[Pyrex] Type casting issues and strict aliasing

[Stefan Behnel]

Hi,

I've run into a couple of strict aliasing issues with type casts for the vtab
struct. Can anyone with more C experience than me comment on these?

For the "r_jiba1" test case in

http://hg.cython.org/cython-devel/file/e21391d5f23a/tests/run/r_jiba1.pyx

Cython currently generates this in the tp_new function of the Norwegian class:

  *(struct __pyx_vtabstruct_7r_jiba1_Norwegian **)&p->__pyx_base.__pyx_vtab =
        __pyx_vtabptr_7r_jiba1_Norwegian;

which results in this strict aliasing warning with gcc 4.1:

  r_jiba1.c: In function '__pyx_tp_new_7r_jiba1_Norwegian':
  r_jiba1.c:596: warning: dereferencing type-punned pointer will break
          strict-aliasing rules

It goes away when I change it to this (patch is attached):

  p->__pyx_base.__pyx_vtab =
   (struct __pyx_vtabstruct_7r_jiba1_Parrot*)__pyx_vtabptr_7r_jiba1_Norwegian;

Parrot is the base type here. Since these changes usually tend to break some
compiler somewhere, I hesitate to apply the patch right away, though...


Similar things happen when the C methods are initialised in the module init
function. Currently, we have:

  *(void(**)(void))&__pyx_vtable_7r_jiba1_Parrot.describe =
                    (void(*)(void))__pyx_f_7r_jiba1_6Parrot_describe;
  *(void(**)(void))&__pyx_vtable_7r_jiba1_Norwegian.__pyx_base.describe =
                    (void(*)(void))__pyx_f_7r_jiba1_9Norwegian_describe;

and the resulting warning is:

  r_jiba1.c: In function 'PyInit_r_jiba1':
  r_jiba1.c:800: warning: dereferencing type-punned pointer will break
          strict-aliasing rules
  r_jiba1.c:807: warning: dereferencing type-punned pointer will break
          strict-aliasing rules

The problem here is mostly the "self" argument, which has the type of the
concrete class for each subtype. The code that generates this in the
generate_exttype_vtable_init_code() function in ModuleNode.py has the type
casts hard-coded. What would be the right thing to do here? Cast one of the
functions (rhs?) to the type of the other?

Any help is appreciated.

Thanks,
Stefan

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