GCC -O2 and __attribute__((weak))

Question

It looks like GCC with -O2 and __attribute__((weak)) produces different results depending on how you reference your weak symbols. Consider this:

$ cat weak.c

#include <stdio.h>

extern const int weaksym1;
const int weaksym1 __attribute__(( weak )) = 0;

extern const int weaksym2;
const int weaksym2 __attribute__(( weak )) = 0;

extern int weaksym3;
int weaksym3 __attribute__(( weak )) = 0;

void testweak(void)
{
    if ( weaksym1 == 0 )
    {
        printf( "0\n" );
    }
    else
    {
        printf( "1\n" );
    }

    printf( "%d\n", weaksym2 );


    if ( weaksym3 == 0 )
    {
        printf( "0\n" );
    }
    else
    {
        printf( "1\n" );
    }
}

$ cat test.c

extern const int weaksym1;
const int weaksym1 = 1;

extern const int weaksym2;
const int weaksym2 = 1;

extern int weaksym3;
int weaksym3 = 1;

extern void testweak(void);

void main(void)
{
    testweak();
}

$ make

gcc  -c weak.c
gcc  -c test.c
gcc  -o test test.o weak.o

$ ./test

1
1
1

$ make ADD_FLAGS="-O2"

gcc -O2 -c weak.c
gcc -O2 -c test.c
gcc -O2 -o test test.o weak.o

$ ./test

0
1
1

The question is, why the last "./test" produces "0 1 1", not "1 1 1"?

gcc version 5.4.0 (GCC)

Answer 1

Looks like when doing optimizations, the compiler is having trouble with symbols declared const and having the weak definition within the same compilation unit.

You can create a separate c file and move the const weak definitions there, it will work around the problem:

weak_def.c

const int weaksym1 __attribute__(( weak )) = 0;
const int weaksym2 __attribute__(( weak )) = 0;

Same issue described in this question: GCC weak attribute on constant variables