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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 10/31/2022 13:14, Adhemerval Zanella Netto wrote:
> 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?

here the offset part is unsigned long, but the pointer is kept
char *. in other patches the problem was that the pointer
was turned into long.

here unsigned int would be enough, since obstack->alignment_mask
is int, larger alignments are not supported.

the new formula may not be the fastest to compute, but if the
goal is portability then i think it's better than the current
code.

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

i see. i haven't looked at gnulib.

> 
> >  
> >  #include <string.h>
> >