extern const and all that

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Hi,

In the message
 
    http://gcc.gnu.org/ml/gcc/2005-05/msg01265.html

you said

  > Fifth, there is a slight difference between "const" in C and in  C++.
  > In C++, a const variable implicitly has an internal linkage; so a
  > C++ compiler tends to optimize it out when its address is not taken
  > (so no storage is wasted).  This is an issue for the objects
  > automatically generated by the gengtype support machinery.  The are
  > supposed to have external linkage, so we need to explicitly say
  > "extern" in their definitions. 

  Presumably such constants are declared in some header file, with
  external linkage.  It would be better to make that declaration visible
  at the point of definition, rather than marking up the declarations with
  'extern'.

However, if you try to compile the souces (augmented with attched
patch) with g++, you should hit an "undefined reference to symbol"
linker error similar to:

g++   -g -O2 -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -pedantic -Wno-long-long -Wno-variadic-macros -Wold-style-definition -fno-common   -DHAVE_CONFIG_H -DGENERATOR_FILE -o build/genmddeps \
 build/genmddeps.o build/gensupport.o build/dummy-conditions.o build/rtl.o build/read-rtl.o build/ggc-none.o build/min-insn-modes.o \
 build/errors.o ../build-i686-pc-linux-gnu/libiberty/libiberty.a
build/gensupport.o(.text+0xc27): In function `maybe_eval_c_test(char const*)':
/home/gdr/redhat/egcs/gcc/gensupport.c:1122: undefined reference to `insn_elision_unavailable'
build/gensupport.o(.text+0x105d): In function `init_md_reader_args_cb(int, char**, bool (*)(char const*))':
/home/gdr/redhat/egcs/gcc/gensupport.c:975: undefined reference to `n_insn_conditions'
build/gensupport.o(.text+0x108d):/home/gdr/redhat/egcs/gcc/gensupport.c:977: undefined reference to `insn_conditions'
collect2: ld returned 1 exit status


This is an instance of the situations I reported earlier.  
Of course, your suggestion is the natural choice, if we did not have
addiional constraint.  Indeed, the file (for MD gen-support machinery)
that defines those variables is dummy-conditions.c, with the following
comment: 

/* In order to avoid dragging in all the headers that are needed to
   declare things that gensupport.h uses, we duplicate the declaration
   of struct c_test here.  (In particular we do not want to have to
   include tm.h nor rtl.h in this file.)  */
 
Given that constraint, it appears to me that putting "extern" in the
definition is the minimal, viable solution.  

Thoughts?

-- Gaby

Index: ChangeLog
===================================================================
RCS file: /cvs/gcc/gcc/gcc/ChangeLog,v
retrieving revision 2.9112
diff -p -r2.9112 ChangeLog
*** ChangeLog	9 Jun 2005 22:27:41 -0000	2.9112
--- ChangeLog	9 Jun 2005 22:57:51 -0000
***************
*** 1,3 ****
--- 1,9 ----
+ 2005-06-09  Gabriel Dos Reis  <gdr@cs.tamu.edu>
+ 
+ 	* gensupport.c (lookup_predicate): Add explicit cast to
+ 	htab_find() value.
+ 	(init_predicate_table): Use XCNEW.
+ 

Index: gensupport.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/gensupport.c,v
retrieving revision 1.62
diff -p -r1.62 gensupport.c
*** gensupport.c	26 Apr 2005 00:23:54 -0000	1.62
--- gensupport.c	9 Jun 2005 22:57:51 -0000
*************** lookup_predicate (const char *name)
*** 1200,1206 ****
  {
    struct pred_data key;
    key.name = name;
!   return htab_find (predicate_table, &key);
  }
  
  void
--- 1200,1206 ----
  {
    struct pred_data key;
    key.name = name;
!   return (pred_data *) htab_find (predicate_table, &key);
  }
  
  void
*************** init_predicate_table (void)
*** 1289,1295 ****
  
    for (i = 0; i < NUM_KNOWN_OLD_PREDS; i++)
      {
!       pred = xcalloc (sizeof (struct pred_data), 1);
        pred->name = old_preds[i].name;
  
        for (j = 0; old_preds[i].codes[j] != 0; j++)
--- 1289,1295 ----
  
    for (i = 0; i < NUM_KNOWN_OLD_PREDS; i++)
      {
!       pred = XCNEW (struct pred_data);
        pred->name = old_preds[i].name;
  
        for (j = 0; old_preds[i].codes[j] != 0; j++)