src/alpha/ffitarget.h - FFI_TRAMPOLINE_SIZE

src/alpha/ffitarget.h - FFI_TRAMPOLINE_SIZE

[John E. Malmberg]

I do not have a Tru64 system to test this on.

Looking at the code in src/alpha/ffitarget.h and src/alpha/ffi.c, 
something does not look right.

FFI_TRAMPOLINE_SIZE is set to 24 in ffitarget.h

The typedef struct ffi_closure is then set up in ffi.h as:

typedef struct {
   char tramp{FFI_TRAMPOLINE_SIZE];
   ffi_cif * cif;
   void (*fun)...
   void *user_data
} ffi_closure

In ffi.c ffi_prep_closure_loc(), the code then does:

   tramp = (unsigned int *) &closure->tramp[0];
   ....

    *(void **) &tramp[4] = ffi_closure_osf;

But tramp only has 3 longword members per the previous declaration, so 
&tramp[4] is the address of the '*cif' member of the structure which is 
then updated:

    closure->cif = cif;

Am I understanding this correctly?

Regards,
-John

Re: src/alpha/ffitarget.h - FFI_TRAMPOLINE_SIZE

[Jarkko Hietaniemi]

On Wednesday-201504-15 23:20, John E. Malmberg wrote:
>     tramp = (unsigned int *) &closure->tramp[0];
>     ....
>
>      *(void **) &tramp[4] = ffi_closure_osf;
>
> But tramp only has 3 longword members per the previous declaration, so
> &tramp[4] is the address of the '*cif' member of the structure which is
> then updated:

unsigned int is four bytes in alpha.  (LP64, not ILP64).
So writing an 8-byte ptr at int offset 4 or byte offset 16
will update the bytes 16 to 23.  Verified in an alpha, and
for that matter in an x86_64, which is LP64, too.