I didn't realize the code violated strict aliasing. After reading it,
it seems trivial. Somehow I also thought this is only a C++ thing.
Apparently this is "defined as undefined behavior" in C99?
Thanks a lot! You just found a bug in my code.
Regards,
Thanks,
Hendrik Greving
On Wed, Oct 2, 2013 at 3:19 AM, Mikael Pettersson <mikpeli...@gmail.com> wrote:
> Hendrik Greving writes:
> > gcc --version
> > gcc (GCC) 4.8.1
> >
> > (4.7.2 seems to work)
> >
> > gcc -g -fPIC -O2 -Wall -o test test.c
> >
> > ./test
> > type_a is 0x0
> >
> > Correct would be 0x14000. I don't see how the C-code could be
> > ambiguous, I think this is a bug?
> >
> > test.c:
> >
> > #include <stdio.h>
> >
> > typedef struct
> > {
> > unsigned int a:4;
> > unsigned int b:5;
> > unsigned int c:5;
> > unsigned int d:18;
> > } my_comb_t;
> >
> > struct my_s
> > {
> > int l;
> > };
> >
> > my_comb_t *getps_f (struct my_s *a);
> >
> > #define GETPS(x) getps_f(&(x))
> >
> > my_comb_t *
> > getps_f (struct my_s *a)
> > {
> > my_comb_t *p = (my_comb_t *) &(a->l);
> > return p;
>
> This is where the test case is wrong. You need to use a union so that
> gcc can see the type punning, or compile with -fno-strict-aliasing.
>
> Without -fno-strict-aliasing I see different results for -m32 vs -m64,
> but not across gcc versions; with -fno-strict-aliasing all results match.
>
> > }
> >
> > int g_modes = 5;
> >
> > int main(void)
> > {
> > int modes = g_modes;
> > int type_a = 0;
> >
> > if (modes)
> > {
> > struct my_s m;
> > m.l = type_a;
> > my_comb_t *p = GETPS(m);
> > p->d |= modes;
> > type_a = m.l;
> > }
> > printf("type_a is 0x%x\n", type_a);
> > return 0;
> > }
