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Date: Sat, 05 Apr 2008 18:33:00 -0000
Subject: [Bug c++/35835]  New: Compiler fails to recognize match of local "extern" declarations
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From: "oder at eleks dot lviv dot ua" <gcc-bugzilla@gcc.gnu.org>
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Consider following program
======= begin of test.cpp ========
#include <stdio.h>

static inline void SetValue(int i)
{
  extern int g_iValue;

  g_iValue = i;
}

static inline int GetValue()
{
  extern int g_iValue;

  return g_iValue;
}



int g_iValue = 0;


int main()
{
  int iValueSave = GetValue();

  SetValue(1);
  printf("%d\n", GetValue());

  SetValue(iValueSave);

  return 0;
}
======= end of test.cpp ========
I use this apptoach to only publish get/set global variable accessors in header
while keeping variable itself hidden in cpp.

If compiled with optimizations turned on, compiler fails identify two local
"extern" declarations as a single memory object and uses initial value
retrieved for iValueSafe in call to printf as well (as if SetValue() was
unrelated to g_iValue).

If compiled and run, program outputs 0 instead of 1.
osx-leopard:build oder$ g++ -O3 -o test test.cpp
osx-leopard:build oder$ ./test
0

Output of "g++ -v" is:
Using built-in specs.
Target: powerpc-apple-darwin9
Configured with: /var/tmp/gcc/gcc-5478~1/src/configure --disable-checking
-enable-werror --prefix=/usr --mandir=/share/man
--enable-languages=c,objc,c++,obj-c++
--program-transform-name=/^[cg][^.-]*$/s/$/-4.0/
--with-gxx-include-dir=/include/c++/4.0.0 --with-slibdir=/usr/lib
--build=powerpc-apple-darwin9 --host=powerpc-apple-darwin9
--target=powerpc-apple-darwin9
Thread model: posix
gcc version 4.0.1 (Apple Inc. build 5478)


-- 
           Summary: Compiler fails to recognize match of local "extern"
                    declarations
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: oder at eleks dot lviv dot ua
 GCC build triplet: ppc
  GCC host triplet: ppc
GCC target triplet: ppc


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35835