Generic Cmm optimiser

{-
Here we do:

  (a) Constant folding
  (c) Position independent code and dynamic linking
        (i)  introduce the appropriate indirections
             and position independent refs
        (ii) compile a list of imported symbols
  (d) Some arch-specific optimizations

(a) will be moving to the new Hoopl pipeline, however, (c) and
(d) are only needed by the native backend and will continue to live
here.

Ideas for other things we could do (put these in Hoopl please!):

  - shortcut jumps-to-jumps
  - simple CSE: if an expr is assigned to a temp, then replace later occs of
    that expr with the temp, until the expr is no longer valid (can push through
    temp assignments, and certain assigns to mem...)
-}

cmmToCmm :: NCGConfig -> RawCmmDecl -> (RawCmmDecl, [CLabel])
cmmToCmm _ top@(CmmData _ _) = (top, [])
cmmToCmm config (CmmProc info lbl live graph)
    = runCmmOpt config $
      do blocks' <- mapM cmmBlockConFold (toBlockList graph)
         return $ CmmProc info lbl live (ofBlockList (g_entry graph) blocks')

type OptMResult a = (# a, [CLabel] #)

pattern OptMResult :: a -> b -> (# a, b #)
pattern OptMResult x y = (# x, y #)
{-# COMPLETE OptMResult #-}

newtype CmmOptM a = CmmOptM (NCGConfig -> [CLabel] -> OptMResult a)
    deriving (Functor)

instance Applicative CmmOptM where
    pure x = CmmOptM $ \_ imports -> OptMResult x imports
    (<*>) = ap

instance Monad CmmOptM where
  (CmmOptM f) >>= g =
    CmmOptM $ \config imports0 ->
                case f config imports0 of
                  OptMResult x imports1 ->
                    case g x of
                      CmmOptM g' -> g' config imports1

instance CmmMakeDynamicReferenceM CmmOptM where
    addImport = addImportCmmOpt

addImportCmmOpt :: CLabel -> CmmOptM ()
addImportCmmOpt lbl = CmmOptM $ \_ imports -> OptMResult () (lbl:imports)

getCmmOptConfig :: CmmOptM NCGConfig
getCmmOptConfig = CmmOptM $ \config imports -> OptMResult config imports

runCmmOpt :: NCGConfig -> CmmOptM a -> (a, [CLabel])
runCmmOpt config (CmmOptM f) =
  case f config [] of
    OptMResult result imports -> (result, imports)

cmmBlockConFold :: CmmBlock -> CmmOptM CmmBlock
cmmBlockConFold block = do
  let (entry, middle, last) = blockSplit block
      stmts = blockToList middle
  stmts' <- mapM cmmStmtConFold stmts
  last' <- cmmStmtConFold last
  return $ blockJoin entry (blockFromList stmts') last'

-- This does three optimizations, but they're very quick to check, so we don't
-- bother turning them off even when the Hoopl code is active.  Since
-- this is on the old Cmm representation, we can't reuse the code either:
--  * reg = reg      --> nop
--  * if 0 then jump --> nop
--  * if 1 then jump --> jump
-- We might be tempted to skip this step entirely of not Opt_PIC, but
-- there is some PowerPC code for the non-PIC case, which would also
-- have to be separated.
cmmStmtConFold :: CmmNode e x -> CmmOptM (CmmNode e x)
cmmStmtConFold stmt
   = case stmt of
        CmmAssign reg src
           -> do src' <- cmmExprConFold DataReference src
                 return $ case src' of
                   CmmReg reg' | reg == reg' -> CmmComment (fsLit "nop")
                   new_src -> CmmAssign reg new_src

        CmmStore addr src
           -> do addr' <- cmmExprConFold DataReference addr
                 src'  <- cmmExprConFold DataReference src
                 return $ CmmStore addr' src'

        CmmCall { cml_target = addr }
           -> do addr' <- cmmExprConFold JumpReference addr
                 return $ stmt { cml_target = addr' }

        CmmUnsafeForeignCall target regs args
           -> do target' <- case target of
                              ForeignTarget e conv -> do
                                e' <- cmmExprConFold CallReference e
                                return $ ForeignTarget e' conv
                              PrimTarget _ ->
                                return target
                 args' <- mapM (cmmExprConFold DataReference) args
                 return $ CmmUnsafeForeignCall target' regs args'

        CmmCondBranch test true false likely
           -> do test' <- cmmExprConFold DataReference test
                 return $ case test' of
                   CmmLit (CmmInt 0 _) -> CmmBranch false
                   CmmLit (CmmInt _ _) -> CmmBranch true
                   _other -> CmmCondBranch test' true false likely

        CmmSwitch expr ids
           -> do expr' <- cmmExprConFold DataReference expr
                 return $ CmmSwitch expr' ids

        other
           -> return other

cmmExprConFold :: ReferenceKind -> CmmExpr -> CmmOptM CmmExpr
cmmExprConFold referenceKind expr = do
    config <- getCmmOptConfig

    let expr' = if not (ncgDoConstantFolding config)
                    then expr
                    else cmmExprCon config expr

    cmmExprNative referenceKind expr'

cmmExprCon :: NCGConfig -> CmmExpr -> CmmExpr
cmmExprCon config (CmmLoad addr rep) = CmmLoad (cmmExprCon config addr) rep
cmmExprCon config (CmmMachOp mop args)
    = cmmMachOpFold (ncgPlatform config) mop (map (cmmExprCon config) args)
cmmExprCon _ other = other

-- handles both PIC and non-PIC cases... a very strange mixture
-- of things to do.
cmmExprNative :: ReferenceKind -> CmmExpr -> CmmOptM CmmExpr
cmmExprNative referenceKind expr = do
     config <- getCmmOptConfig
     let platform = ncgPlatform config
         arch = platformArch platform
     case expr of
        CmmLoad addr rep
          -> do addr' <- cmmExprNative DataReference addr
                return $ CmmLoad addr' rep

        CmmMachOp mop args
          -> do args' <- mapM (cmmExprNative DataReference) args
                return $ CmmMachOp mop args'

        CmmLit (CmmBlock id)
          -> cmmExprNative referenceKind (CmmLit (CmmLabel (infoTblLbl id)))
          -- we must convert block Ids to CLabels here, because we
          -- might have to do the PIC transformation.  Hence we must
          -- not modify BlockIds beyond this point.

        CmmLit (CmmLabel lbl)
          -> cmmMakeDynamicReference config referenceKind lbl
        CmmLit (CmmLabelOff lbl off)
          -> do dynRef <- cmmMakeDynamicReference config referenceKind lbl
                -- need to optimize here, since it's late
                return $ cmmMachOpFold platform (MO_Add (wordWidth platform)) [
                    dynRef,
                    (CmmLit $ CmmInt (fromIntegral off) (wordWidth platform))
                  ]

        -- On powerpc (non-PIC), it's easier to jump directly to a label than
        -- to use the register table, so we replace these registers
        -- with the corresponding labels:
        CmmReg (CmmGlobal EagerBlackholeInfo)
          | arch == ArchPPC && not (ncgPIC config)
          -> cmmExprNative referenceKind $
             CmmLit (CmmLabel (mkCmmCodeLabel rtsUnitId (fsLit "__stg_EAGER_BLACKHOLE_info")))
        CmmReg (CmmGlobal GCEnter1)
          | arch == ArchPPC && not (ncgPIC config)
          -> cmmExprNative referenceKind $
             CmmLit (CmmLabel (mkCmmCodeLabel rtsUnitId (fsLit "__stg_gc_enter_1")))
        CmmReg (CmmGlobal GCFun)
          | arch == ArchPPC && not (ncgPIC config)
          -> cmmExprNative referenceKind $
             CmmLit (CmmLabel (mkCmmCodeLabel rtsUnitId (fsLit "__stg_gc_fun")))

        other
           -> return other

-- | Initialize the native code generator configuration from the DynFlags
initNCGConfig :: DynFlags -> Module -> NCGConfig
initNCGConfig dflags this_mod = NCGConfig
   { ncgPlatform              = targetPlatform dflags
   , ncgThisModule            = this_mod
   , ncgAsmContext            = initSDocContext dflags (PprCode AsmStyle)
   , ncgProcAlignment         = cmmProcAlignment dflags
   , ncgExternalDynamicRefs   = gopt Opt_ExternalDynamicRefs dflags
   , ncgPIC                   = positionIndependent dflags
   , ncgInlineThresholdMemcpy = fromIntegral $ maxInlineMemcpyInsns dflags
   , ncgInlineThresholdMemset = fromIntegral $ maxInlineMemsetInsns dflags
   , ncgSplitSections         = gopt Opt_SplitSections dflags
   , ncgRegsIterative         = gopt Opt_RegsIterative dflags
   , ncgAsmLinting            = gopt Opt_DoAsmLinting dflags
   , ncgCfgWeights            = cfgWeights dflags
   , ncgCfgBlockLayout        = gopt Opt_CfgBlocklayout dflags
   , ncgCfgWeightlessLayout   = gopt Opt_WeightlessBlocklayout dflags

     -- With -O1 and greater, the cmmSink pass does constant-folding, so
     -- we don't need to do it again in the native code generator.
   , ncgDoConstantFolding     = optLevel dflags < 1

   , ncgDumpRegAllocStages    = dopt Opt_D_dump_asm_regalloc_stages dflags
   , ncgDumpAsmStats          = dopt Opt_D_dump_asm_stats dflags
   , ncgDumpAsmConflicts      = dopt Opt_D_dump_asm_conflicts dflags
   , ncgBmiVersion            = case platformArch (targetPlatform dflags) of
                                 ArchX86_64 -> bmiVersion dflags
                                 ArchX86    -> bmiVersion dflags
                                 _          -> Nothing

     -- We assume  SSE1 and SSE2 operations are available on both
     -- x86 and x86_64. Historically we didn't default to SSE2 and
     -- SSE1 on x86, which results in defacto nondeterminism for how
     -- rounding behaves in the associated x87 floating point instructions
     -- because variations in the spill/fpu stack placement of arguments for
     -- operations would change the precision and final result of what
     -- would otherwise be the same expressions with respect to single or
     -- double precision IEEE floating point computations.
   , ncgSseVersion =
      let v | sseVersion dflags < Just SSE2 = Just SSE2
            | otherwise                     = sseVersion dflags
      in case platformArch (targetPlatform dflags) of
            ArchX86_64 -> v
            ArchX86    -> v
            _          -> Nothing

   , ncgDwarfEnabled        = debugLevel dflags >  0
   , ncgDwarfUnwindings     = debugLevel dflags >= 1
   , ncgExposeInternalSymbols = gopt Opt_ExposeInternalSymbols dflags
   , ncgDwarfStripBlockInfo = debugLevel dflags <  2 -- We strip out block information when running with -g0 or -g1.
   , ncgDwarfSourceNotes    = debugLevel dflags >= 3 -- We produce GHC-specific source-note DIEs only with -g3
   }