    {-
      This module handles generation of position independent code and
      dynamic-linking related issues for the native code generator.

      This depends both the architecture and OS, so we define it here
      instead of in one of the architecture specific modules.

      Things outside this module which are related to this:

      + module CLabel
        - PIC base label (pretty printed as local label 1)
        - DynamicLinkerLabels - several kinds:
            CodeStub, SymbolPtr, GotSymbolPtr, GotSymbolOffset
        - labelDynamic predicate
      + module Cmm
        - The GlobalReg datatype has a PicBaseReg constructor
        - The CmmLit datatype has a CmmLabelDiffOff constructor
      + codeGen & RTS
        - When tablesNextToCode, no absolute addresses are stored in info tables
          any more. Instead, offsets from the info label are used.
        - For Win32 only, SRTs might contain addresses of __imp_ symbol pointers
          because Win32 doesn't support external references in data sections.
          TODO: make sure this still works, it might be bitrotted
      + NCG
        - The cmmToCmm pass in AsmCodeGen calls cmmMakeDynamicReference for all
          labels.
        - nativeCodeGen calls pprImportedSymbol and pprGotDeclaration to output
          all the necessary stuff for imported symbols.
        - The NCG monad keeps track of a list of imported symbols.
        - MachCodeGen invokes initializePicBase to generate code to initialize
          the PIC base register when needed.
        - MachCodeGen calls cmmMakeDynamicReference whenever it uses a CLabel
          that wasn't in the original Cmm code (e.g. floating point literals).
      + The Mangler
        - The mangler converts absolure refs to relative refs in info tables
        - Symbol pointers, stub code and PIC calculations that are generated
          by GCC are left intact by the mangler (so far only on ppc-darwin
          and ppc-linux).
    -}

    module PIC (
            cmmMakeDynamicReference,
            ReferenceKind(..),
            needImportedSymbols,
            pprImportedSymbol,
            pprGotDeclaration,

            initializePicBase_ppc,
            initializePicBase_x86
    )

    where

    import qualified PPC.Instr	as PPC
    import qualified PPC.Regs	as PPC

    import qualified X86.Instr	as X86

    import Platform
    import Instruction
    import Size
    import Reg
    import NCGMonad

    import Cmm
    import CLabel           ( CLabel, ForeignLabelSource(..), pprCLabel,
                              mkDynamicLinkerLabel, DynamicLinkerLabelInfo(..),
                              dynamicLinkerLabelInfo, mkPicBaseLabel,
                              labelDynamic, externallyVisibleCLabel )

    import CLabel           ( mkForeignLabel )

    import StaticFlags	( opt_PIC, opt_Static )
    import BasicTypes

    import Pretty
    import qualified Outputable

    import Panic            ( panic )
    import DynFlags
    import FastString

    --------------------------------------------------------------------------------
    -- It gets called by the cmmToCmm pass for every CmmLabel in the Cmm
    -- code. It does The Right Thing(tm) to convert the CmmLabel into a
    -- position-independent, dynamic-linking-aware reference to the thing
    -- in question.
    -- Note that this also has to be called from MachCodeGen in order to
    -- access static data like floating point literals (labels that were
    -- created after the cmmToCmm pass).
    -- The function must run in a monad that can keep track of imported symbols
    -- A function for recording an imported symbol must be passed in:
    -- - addImportCmmOpt for the CmmOptM monad
    -- - addImportNat for the NatM monad.

    data ReferenceKind
    	= DataReference
            | CallReference
            | JumpReference
            deriving(Eq)

    cmmMakeDynamicReference, cmmMakeDynamicReference'
      :: Monad m => DynFlags
                 -> (CLabel -> m ())  -- a monad & a function
                                      -- used for recording imported symbols
                 -> ReferenceKind     -- whether this is the target of a jump
                 -> CLabel            -- the label
                 -> m CmmExpr

    cmmMakeDynamicReference = cmmMakeDynamicReference'

    cmmMakeDynamicReference' dflags addImport referenceKind lbl
      | Just _ <- dynamicLinkerLabelInfo lbl
      = return $ CmmLit $ CmmLabel lbl   -- already processed it, pass through

      | otherwise
      = case howToAccessLabel
      		dflags
    		(platformArch $ targetPlatform dflags)
    		(platformOS   $ targetPlatform dflags)
    		referenceKind lbl of

            AccessViaStub -> do
                  let stub = mkDynamicLinkerLabel CodeStub lbl
                  addImport stub
                  return $ CmmLit $ CmmLabel stub

            AccessViaSymbolPtr -> do
                  let symbolPtr = mkDynamicLinkerLabel SymbolPtr lbl
                  addImport symbolPtr
                  return $ CmmLoad (cmmMakePicReference dflags symbolPtr) bWord

            AccessDirectly -> case referenceKind of
                    -- for data, we might have to make some calculations:
                  DataReference -> return $ cmmMakePicReference dflags lbl
                    -- all currently supported processors support
                    -- PC-relative branch and call instructions,
                    -- so just jump there if it's a call or a jump
                  _ -> return $ CmmLit $ CmmLabel lbl

    -- -----------------------------------------------------------------------------
    -- Create a position independent reference to a label.
    -- (but do not bother with dynamic linking).
    -- We calculate the label's address by adding some (platform-dependent)
    -- offset to our base register; this offset is calculated by
    -- the function picRelative in the platform-dependent part below.

    cmmMakePicReference :: DynFlags -> CLabel -> CmmExpr
    cmmMakePicReference dflags lbl

            -- Windows doesn't need PIC,
            -- everything gets relocated at runtime
    	| OSMinGW32	<- platformOS $ targetPlatform dflags
    	= CmmLit $ CmmLabel lbl

    	| (opt_PIC || not opt_Static) && absoluteLabel lbl
    	= CmmMachOp (MO_Add wordWidth)
    		[ CmmReg (CmmGlobal PicBaseReg)
    		, CmmLit $ picRelative
    				(platformArch 	$ targetPlatform dflags)
    				(platformOS	$ targetPlatform dflags)
    				lbl ]

    	| otherwise
    	= CmmLit $ CmmLabel lbl

    absoluteLabel :: CLabel -> Bool
    absoluteLabel lbl
     = case dynamicLinkerLabelInfo lbl of
    	Just (GotSymbolPtr, _)		-> False
    	Just (GotSymbolOffset, _)	-> False
    	_ 				-> True

    --------------------------------------------------------------------------------
    -- Knowledge about how special dynamic linker labels like symbol
    -- pointers, code stubs and GOT offsets look like is located in the
    -- module CLabel.

    -- We have to decide which labels need to be accessed
    -- indirectly or via a piece of stub code.
    data LabelAccessStyle
    	= AccessViaStub
    	| AccessViaSymbolPtr
    	| AccessDirectly

    howToAccessLabel
    	:: DynFlags -> Arch -> OS -> ReferenceKind -> CLabel -> LabelAccessStyle

    -- Windows
    -- In Windows speak, a "module" is a set of objects linked into the
    -- same Portable Exectuable (PE) file. (both .exe and .dll files are PEs).
    --
    -- If we're compiling a multi-module program then symbols from other modules
    -- are accessed by a symbol pointer named __imp_SYMBOL. At runtime we have the
    -- following.
    --
    --   (in the local module)
    --     __imp_SYMBOL: addr of SYMBOL
    --
    --   (in the other module)
    --     SYMBOL: the real function / data.
    --
    -- To access the function at SYMBOL from our local module, we just need to
    -- dereference the local __imp_SYMBOL.
    --
    -- If opt_Static is set then we assume that all our code will be linked
    -- into the same .exe file. In this case we always access symbols directly,
    -- and never use __imp_SYMBOL.
    --
    howToAccessLabel dflags _ OSMinGW32 _ lbl

    	-- Assume all symbols will be in the same PE, so just access them directly.
    	| opt_Static
    	= AccessDirectly

    	-- If the target symbol is in another PE we need to access it via the
    	--	appropriate __imp_SYMBOL pointer.
    	| labelDynamic (thisPackage dflags) lbl
    	= AccessViaSymbolPtr

    	-- Target symbol is in the same PE as the caller, so just access it directly.
    	| otherwise
    	= AccessDirectly

    -- Mach-O (Darwin, Mac OS X)
    --
    -- Indirect access is required in the following cases:
    --  * things imported from a dynamic library
    --  * (not on x86_64) data from a different module, if we're generating PIC code
    -- It is always possible to access something indirectly,
    -- even when it's not necessary.
    --
    howToAccessLabel dflags arch OSDarwin DataReference lbl
    	-- data access to a dynamic library goes via a symbol pointer
    	| labelDynamic (thisPackage dflags) lbl
    	= AccessViaSymbolPtr

    	-- when generating PIC code, all cross-module data references must
    	-- must go via a symbol pointer, too, because the assembler
    	-- cannot generate code for a label difference where one
    	-- label is undefined. Doesn't apply t x86_64.
    	-- Unfortunately, we don't know whether it's cross-module,
    	-- so we do it for all externally visible labels.
    	-- This is a slight waste of time and space, but otherwise
    	-- we'd need to pass the current Module all the way in to
    	-- this function.
    	| arch /= ArchX86_64
    	, opt_PIC && externallyVisibleCLabel lbl
    	= AccessViaSymbolPtr

    	| otherwise
    	= AccessDirectly

    howToAccessLabel dflags arch OSDarwin JumpReference lbl
    	-- dyld code stubs don't work for tailcalls because the
    	-- stack alignment is only right for regular calls.
    	-- Therefore, we have to go via a symbol pointer:
    	| arch == ArchX86 || arch == ArchX86_64
    	, labelDynamic (thisPackage dflags) lbl
    	= AccessViaSymbolPtr

    howToAccessLabel dflags arch OSDarwin _ lbl
    	-- Code stubs are the usual method of choice for imported code;
    	-- not needed on x86_64 because Apple's new linker, ld64, generates
    	-- them automatically.
    	| arch /= ArchX86_64
    	, labelDynamic (thisPackage dflags) lbl
    	= AccessViaStub

    	| otherwise
    	= AccessDirectly

    -- ELF (Linux)
    --
    -- ELF tries to pretend to the main application code that dynamic linking does
    -- not exist. While this may sound convenient, it tends to mess things up in
    -- very bad ways, so we have to be careful when we generate code for the main
    -- program (-dynamic but no -fPIC).
    --
    -- Indirect access is required for references to imported symbols
    -- from position independent code. It is also required from the main program
    -- when dynamic libraries containing Haskell code are used.

    howToAccessLabel _ ArchPPC_64 os kind _
    	| osElfTarget os
    	= if kind == DataReference
    	    -- ELF PPC64 (powerpc64-linux), AIX, MacOS 9, BeOS/PPC
                then AccessViaSymbolPtr
    	    -- actually, .label instead of label
                else AccessDirectly

    howToAccessLabel _ _ os _ _
    	-- no PIC -> the dynamic linker does everything for us;
    	--           if we don't dynamically link to Haskell code,
    	--           it actually manages to do so without messing thins up.
    	| osElfTarget os
    	, not opt_PIC && opt_Static
    	= AccessDirectly

    howToAccessLabel dflags arch os DataReference lbl
    	| osElfTarget os
    	= case () of
    	    -- A dynamic label needs to be accessed via a symbol pointer.
              _ | labelDynamic (thisPackage dflags) lbl
    	    -> AccessViaSymbolPtr

     	    -- For PowerPC32 -fPIC, we have to access even static data
    	    -- via a symbol pointer (see below for an explanation why
    	    -- PowerPC32 Linux is especially broken).
    	    | arch == ArchPPC
    	    , opt_PIC
    	    -> AccessViaSymbolPtr

    	    | otherwise
    	    -> AccessDirectly

    	-- In most cases, we have to avoid symbol stubs on ELF, for the following reasons:
    	--   on i386, the position-independent symbol stubs in the Procedure Linkage Table
    	--   require the address of the GOT to be loaded into register %ebx on entry.
    	--   The linker will take any reference to the symbol stub as a hint that
    	--   the label in question is a code label. When linking executables, this
    	--   will cause the linker to replace even data references to the label with
    	--   references to the symbol stub.

    	-- This leaves calling a (foreign) function from non-PIC code
    	-- (AccessDirectly, because we get an implicit symbol stub)
    	-- and calling functions from PIC code on non-i386 platforms (via a symbol stub)

    howToAccessLabel dflags arch os CallReference lbl
    	| osElfTarget os
    	, labelDynamic (thisPackage dflags) lbl && not opt_PIC
    	= AccessDirectly

    	| osElfTarget os
    	, arch /= ArchX86
    	, labelDynamic (thisPackage dflags) lbl && opt_PIC
    	= AccessViaStub

    howToAccessLabel dflags _ os _ lbl
    	| osElfTarget os
    	= if labelDynamic (thisPackage dflags) lbl
    	    then AccessViaSymbolPtr
    	    else AccessDirectly

    -- all other platforms
    howToAccessLabel _ _ _ _ _
    	| not opt_PIC
    	= AccessDirectly

    	| otherwise
    	= panic "howToAccessLabel: PIC not defined for this platform"

    -- -------------------------------------------------------------------
    -- | Says what we we have to add to our 'PIC base register' in order to
    -- 	get the address of a label.

    picRelative :: Arch -> OS -> CLabel -> CmmLit

    -- Darwin, but not x86_64:
    -- The PIC base register points to the PIC base label at the beginning
    -- of the current CmmTop. We just have to use a label difference to
    -- get the offset.
    -- We have already made sure that all labels that are not from the current
    -- module are accessed indirectly ('as' can't calculate differences between
    -- undefined labels).
    picRelative arch OSDarwin lbl
    	| arch /= ArchX86_64
    	= CmmLabelDiffOff lbl mkPicBaseLabel 0

    -- PowerPC Linux:
    -- The PIC base register points to our fake GOT. Use a label difference
    -- to get the offset.
    -- We have made sure that *everything* is accessed indirectly, so this
    -- is only used for offsets from the GOT to symbol pointers inside the
    -- GOT.
    picRelative ArchPPC os lbl
    	| osElfTarget os
    	= CmmLabelDiffOff lbl gotLabel 0

    -- Most Linux versions:
    -- The PIC base register points to the GOT. Use foo@got for symbol
    -- pointers, and foo@gotoff for everything else.
    -- Linux and Darwin on x86_64:
    -- The PIC base register is %rip, we use foo@gotpcrel for symbol pointers,
    -- and a GotSymbolOffset label for other things.
    -- For reasons of tradition, the symbol offset label is written as a plain label.
    picRelative arch os lbl
    	| osElfTarget os || (os == OSDarwin && arch == ArchX86_64)
    	= let	result
    			| Just (SymbolPtr, lbl') <- dynamicLinkerLabelInfo lbl
      			= CmmLabel $ mkDynamicLinkerLabel GotSymbolPtr lbl'

    			| otherwise
    			= CmmLabel $ mkDynamicLinkerLabel GotSymbolOffset lbl

    	  in	result

    picRelative _ _ _
    	= panic "PositionIndependentCode.picRelative undefined for this platform"

    --------------------------------------------------------------------------------

    -- utility function for pretty-printing asm-labels,
    -- copied from PprMach
    --
    asmSDoc :: Outputable.SDoc -> Doc
    asmSDoc d
    	= Outputable.withPprStyleDoc
    		(Outputable.mkCodeStyle Outputable.AsmStyle) d

    pprCLabel_asm :: CLabel -> Doc
    pprCLabel_asm l
    	= asmSDoc (pprCLabel l)

    needImportedSymbols :: Arch -> OS -> Bool
    needImportedSymbols arch os
    	| os 	== OSDarwin
    	, arch 	/= ArchX86_64
    	= True

    	-- PowerPC Linux: -fPIC or -dynamic
    	| osElfTarget os
    	, arch	== ArchPPC
    	= opt_PIC || not opt_Static

    	-- i386 (and others?): -dynamic but not -fPIC
    	| osElfTarget os
    	, arch	/= ArchPPC_64
    	= not opt_Static && not opt_PIC

    	| otherwise
    	= False

    -- gotLabel
    -- The label used to refer to our "fake GOT" from
    -- position-independent code.
    gotLabel :: CLabel
    gotLabel
    	-- HACK: this label isn't really foreign
    	= mkForeignLabel
    		(fsLit ".LCTOC1")
    		Nothing ForeignLabelInThisPackage IsData

    --------------------------------------------------------------------------------
    -- We don't need to declare any offset tables.
    -- However, for PIC on x86, we need a small helper function.
    pprGotDeclaration :: Arch -> OS -> Doc
    pprGotDeclaration ArchX86 OSDarwin
    	| opt_PIC
    	= vcat [
    	        ptext (sLit ".section __TEXT,__textcoal_nt,coalesced,no_toc"),
    	        ptext (sLit ".weak_definition ___i686.get_pc_thunk.ax"),
    	        ptext (sLit ".private_extern ___i686.get_pc_thunk.ax"),
    	        ptext (sLit "___i686.get_pc_thunk.ax:"),
    		ptext (sLit "\tmovl (%esp), %eax"),
    		ptext (sLit "\tret") ]

    pprGotDeclaration _ OSDarwin
    	= Pretty.empty

    -- pprGotDeclaration
    -- Output whatever needs to be output once per .s file.
    -- The .LCTOC1 label is defined to point 32768 bytes into the table,
    -- to make the most of the PPC's 16-bit displacements.
    -- Only needed for PIC.
    pprGotDeclaration arch os
    	| osElfTarget os
    	, arch	/= ArchPPC_64
    	, not opt_PIC
    	= Pretty.empty

    	| osElfTarget os
    	, arch	/= ArchPPC_64
    	= vcat [
    		ptext (sLit ".section \".got2\",\"aw\""),
    		ptext (sLit ".LCTOC1 = .+32768") ]

    pprGotDeclaration _ _
    	= panic "pprGotDeclaration: no match"

    --------------------------------------------------------------------------------
    -- On Darwin, we have to generate our own stub code for lazy binding..
    -- For each processor architecture, there are two versions, one for PIC
    -- and one for non-PIC.
    --
    -- Whenever you change something in this assembler output, make sure
    -- the splitter in driver/split/ghc-split.lprl recognizes the new output

    pprImportedSymbol :: Arch -> OS -> CLabel -> Doc
    pprImportedSymbol ArchPPC OSDarwin importedLbl
    	| Just (CodeStub, lbl) <- dynamicLinkerLabelInfo importedLbl
    	= case opt_PIC of
               False ->
                vcat [
                    ptext (sLit ".symbol_stub"),
                    ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$stub:"),
                        ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
                        ptext (sLit "\tlis r11,ha16(L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr)"),
                        ptext (sLit "\tlwz r12,lo16(L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr)(r11)"),
                        ptext (sLit "\tmtctr r12"),
                        ptext (sLit "\taddi r11,r11,lo16(L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr)"),
                        ptext (sLit "\tbctr")
                ]
               True ->
                vcat [
                    ptext (sLit ".section __TEXT,__picsymbolstub1,")
                      <> ptext (sLit "symbol_stubs,pure_instructions,32"),
                    ptext (sLit "\t.align 2"),
                    ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$stub:"),
                        ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
                        ptext (sLit "\tmflr r0"),
                        ptext (sLit "\tbcl 20,31,L0$") <> pprCLabel_asm lbl,
                    ptext (sLit "L0$") <> pprCLabel_asm lbl <> char ':',
                        ptext (sLit "\tmflr r11"),
                        ptext (sLit "\taddis r11,r11,ha16(L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr-L0$") <> pprCLabel_asm lbl <> char ')',
                        ptext (sLit "\tmtlr r0"),
                        ptext (sLit "\tlwzu r12,lo16(L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr-L0$") <> pprCLabel_asm lbl
                            <> ptext (sLit ")(r11)"),
                        ptext (sLit "\tmtctr r12"),
                        ptext (sLit "\tbctr")
                ]
    	  $+$ vcat [
             	ptext (sLit ".lazy_symbol_pointer"),
    	        ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$lazy_ptr:"),
    		ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
    	        ptext (sLit "\t.long dyld_stub_binding_helper")]

    	| Just (SymbolPtr, lbl) <- dynamicLinkerLabelInfo importedLbl
    	= vcat [
    	        ptext (sLit ".non_lazy_symbol_pointer"),
    	        char 'L' <> pprCLabel_asm lbl <> ptext (sLit "$non_lazy_ptr:"),
    		ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
    		ptext (sLit "\t.long\t0")]

    	| otherwise
    	= empty

    pprImportedSymbol ArchX86 OSDarwin importedLbl
    	| Just (CodeStub, lbl) <- dynamicLinkerLabelInfo importedLbl
    	= case opt_PIC of
               False ->
                vcat [
                    ptext (sLit ".symbol_stub"),
                    ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$stub:"),
                        ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
                        ptext (sLit "\tjmp *L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr"),
                    ptext (sLit "L") <> pprCLabel_asm lbl
                        <> ptext (sLit "$stub_binder:"),
                        ptext (sLit "\tpushl $L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr"),
                        ptext (sLit "\tjmp dyld_stub_binding_helper")
                ]
               True ->
                vcat [
                    ptext (sLit ".section __TEXT,__picsymbolstub2,")
                        <> ptext (sLit "symbol_stubs,pure_instructions,25"),
                    ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$stub:"),
                        ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
                        ptext (sLit "\tcall ___i686.get_pc_thunk.ax"),
                    ptext (sLit "1:"),
                        ptext (sLit "\tmovl L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr-1b(%eax),%edx"),
                        ptext (sLit "\tjmp *%edx"),
                    ptext (sLit "L") <> pprCLabel_asm lbl
                        <> ptext (sLit "$stub_binder:"),
                        ptext (sLit "\tlea L") <> pprCLabel_asm lbl
                            <> ptext (sLit "$lazy_ptr-1b(%eax),%eax"),
                        ptext (sLit "\tpushl %eax"),
                        ptext (sLit "\tjmp dyld_stub_binding_helper")
                ]
    	  $+$ vcat [        ptext (sLit ".section __DATA, __la_sym_ptr")
                        <> (if opt_PIC then int 2 else int 3)
                        <> ptext (sLit ",lazy_symbol_pointers"),
    	        ptext (sLit "L") <> pprCLabel_asm lbl <> ptext (sLit "$lazy_ptr:"),
    	            ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
    	            ptext (sLit "\t.long L") <> pprCLabel_asm lbl
                        <> ptext (sLit "$stub_binder")]

    	| Just (SymbolPtr, lbl) <- dynamicLinkerLabelInfo importedLbl
    	= vcat [
    	        ptext (sLit ".non_lazy_symbol_pointer"),
    	        char 'L' <> pprCLabel_asm lbl <> ptext (sLit "$non_lazy_ptr:"),
    		ptext (sLit "\t.indirect_symbol") <+> pprCLabel_asm lbl,
    		ptext (sLit "\t.long\t0")]

    	| otherwise
    	= empty

    pprImportedSymbol _ OSDarwin _
    	= empty

    -- ELF / Linux
    --
    -- In theory, we don't need to generate any stubs or symbol pointers
    -- by hand for Linux.
    --
    -- Reality differs from this in two areas.
    --
    -- 1) If we just use a dynamically imported symbol directly in a read-only
    --    section of the main executable (as GCC does), ld generates R_*_COPY
    --    relocations, which are fundamentally incompatible with reversed info
    --    tables. Therefore, we need a table of imported addresses in a writable
    --    section.
    --    The "official" GOT mechanism (label@got) isn't intended to be used
    --    in position dependent code, so we have to create our own "fake GOT"
    --    when not opt_PIC && not opt_Static.
    --
    -- 2) PowerPC Linux is just plain broken.
    --    While it's theoretically possible to use GOT offsets larger
    --    than 16 bit, the standard crt*.o files don't, which leads to
    --    linker errors as soon as the GOT size exceeds 16 bit.
    --    Also, the assembler doesn't support @gotoff labels.
    --    In order to be able to use a larger GOT, we have to circumvent the
    --    entire GOT mechanism and do it ourselves (this is also what GCC does).

    -- When needImportedSymbols is defined,
    -- the NCG will keep track of all DynamicLinkerLabels it uses
    -- and output each of them using pprImportedSymbol.

    pprImportedSymbol ArchPPC_64 os _
    	| osElfTarget os
    	= empty

    pprImportedSymbol _ os importedLbl
    	| osElfTarget os
    	= case dynamicLinkerLabelInfo importedLbl of
    	    Just (SymbolPtr, lbl)
    	      -> let symbolSize = case wordWidth of
    		         W32 -> sLit "\t.long"
    		         W64 -> sLit "\t.quad"
    		         _ -> panic "Unknown wordRep in pprImportedSymbol"

    	         in vcat [
    	              ptext (sLit ".section \".got2\", \"aw\""),
    	              ptext (sLit ".LC_") <> pprCLabel_asm lbl <> char ':',
    	              ptext symbolSize <+> pprCLabel_asm lbl ]

    	    -- PLT code stubs are generated automatically by the dynamic linker.
    	    _ -> empty

    pprImportedSymbol _ _ _
    	= panic "PIC.pprImportedSymbol: no match"

    --------------------------------------------------------------------------------
    -- Generate code to calculate the address that should be put in the
    -- PIC base register.
    -- This is called by MachCodeGen for every CmmProc that accessed the
    -- PIC base register. It adds the appropriate instructions to the
    -- top of the CmmProc.

    -- It is assumed that the first NatCmmTop in the input list is a Proc
    -- and the rest are CmmDatas.

    -- Darwin is simple: just fetch the address of a local label.
    -- The FETCHPC pseudo-instruction is expanded to multiple instructions
    -- during pretty-printing so that we don't have to deal with the
    -- local label:

    -- PowerPC version:
    --          bcl 20,31,1f.
    --      1:  mflr picReg

    -- i386 version:
    --          call 1f
    --      1:  popl %picReg

    -- Get a pointer to our own fake GOT, which is defined on a per-module basis.
    -- This is exactly how GCC does it, and it's quite horrible:
    -- We first fetch the address of a local label (mkPicBaseLabel).
    -- Then we add a 16-bit offset to that to get the address of a .long that we
    -- define in .text space right next to the proc. This .long literal contains
    -- the (32-bit) offset from our local label to our global offset table
    -- (.LCTOC1 aka gotOffLabel).

    initializePicBase_ppc
    	:: Arch -> OS -> Reg
    	-> [NatCmmTop PPC.Instr]
    	-> NatM [NatCmmTop PPC.Instr]

    initializePicBase_ppc ArchPPC os picReg
        (CmmProc info lab params (ListGraph blocks) : statics)
        | osElfTarget os
        = do
            gotOffLabel <- getNewLabelNat
            tmp <- getNewRegNat $ intSize wordWidth
            let
                gotOffset = CmmData Text [
                                CmmDataLabel gotOffLabel,
    			    CmmStaticLit (CmmLabelDiffOff gotLabel
                            	                          mkPicBaseLabel
    				                          0)
                            ]
                offsetToOffset
    	    		= PPC.ImmConstantDiff
    				(PPC.ImmCLbl gotOffLabel)
    				(PPC.ImmCLbl mkPicBaseLabel)

                BasicBlock bID insns
    	    		= head blocks

                b' = BasicBlock bID (PPC.FETCHPC picReg
                                   : PPC.LD PPC.archWordSize tmp
                                        (PPC.AddrRegImm picReg offsetToOffset)
                                   : PPC.ADD picReg picReg (PPC.RIReg tmp)
                                   : insns)

            return (CmmProc info lab params (ListGraph (b' : tail blocks)) : gotOffset : statics)

    initializePicBase_ppc ArchPPC OSDarwin picReg
    	(CmmProc info lab params (ListGraph blocks) : statics)
    	= return (CmmProc info lab params (ListGraph (b':tail blocks)) : statics)

    	where 	BasicBlock bID insns = head blocks
              	b' = BasicBlock bID (PPC.FETCHPC picReg : insns)

    initializePicBase_ppc _ _ _ _
    	= panic "initializePicBase_ppc: not needed"

    -- We cheat a bit here by defining a pseudo-instruction named FETCHGOT
    -- which pretty-prints as:
    --              call 1f
    -- 1:           popl %picReg
    --              addl __GLOBAL_OFFSET_TABLE__+.-1b, %picReg
    -- (See PprMach.lhs)

    initializePicBase_x86
    	:: Arch -> OS -> Reg
    	-> [NatCmmTop X86.Instr]
    	-> NatM [NatCmmTop X86.Instr]

    initializePicBase_x86 ArchX86 os picReg
    	(CmmProc info lab params (ListGraph blocks) : statics)
        | osElfTarget os
        = return (CmmProc info lab params (ListGraph (b':tail blocks)) : statics)
        where BasicBlock bID insns = head blocks
              b' = BasicBlock bID (X86.FETCHGOT picReg : insns)

    initializePicBase_x86 ArchX86 OSDarwin picReg
    	(CmmProc info lab params (ListGraph blocks) : statics)
    	= return (CmmProc info lab params (ListGraph (b':tail blocks)) : statics)

    	where 	BasicBlock bID insns = head blocks
              	b' = BasicBlock bID (X86.FETCHPC picReg : insns)

    initializePicBase_x86 _ _ _ _
    	= panic "initializePicBase_x86: not needed"