X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FnativeGen%2FPIC.hs;fp=compiler%2FnativeGen%2FPIC.hs;h=98e4f9f2d58f47e85cce39fa213f8323e86829eb;hb=b04a210e26ca57242fd052f2aa91011a80b76299;hp=0000000000000000000000000000000000000000;hpb=77ed23d51b968505b3ad8541c075657ae94f0ea3;p=ghc-hetmet.git

diff --git a/compiler/nativeGen/PIC.hs b/compiler/nativeGen/PIC.hs
new file mode 100644
index 0000000..98e4f9f
--- /dev/null
+++ b/compiler/nativeGen/PIC.hs
@@ -0,0 +1,746 @@
+{-
+  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, 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
+  :: 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 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
+-- 
+-- We need to use access *exactly* those things that
+-- are imported from a DLL via an __imp_* label.
+-- There are no stubs for imported code.
+--
+howToAccessLabel dflags _ OSMinGW32 _ lbl
+	| labelDynamic (thisPackage dflags) lbl	
+	= AccessViaSymbolPtr
+
+	| 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 OSLinux kind _
+
+	-- ELF PPC64 (powerpc64-linux), AIX, MacOS 9, BeOS/PPC
+	| DataReference	<- kind
+	= AccessViaSymbolPtr
+	
+	-- actually, .label instead of label
+	| otherwise
+	= AccessDirectly 
+
+howToAccessLabel _ _ OSLinux _ _
+	-- 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.
+	| not opt_PIC && opt_Static 
+	= AccessDirectly
+
+howToAccessLabel dflags arch OSLinux DataReference lbl
+	-- 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 OSLinux CallReference lbl
+	| labelDynamic (thisPackage dflags) lbl && not opt_PIC
+	= AccessDirectly
+
+	| arch /= ArchX86
+	, labelDynamic (thisPackage dflags) lbl && opt_PIC
+	= AccessViaStub
+
+howToAccessLabel dflags _ OSLinux _ lbl
+	| labelDynamic (thisPackage dflags) lbl 
+	= AccessViaSymbolPtr
+
+	| otherwise 
+	= 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 OSLinux lbl
+	= 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
+	| os == OSLinux || (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
+	| os	== OSLinux
+	, arch	== ArchPPC
+	= opt_PIC || not opt_Static
+
+	-- i386 (and others?): -dynamic but not -fPIC
+	| os	== OSLinux
+	, 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 
+	= mkForeignLabel -- HACK: it's not really foreign
+        	(fsLit ".LCTOC1") Nothing False 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") ]
+
+	| otherwise
+	= 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 OSLinux
+	| arch	/= ArchPPC_64
+	, not opt_PIC 
+	= Pretty.empty
+
+	| 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_PCI && 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 OSLinux _
+	= empty
+
+pprImportedSymbol _ OSLinux importedLbl
+	| Just (SymbolPtr, lbl)	<- dynamicLinkerLabelInfo importedLbl
+	= 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.
+	| otherwise = 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 OSLinux picReg
+    (CmmProc info lab params (ListGraph blocks) : statics)
+    = 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 OSLinux 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.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"
+