[PATCH 10/20] malloc: Fix alignment logic in obstack

[Adhemerval Zanella Netto]

On 27/10/22 12:33, Szabolcs Nagy via Libc-alpha wrote:
> If sizeof(ptrdiff_t) < sizeof(void*) the alignment logic was wrong:
> incorrectly assumed that base was already sufficiently aligned.
> 
> Use more robust alignment logic: this one should work on any target.
> Note: this is an installed header so it must be namespace clean and
> portable hence it uses unsigned long for the alignment offset.
> ---
>  malloc/obstack.h | 19 +++----------------
>  1 file changed, 3 insertions(+), 16 deletions(-)
> 
> diff --git a/malloc/obstack.h b/malloc/obstack.h
> index 4b01cdfe4d..1cf18e5464 100644
> --- a/malloc/obstack.h
> +++ b/malloc/obstack.h
> @@ -116,22 +116,9 @@
>  # define PTR_INT_TYPE ptrdiff_t
>  #endif
>  
> -/* If B is the base of an object addressed by P, return the result of
> -   aligning P to the next multiple of A + 1.  B and P must be of type
> -   char *.  A + 1 must be a power of 2.  */
> -
> -#define __BPTR_ALIGN(B, P, A) ((B) + (((P) - (B) + (A)) & ~(A)))
> -
> -/* Similar to _BPTR_ALIGN (B, P, A), except optimize the common case
> -   where pointers can be converted to integers, aligned as integers,
> -   and converted back again.  If PTR_INT_TYPE is narrower than a
> -   pointer (e.g., the AS/400), play it safe and compute the alignment
> -   relative to B.  Otherwise, use the faster strategy of computing the
> -   alignment relative to 0.  */
> -
> -#define __PTR_ALIGN(B, P, A)						      \
> -  __BPTR_ALIGN (sizeof (PTR_INT_TYPE) < sizeof (void *) ? (B) : (char *) 0, \
> -		P, A)
> +/* Align P to the next multiple of A + 1, where A + 1 is a power of 2,
> +   A fits into unsigned long and P has type char *.  */
> +#define __PTR_ALIGN(B, P, A) ((P) + (-(unsigned long)(P) & (A)))

Shouldn't you use uintptr_t here to be consistent with your other changes
that exactly change using long to cast from pointers?

It would be good to check with gnulib as well, since this header is also
shared with it.

>  
>  #include <string.h>
>