GitLab

David Eichmann pushed to branch wip/27162/rts-explicit-dllimport at Glasgow Haskell Compiler / GHC

Commits:

d28c0873

by Duncan Coutts at 2026-04-15T18:31:32+01:00

Use __attribute__((dllimport)) for external RTS symbol declarations

This is needed to be hygenic about DLL symbol imports and exports.
The attribute is ignored on platforms other than Windows.

Use of the attribute however means that external data symbols do not
have a compile-time constant address (they are loaded using an
indirection). This means we have to adjust the rtsSyms initial linker
table so that it is a local constant in a function, rather than a global
constant. We now define it within a function that pre-populates the
symbol table with the RTS symbols.

3 changed files:

rts/Linker.c
rts/RtsSymbols.c
rts/RtsSymbols.h

Changes:

rts/Linker.c

@@ -478,16 +478,7 @@ initLinker_ (int retain_cafs)
      symhash = allocStrHashTable();
      /* populate the symbol table with stuff from the RTS */
 -    IF_DEBUG(linker, debugBelch("populating linker symbol table with built-in RTS symbols\n"));
 -    for (const RtsSymbolVal *sym = rtsSyms; sym->lbl != NULL; sym++) {
 -        IF_DEBUG(linker, debugBelch("initLinker: inserting rts symbol %s, %p\n", sym->lbl, sym->addr));
 -        if (! ghciInsertSymbolTable(WSTR("(GHCi built-in symbols)"),
 -                                    symhash, sym->lbl, sym->addr,
 -                                    sym->strength, sym->type, 0, NULL)) {
 -            barf("ghciInsertSymbolTable failed");
 -        }
 -    }
 -    IF_DEBUG(linker, debugBelch("done with built-in RTS symbols\n"));
 +    initLinkerRtsSyms(symhash);
      /* Add extra symbols. rtsExtraSyms() is a weakly defined symbol in the rts,
       * that can be overrided by linking in an object with a corresponding

rts/RtsSymbols.c

@@ -9,6 +9,7 @@
  #include "ghcplatform.h"
  #include "Rts.h"
  #include "RtsSymbols.h"
 +#include "LinkerInternals.h"
  #include "TopHandler.h"
  #include "HsFFI.h"
@@ -51,6 +52,18 @@ extern char **environ;
  /* -----------------------------------------------------------------------------
   * Symbols to be inserted into the RTS symbol table.
 + *
 + * Note [Naming Scheme for Symbol Macros]
 + *
 + * SymI_*: symbol is internal to the RTS. It resides in an object
 + *         file/library that is statically.
 + * SymE_*: symbol is external to the RTS library. It might be linked
 + *         dynamically.
 + *
 + * Sym*_HasProto  : the symbol prototype is imported in an include file
 + *                  or defined explicitly
 + * Sym*_NeedsProto: the symbol is undefined and we add a dummy
 + *                  default proto extern void sym(void);
   */
  #define Maybe_Stable_Names      SymI_HasProto(stg_mkWeakzh)                   \
@@ -1144,12 +1157,21 @@ extern char **environ;
        SymI_HasProto(hs_word2float64)
 -/* entirely bogus claims about types of these symbols */
 -#define SymI_NeedsProto(vvv)  extern void vvv(void);
 -#define SymI_NeedsDataProto(vvv)  extern StgWord vvv[];
 -#define SymE_NeedsProto(vvv)  SymI_NeedsProto(vvv);
 -#define SymE_NeedsDataProto(vvv)  SymI_NeedsDataProto(vvv);
 -#define SymE_HasProto(vvv)    SymI_HasProto(vvv);
 +/* Declare prototypes for the symbols that need it, so we can refer
 + * to them in the rtsSyms table below.
 + *
 + * In particular, for the external ones (SymE_*) we use the dllimport attribute
 + * to indicate that (on Windows) they come from external DLLs. This attribute
 + * is ignored on other platforms.
 + *
 + * The claims about the types of these symbols are entirely bogus.
 + */
 +#define SymI_NeedsProto(vvv)      extern                            void vvv(void);
 +#define SymI_NeedsDataProto(vvv)  extern                            StgWord vvv[];
 +#define SymE_NeedsProto(vvv)      extern __attribute__((dllimport)) void vvv(void);
 +#define SymE_NeedsDataProto(vvv)  extern __attribute__((dllimport)) StgWord vvv[];
++
 +#define SymE_HasProto(vvv) /**/
  #define SymI_HasProto(vvv) /**/
  #define SymI_HasDataProto(vvv) /**/
  #define SymI_HasProto_redirect(vvv,xxx,strength,ty) /**/
@@ -1178,6 +1200,8 @@ RTS_SYMBOLS_PRIM
  #undef SymE_NeedsProto
  #undef SymE_NeedsDataProto
 +/* See Note [Naming Scheme for Symbol Macros] */
++
  #define SymI_HasProto(vvv) { MAYBE_LEADING_UNDERSCORE_STR(#vvv), \
                      (void*)(&(vvv)), STRENGTH_NORMAL, SYM_TYPE_CODE },
  #define SymI_HasDataProto(vvv) { MAYBE_LEADING_UNDERSCORE_STR(#vvv), \
@@ -1198,7 +1222,16 @@ RTS_SYMBOLS_PRIM
      { MAYBE_LEADING_UNDERSCORE_STR(#vvv),    \
        (void*)(&(xxx)), strength, ty },
 -RtsSymbolVal rtsSyms[] = {
++
 +/* Populate the symbol table with stuff from the RTS. */
 +void initLinkerRtsSyms (StrHashTable *symhash) {
++
 +    /* The address of data symbols with the dllimport attribute are not
 +     * compile-time constants and so cannot be used in constant initialisers.
 +     * For this reason, rtsSyms is a local variable within this function
 +     * rather than a global constant (as it was historically).
 +     */
 +    const RtsSymbolVal rtsSyms[] = {
        RTS_SYMBOLS
        RTS_RET_SYMBOLS
        RTS_POSIX_ONLY_SYMBOLS
@@ -1213,7 +1246,19 @@ RtsSymbolVal rtsSyms[] = {
        RTS_SYMBOLS_PRIM
        SymI_HasDataProto(nonmoving_write_barrier_enabled)
        { 0, 0, STRENGTH_NORMAL, SYM_TYPE_CODE } /* sentinel */
 -};
 +    };
++
 +    IF_DEBUG(linker, debugBelch("populating linker symbol table with built-in RTS symbols\n"));
 +    for (const RtsSymbolVal *sym = rtsSyms; sym->lbl != NULL; sym++) {
 +        IF_DEBUG(linker, debugBelch("initLinker: inserting rts symbol %s, %p\n", sym->lbl, sym->addr));
 +        if (! ghciInsertSymbolTable(WSTR("(GHCi built-in symbols)"),
 +                                    symhash, sym->lbl, sym->addr,
 +                                    sym->strength, sym->type, 0, NULL)) {
 +            barf("ghciInsertSymbolTable failed");
 +        }
 +    }
 +    IF_DEBUG(linker, debugBelch("done with built-in RTS symbols\n"));
 +}
  // Note [Extra RTS symbols]

rts/RtsSymbols.h

@@ -9,6 +9,7 @@
  #pragma once
  #include "ghcautoconf.h"
 +#include "Hash.h"
  #if defined(LEADING_UNDERSCORE)
  #define MAYBE_LEADING_UNDERSCORE_STR(s) ("_" s)
@@ -46,7 +47,7 @@ typedef struct _RtsSymbolVal {
      SymType type;
  } RtsSymbolVal;
 -extern RtsSymbolVal rtsSyms[];
 +void initLinkerRtsSyms (StrHashTable *symhash);
  extern RtsSymbolVal* __attribute__((weak)) rtsExtraSyms(void);