Re: POINTER_ALIGNED_P (was: Re: CVS commit: src/sys)

[christos%zoulas.com@localhost (Christos Zoulas)]

On Feb 17,  2:20am, uwe%stderr.spb.ru@localhost (Valery Ushakov) wrote:
-- Subject: Re: POINTER_ALIGNED_P (was: Re: CVS commit: src/sys)

| Well, it was you who did the definion of ALIGNED_POINTER centralized
| and overridable :)
| 
|   revision 1.400
|   date: 2012-01-25 00:03:36 +0400;  author: christos;  state: Exp;  lines: +26 -1;
|   Use and define ALIGN() ALIGN_POINTER() and STACK_ALIGN() consistently,
|   and avoid definining them in 10 different places if not needed.

If you look a bit deeper into that change, it merged many MD copies
of this macro into one, and I needed the override for the archs that
were different.

| ALIGNED_POINTER is overriden on x86 to be always true.  Surprisingly
| it is not overriden for m68k and vax that are __NO_STRICT_ALIGNMENT.
| That is most likely an oversight, but that will probably require some
| cvs archaeology to confirm.  Some uses of ALIGNED_POINTER are inside
| an __NO_STRICT_ALIGNMENT #ifdef.

This is a mess as you can see. The problem is that in each case we
need to determine if this macro is used to test alignment to determine
access restrictions on certain architectures (most cases), or it
is done for performance/protocol requirements.

I hope that nothing falls into the second category and then we can
use a single macro that uses a combination of __NO_STRICT_ALIGNMENT
and __alignof() which my guess is that it did not exist at the time
the macro was invented, and thus it used sizeof() and limited it to
integral types.

| We don't even seem to be sure about its semantics, as far as I can
| tell (see bus space comments in my mail).
| 
| That's even more of a reason to stop doing aimless random changes
| without getting some kind of understanding first.  The last thing we
| need is ALIGNED_POINTER and POINTER_ALIGNED macros with slighly
| different semantics both of which are counter-intuitive to begin with
| (and riastradh@ even had to add a verbose comment for that).

This change was not aimless. I wanted to remove the multiple copies of
the m_copyup/m_pullup code into one function. To do that I needed the
alignment as a value, not a predicate macro (that was again copied in
~10 places). When I tried to centralize it, I wanted to do the minimal
change so I would not screw it up (and I did end up screwing it up).

This is a good opportunity now to clean this up even more and provide
sane alignment macros.

christos