/**
 * Compiler implementation of the
 * $(LINK2 https://www.dlang.org, D programming language).
 *
 * Copyright:   Copyright (C) 1994-1998 by Symantec
 *              Copyright (C) 2000-2022 by The D Language Foundation, All Rights Reserved
 * Authors:     $(LINK2 https://www.digitalmars.com, Walter Bright)
 * License:     $(LINK2 https://www.boost.org/LICENSE_1_0.txt, Boost License 1.0)
 * Source:      $(LINK2 https://github.com/dlang/dmd/blob/master/src/dmd/backend/obj.d, backend/obj.d)
 */

module dmd.backend.obj;

// Online documentation: https://dlang.org/phobos/dmd_backend_obj.html

/* Interface to object file format
 */

import dmd.backend.cdef;
import dmd.backend.cc;
import dmd.backend.code;
import dmd.backend.el;

import dmd.common.outbuffer;

extern (C++):

nothrow:

version (SPP)
    version = STUB;
else version (HTOD)
    version = STUB;
else version (Windows)
{
}
else version (Posix)
{
}
else
    static assert(0, "unsupported version");

/******************************************************************/

/* Functions common to all object formats
 */
mixin(ObjMemDecl("Obj $Obj_init(OutBuffer *, const(char)* filename, const(char)* csegname)"));
mixin(ObjMemDecl("void $Obj_initfile(const(char)* filename, const(char)* csegname, const(char)* modname)"));
mixin(ObjMemDecl("void $Obj_termfile()"));
mixin(ObjMemDecl("void $Obj_term(const(char)* objfilename)"));
mixin(ObjMemDecl("void $Obj_linnum(Srcpos srcpos, int seg, targ_size_t offset)"));
mixin(ObjMemDecl("int  $Obj_codeseg(const char *name,int suffix)"));
mixin(ObjMemDecl("void $Obj_startaddress(Symbol *)"));
mixin(ObjMemDecl("bool $Obj_includelib(const(char)* )"));
mixin(ObjMemDecl("bool $Obj_linkerdirective(const(char)* )"));
mixin(ObjMemDecl("bool $Obj_allowZeroSize()"));
mixin(ObjMemDecl("void $Obj_exestr(const(char)* p)"));
mixin(ObjMemDecl("void $Obj_user(const(char)* p)"));
mixin(ObjMemDecl("void $Obj_compiler()"));
mixin(ObjMemDecl("void $Obj_wkext(Symbol *,Symbol *)"));
mixin(ObjMemDecl("void $Obj_alias(const(char)* n1,const(char)* n2)"));
mixin(ObjMemDecl("void $Obj_staticctor(Symbol *s,int dtor,int seg)"));
mixin(ObjMemDecl("void $Obj_staticdtor(Symbol *s)"));
mixin(ObjMemDecl("void $Obj_setModuleCtorDtor(Symbol *s, bool isCtor)"));
mixin(ObjMemDecl("void $Obj_ehtables(Symbol *sfunc,uint size,Symbol *ehsym)"));
mixin(ObjMemDecl("void $Obj_ehsections()"));
mixin(ObjMemDecl("void $Obj_moduleinfo(Symbol *scc)"));
mixin(ObjMemDecl("int  $Obj_comdat(Symbol *)"));
mixin(ObjMemDecl("int  $Obj_comdatsize(Symbol *, targ_size_t symsize)"));
mixin(ObjMemDecl("int  $Obj_readonly_comdat(Symbol *s)"));
mixin(ObjMemDecl("void $Obj_setcodeseg(int seg)"));
mixin(ObjMemDecl("seg_data* $Obj_tlsseg()"));
mixin(ObjMemDecl("seg_data* $Obj_tlsseg_bss()"));
mixin(ObjMemDecl("seg_data* $Obj_tlsseg_data()"));
mixin(ObjMemDecl("void $Obj_export_symbol(Symbol *s, uint argsize)"));
mixin(ObjMemDecl("void $Obj_pubdef(int seg, Symbol *s, targ_size_t offset)"));
mixin(ObjMemDecl("void $Obj_pubdefsize(int seg, Symbol *s, targ_size_t offset, targ_size_t symsize)"));
mixin(ObjMemDecl("int  $Obj_external_def(const(char)*)"));
mixin(ObjMemDecl("int  $Obj_data_start(Symbol *sdata, targ_size_t datasize, int seg)"));
mixin(ObjMemDecl("int  $Obj_external(Symbol *)"));
mixin(ObjMemDecl("int  $Obj_common_block(Symbol *s, targ_size_t size, targ_size_t count)"));
mixin(ObjMemDecl("int  $Obj_common_block(Symbol *s, int flag, targ_size_t size, targ_size_t count)"));
mixin(ObjMemDecl("void $Obj_lidata(int seg, targ_size_t offset, targ_size_t count)"));
mixin(ObjMemDecl("void $Obj_write_zeros(seg_data *pseg, targ_size_t count)"));
mixin(ObjMemDecl("void $Obj_write_byte(seg_data *pseg, uint _byte)"));
mixin(ObjMemDecl("void $Obj_write_bytes(seg_data *pseg, uint nbytes, void *p)"));
mixin(ObjMemDecl("void $Obj_byte(int seg, targ_size_t offset, uint _byte)"));
mixin(ObjMemDecl("uint $Obj_bytes(int seg, targ_size_t offset, uint nbytes, void *p)"));
mixin(ObjMemDecl("void $Obj_reftodatseg(int seg, targ_size_t offset, targ_size_t val, uint targetdatum, int flags)"));
mixin(ObjMemDecl("void $Obj_reftocodeseg(int seg, targ_size_t offset, targ_size_t val)"));
mixin(ObjMemDecl("int  $Obj_reftoident(int seg, targ_size_t offset, Symbol *s, targ_size_t val, int flags)"));
mixin(ObjMemDecl("void $Obj_far16thunk(Symbol *s)"));
mixin(ObjMemDecl("void $Obj_fltused()"));
mixin(ObjMemDecl("int  $Obj_data_readonly(char *p, int len, int *pseg)"));
mixin(ObjMemDecl("int  $Obj_data_readonly(char *p, int len)"));
mixin(ObjMemDecl("int  $Obj_string_literal_segment(uint sz)"));
mixin(ObjMemDecl("Symbol* $Obj_sym_cdata(tym_t, char *, int)"));
mixin(ObjMemDecl("void $Obj_func_start(Symbol *sfunc)"));
mixin(ObjMemDecl("void $Obj_func_term(Symbol *sfunc)"));
mixin(ObjMemDecl("void $Obj_write_pointerRef(Symbol* s, uint off)"));
mixin(ObjMemDecl("int  $Obj_jmpTableSegment(Symbol* s)"));
mixin(ObjMemDecl("Symbol* $Obj_tlv_bootstrap()"));

/******************************************************************/

size_t OmfObj_mangle(Symbol *s,char *dest);
void OmfObj_import(elem *e);
void OmfObj_dosseg();
void OmfObj_lzext(Symbol *,Symbol *);
void OmfObj_theadr(const(char)* modname);
void OmfObj_segment_group(targ_size_t codesize, targ_size_t datasize, targ_size_t cdatasize, targ_size_t udatasize);
int  OmfObj_fardata(char *name, targ_size_t size, targ_size_t *poffset);
void OmfObj_ledata(int seg, targ_size_t offset, targ_size_t data, uint lcfd, uint idx1, uint idx2);
void OmfObj_write_long(int seg, targ_size_t offset, uint data, uint lcfd, uint idx1, uint idx2);
void OmfObj_reftofarseg(int seg, targ_size_t offset, targ_size_t val, int farseg, int flags);
int  OmfObj_seg_debugT();           // where the symbolic debug type data goes

/******************************************************************/

int  MsCoffObj_getsegment(const(char)* sectname, uint flags);
int  MsCoffObj_getsegment2(uint shtidx);
uint MsCoffObj_addScnhdr(const(char)* scnhdr_name, uint flags);
void MsCoffObj_addrel(int seg, targ_size_t offset, Symbol *targsym, uint targseg, int rtype, int val);
int  MsCoffObj_seg_drectve();
int  MsCoffObj_seg_pdata();
int  MsCoffObj_seg_xdata();
int  MsCoffObj_seg_pdata_comdat(Symbol *sfunc);
int  MsCoffObj_seg_xdata_comdat(Symbol *sfunc);
int  MsCoffObj_seg_debugS();
int  MsCoffObj_seg_debugS_comdat(Symbol *sfunc);
int  MsCoffObj_seg_debugT();           // where the symbolic debug type data goes

/******************************************************************/

void ElfObj_dosseg();
size_t ElfObj_mangle(Symbol* s, char* dest);
void ElfObj_import(elem* e);
void ElfObj_lzext(Symbol*, Symbol*);
void ElfObj_theadr(const(char)* modname);
void ElfObj_segment_group(targ_size_t codesize, targ_size_t datasize, targ_size_t cdatasize, targ_size_t udatasize);
int ElfObj_fardata(char *name, targ_size_t size, targ_size_t* poffset);
void ElfObj_ledata(int seg, targ_size_t offset, targ_size_t data, uint lcfd, uint idx1, uint idx2);
void ElfObj_reftofarseg(int seg, targ_size_t offset, targ_size_t val, int farseg, int flags);
void ElfObj_gotref(Symbol* s);
uint ElfObj_addstr(OutBuffer* strtab, const(char)*);
Symbol* ElfObj_getGOTsym();
void ElfObj_refGOTsym();
int ElfObj_getsegment(const(char)* sectname, const(char)* suffix, int type, int flags, int align_);
void ElfObj_addrel(int seg, targ_size_t offset, uint type, uint symidx, targ_size_t val);
size_t ElfObj_writerel(int targseg, size_t offset, uint type, uint symidx, targ_size_t val);

/******************************************************************/

void MachObj_dosseg();
size_t MachObj_mangle(Symbol *s,char *dest);
void MachObj_import(elem *e);
void MachObj_lzext(Symbol *,Symbol *);
void MachObj_theadr(const(char)* modname);
void MachObj_segment_group(targ_size_t codesize, targ_size_t datasize, targ_size_t cdatasize, targ_size_t udatasize);
int MachObj_fardata(char *name, targ_size_t size, targ_size_t *poffset);
void MachObj_ledata(int seg, targ_size_t offset, targ_size_t data, uint lcfd, uint idx1, uint idx2);
void MachObj_reftofarseg(int seg, targ_size_t offset, targ_size_t val, int farseg, int flags);
void MachObj_gotref(Symbol *s);
uint MachObj_addstr(OutBuffer *strtab, const(char)* );
Symbol* MachObj_getGOTsym();
void MachObj_refGOTsym();
int MachObj_getsegment(const(char)* sectname, const(char)* segname, int align_, int flags);
void MachObj_addrel(int seg, targ_size_t offset, Symbol* targsym, uint targseg, int rtype, int val);

/******************************************************************/

version (STUB)
{
    public import stubobj;
}
else
{
    /*******************************************
     * Generic interface to the four object module file formats supported.
     * Instead of using virtual functions (i.e. virtual dispatch) it uses
     * static dispatch. Since config.objfmt never changes after initialization
     * of the compiler, static branch prediction should make it faster than
     * virtual dispatch.
     *
     * Making static dispatch work requires tediously repetitive boilerplate,
     * which we accomplish via string mixins.
     */
    class Obj
    {
      static
      {
        nothrow:

        Obj init(OutBuffer* objbuf, const(char)* filename, const(char)* csegname)
        {
            mixin(genRetVal("init(objbuf, filename, csegname)"));
        }

        void initfile(const(char)* filename, const(char)* csegname, const(char)* modname)
        {
            mixin(genRetVoid("initfile(filename, csegname, modname)"));
        }

        void termfile()
        {
            mixin(genRetVoid("termfile()"));
        }

        void term(const(char)* objfilename)
        {
            mixin(genRetVoid("term(objfilename)"));
        }

        size_t mangle(Symbol *s,char *dest)
        {
            assert(config.objfmt == OBJ_OMF);
            return OmfObj_mangle(s, dest);
        }

        void _import(elem *e)
        {
            assert(config.objfmt == OBJ_OMF);
            return OmfObj_import(e);
        }

        void linnum(Srcpos srcpos, int seg, targ_size_t offset)
        {
            mixin(genRetVoid("linnum(srcpos, seg, offset)"));
        }

        int codeseg(const char *name,int suffix)
        {
            mixin(genRetVal("codeseg(name, suffix)"));
        }

        void dosseg()
        {
            assert(config.objfmt == OBJ_OMF);
            return OmfObj_dosseg();
        }

        void startaddress(Symbol *s)
        {
            mixin(genRetVoid("startaddress(s)"));
        }

        bool includelib(const(char)* name)
        {
            mixin(genRetVal("includelib(name)"));
        }

        bool linkerdirective(const(char)* p)
        {
            mixin(genRetVal("linkerdirective(p)"));
        }

        bool allowZeroSize()
        {
            mixin(genRetVal("allowZeroSize()"));
        }

        void exestr(const(char)* p)
        {
            mixin(genRetVoid("exestr(p)"));
        }

        void user(const(char)* p)
        {
            mixin(genRetVoid("user(p)"));
        }

        void compiler()
        {
            mixin(genRetVoid("compiler()"));
        }

        void wkext(Symbol* s1, Symbol* s2)
        {
            mixin(genRetVoid("wkext(s1, s2)"));
        }

        void lzext(Symbol* s1, Symbol* s2)
        {
            assert(config.objfmt == OBJ_OMF);
            OmfObj_lzext(s1, s2);
        }

        void _alias(const(char)* n1,const(char)* n2)
        {
            mixin(genRetVoid("alias(n1, n2)"));
        }

        void theadr(const(char)* modname)
        {
            assert(config.objfmt == OBJ_OMF);
            OmfObj_theadr(modname);
        }

        void segment_group(targ_size_t codesize, targ_size_t datasize, targ_size_t cdatasize, targ_size_t udatasize)
        {
            assert(config.objfmt == OBJ_OMF);
            OmfObj_segment_group(codesize, datasize, cdatasize, udatasize);
        }

        void staticctor(Symbol *s,int dtor,int seg)
        {
            mixin(genRetVoid("staticctor(s, dtor, seg)"));
        }

        void staticdtor(Symbol *s)
        {
            mixin(genRetVoid("staticdtor(s)"));
        }

        void setModuleCtorDtor(Symbol *s, bool isCtor)
        {
            mixin(genRetVoid("setModuleCtorDtor(s, isCtor)"));
        }

        void ehtables(Symbol *sfunc,uint size,Symbol *ehsym)
        {
            mixin(genRetVoid("ehtables(sfunc, size, ehsym)"));
        }

        void ehsections()
        {
            mixin(genRetVoid("ehsections()"));
        }

        void moduleinfo(Symbol *scc)
        {
            mixin(genRetVoid("moduleinfo(scc)"));
        }

        int comdat(Symbol *s)
        {
            mixin(genRetVal("comdat(s)"));
        }

        int comdatsize(Symbol *s, targ_size_t symsize)
        {
            mixin(genRetVal("comdatsize(s, symsize)"));
        }

        int readonly_comdat(Symbol *s)
        {
            mixin(genRetVal("comdat(s)"));
        }

        void setcodeseg(int seg)
        {
            mixin(genRetVoid("setcodeseg(seg)"));
        }

        seg_data *tlsseg()
        {
            mixin(genRetVal("tlsseg()"));
        }

        seg_data *tlsseg_bss()
        {
            mixin(genRetVal("tlsseg_bss()"));
        }

        seg_data *tlsseg_data()
        {
            mixin(genRetVal("tlsseg_data()"));
        }

        int  fardata(char *name, targ_size_t size, targ_size_t *poffset)
        {
            assert(config.objfmt == OBJ_OMF);
            return OmfObj_fardata(name, size, poffset);
        }

        void export_symbol(Symbol *s, uint argsize)
        {
            mixin(genRetVoid("export_symbol(s, argsize)"));
        }

        void pubdef(int seg, Symbol *s, targ_size_t offset)
        {
            mixin(genRetVoid("pubdef(seg, s, offset)"));
        }

        void pubdefsize(int seg, Symbol *s, targ_size_t offset, targ_size_t symsize)
        {
            mixin(genRetVoid("pubdefsize(seg, s, offset, symsize)"));
        }

        int external_def(const(char)* name)
        {
            mixin(genRetVal("external_def(name)"));
        }

        int data_start(Symbol *sdata, targ_size_t datasize, int seg)
        {
            mixin(genRetVal("data_start(sdata, datasize, seg)"));
        }

        int external(Symbol *s)
        {
            mixin(genRetVal("external(s)"));
        }

        int common_block(Symbol *s, targ_size_t size, targ_size_t count)
        {
            mixin(genRetVal("common_block(s, size, count)"));
        }

        int common_block(Symbol *s, int flag, targ_size_t size, targ_size_t count)
        {
            mixin(genRetVal("common_block(s, flag, size, count)"));
        }

        void lidata(int seg, targ_size_t offset, targ_size_t count)
        {
            mixin(genRetVoid("lidata(seg, offset, count)"));
        }

        void write_zeros(seg_data *pseg, targ_size_t count)
        {
            mixin(genRetVoid("write_zeros(pseg, count)"));
        }

        void write_byte(seg_data *pseg, uint _byte)
        {
            mixin(genRetVoid("write_byte(pseg, _byte)"));
        }

        void write_bytes(seg_data *pseg, uint nbytes, void *p)
        {
            mixin(genRetVoid("write_bytes(pseg, nbytes, p)"));
        }

        void _byte(int seg, targ_size_t offset, uint _byte)
        {
            mixin(genRetVoid("byte(seg, offset, _byte)"));
        }

        uint bytes(int seg, targ_size_t offset, uint nbytes, void *p)
        {
            mixin(genRetVal("bytes(seg, offset, nbytes, p)"));
        }

        void ledata(int seg, targ_size_t offset, targ_size_t data, uint lcfd, uint idx1, uint idx2)
        {
            assert(config.objfmt == OBJ_OMF);
            OmfObj_ledata(seg, offset, data, lcfd, idx1, idx2);
        }

        void reftodatseg(int seg, targ_size_t offset, targ_size_t val, uint targetdatum, int flags)
        {
            mixin(genRetVoid("reftodatseg(seg, offset, val, targetdatum, flags)"));
        }

        void reftofarseg(int seg, targ_size_t offset, targ_size_t val, int farseg, int flags)
        {
            assert(config.objfmt == OBJ_OMF);
            OmfObj_reftofarseg(seg, offset, val, farseg, flags);
        }

        void reftocodeseg(int seg, targ_size_t offset, targ_size_t val)
        {
            mixin(genRetVoid("reftocodeseg(seg, offset, val)"));
        }

        int reftoident(int seg, targ_size_t offset, Symbol *s, targ_size_t val, int flags)
        {
            mixin(genRetVal("reftoident(seg, offset, s, val, flags)"));
        }

        void far16thunk(Symbol *s)
        {
            mixin(genRetVoid("far16thunk(s)"));
        }

        void fltused()
        {
            mixin(genRetVoid("fltused()"));
        }

        int data_readonly(char *p, int len, int *pseg)
        {
            mixin(genRetVal("data_readonly(p, len, pseg)"));
        }

        int data_readonly(char *p, int len)
        {
            mixin(genRetVal("data_readonly(p, len)"));
        }

        int string_literal_segment(uint sz)
        {
            mixin(genRetVal("string_literal_segment(sz)"));
        }

        Symbol *sym_cdata(tym_t ty, char *p, int len)
        {
            mixin(genRetVal("sym_cdata(ty, p, len)"));
        }

        void func_start(Symbol *sfunc)
        {
            mixin(genRetVoid("func_start(sfunc)"));
        }

        void func_term(Symbol *sfunc)
        {
            mixin(genRetVoid("func_term(sfunc)"));
        }

        void write_pointerRef(Symbol* s, uint off)
        {
            mixin(genRetVoid("write_pointerRef(s, off)"));
        }

        int jmpTableSegment(Symbol* s)
        {
            mixin(genRetVal("jmpTableSegment(s)"));
        }

        Symbol *tlv_bootstrap()
        {
            mixin(genRetVal("tlv_bootstrap()"));
        }

        void gotref(Symbol *s)
        {
            switch (config.objfmt)
            {
                case OBJ_ELF:     ElfObj_gotref(s); break;
                case OBJ_MACH:   MachObj_gotref(s); break;
                default:         assert(0);
            }
        }

        Symbol *getGOTsym()
        {
            switch (config.objfmt)
            {
                case OBJ_ELF:    return  ElfObj_getGOTsym();
                case OBJ_MACH:   return MachObj_getGOTsym();
                default:         assert(0);
            }
        }

        void refGOTsym()
        {
            switch (config.objfmt)
            {
                case OBJ_ELF:     ElfObj_refGOTsym(); break;
                case OBJ_MACH:   MachObj_refGOTsym(); break;
                default:         assert(0);
            }
        }

        int seg_debugT()           // where the symbolic debug type data goes
        {
            switch (config.objfmt)
            {
                case OBJ_MSCOFF: return MsCoffObj_seg_debugT();
                case OBJ_OMF:    return    OmfObj_seg_debugT();
                default:         assert(0);
            }
        }

        void write_long(int seg, targ_size_t offset, uint data, uint lcfd, uint idx1, uint idx2)
        {
            assert(config.objfmt == OBJ_OMF);
            return OmfObj_write_long(seg, offset, data, lcfd, idx1, idx2);
        }

        uint addstr(OutBuffer *strtab, const(char)* p)
        {
            switch (config.objfmt)
            {
                case OBJ_ELF:    return    ElfObj_addstr(strtab, p);
                case OBJ_MACH:   return   MachObj_addstr(strtab, p);
                default:         assert(0);
            }
        }

        int getsegment(const(char)* sectname, const(char)* segname, int align_, int flags)
        {
            assert(config.objfmt == OBJ_MACH);
            return MachObj_getsegment(sectname, segname, align_, flags);
        }

        int getsegment(const(char)* name, const(char)* suffix, int type, int flags, int  align_)
        {
            assert(config.objfmt == OBJ_ELF);
            return ElfObj_getsegment(name, suffix, type, flags, align_);
        }

        int getsegment(const(char)* sectname, uint flags)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_getsegment(sectname, flags);
        }

        void addrel(int seg, targ_size_t offset, Symbol *targsym, uint targseg, int rtype, int val = 0)
        {
            switch (config.objfmt)
            {
                case OBJ_MSCOFF: return MsCoffObj_addrel(seg, offset, targsym, targseg, rtype, val);
                case OBJ_MACH:   return   MachObj_addrel(seg, offset, targsym, targseg, rtype, val);
                default:         assert(0);
            }
        }

        void addrel(int seg, targ_size_t offset, uint type, uint symidx, targ_size_t val)
        {
            assert(config.objfmt == OBJ_ELF);
            return ElfObj_addrel(seg, offset, type, symidx, val);
        }

        size_t writerel(int targseg, size_t offset, uint type, uint symidx, targ_size_t val)
        {
            assert(config.objfmt == OBJ_ELF);
            return ElfObj_writerel(targseg, offset, type, symidx, val);
        }

        int  getsegment2(uint shtidx)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_getsegment2(shtidx);
        }

        uint addScnhdr(const(char)* scnhdr_name, uint flags)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_addScnhdr(scnhdr_name, flags);
        }

        int  seg_drectve()
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_drectve();
        }

        int  seg_pdata()
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_pdata();
        }

        int  seg_xdata()
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_xdata();
        }

        int  seg_pdata_comdat(Symbol *sfunc)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_pdata_comdat(sfunc);
        }

        int  seg_xdata_comdat(Symbol *sfunc)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_xdata_comdat(sfunc);
        }

        int  seg_debugS()
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_debugS();
        }

        int  seg_debugS_comdat(Symbol *sfunc)
        {
            assert(config.objfmt == OBJ_MSCOFF);
            return MsCoffObj_seg_debugS_comdat(sfunc);
        }
      }
    }
}


extern __gshared Obj objmod;

/*****************************************
 * Use to generate 4 function declarations, one for
 * each object file format supported.
 * Params:
 *      pattern = function declaration
 * Returns:
 *      declarations as a string suitable for mixin
 */
private extern (D)
string ObjMemDecl(string pattern)
{
    string r =
        gen(pattern,    "Omf") ~ ";\n" ~
        gen(pattern, "MsCoff") ~ ";\n" ~
        gen(pattern,    "Elf") ~ ";\n" ~
        gen(pattern,   "Mach") ~ ";\n";
    return r;
}

/****************************************
 * Generate boilerplate for static dispatch that
 * returns a void.
 * Params:
 *      arg = string to insert where ever there's a '$'
 * Returns:
 *      mixin string with static dispatch
 */
private extern (D)
string genRetVoid(string arg)
{
    string pattern =
    "
        switch (config.objfmt)
        {
            case OBJ_ELF:       ElfObj_$; break;
            case OBJ_MSCOFF: MsCoffObj_$; break;
            case OBJ_OMF:       OmfObj_$; break;
            case OBJ_MACH:     MachObj_$; break;
            default:     assert(0);
        }
    ";
    return gen(pattern, arg);
}

/****************************************
 * Generate boilerplate for static dispatch that
 * returns a value. Don't care about type of the value.
 * Params:
 *      arg = string to insert where ever there's a '$'
 * Returns:
 *      mixin string with static dispatch
 */
private extern (D)
string genRetVal(string arg)
{
    string pattern =
    "
        switch (config.objfmt)
        {
            case OBJ_ELF:    return    ElfObj_$;
            case OBJ_MSCOFF: return MsCoffObj_$;
            case OBJ_OMF:    return    OmfObj_$;
            case OBJ_MACH:   return   MachObj_$;
            default:     assert(0);
        }
    ";
    return gen(pattern, arg);
}

/****************************************
 * Generate boilerplate that replaces '$' in pattern with `arg`
 * Params:
 *      pattern = pattern to scan for '$'
 *      arg = string to insert where ever '$' is found
 * Returns:
 *      boilerplate string
 */
private extern (D)
string gen(string pattern, string arg)
{
    size_t count;               // number of '$' in pattern
    foreach (c; pattern)
        count += (c == '$');

    // length of result
    const length = pattern.length - count + arg.length * count;

    char[] result = new char[length];
    size_t i;
    foreach (c; pattern)
    {
        if (c == '$')
        {
            result[i .. i + arg.length] = arg[]; // substitution
            i += arg.length;
        }
        else
        {
            result[i] = c;
            ++i;
        }
    }
    assert(i == length);
    return cast(string)result;
}


