>
>Of course on x86 there is no special requirement any type to be aligned at
all.
>Double do not need to be 8 byte aligned. Ints do not need to be on an even
>address etc. So malloc can return a pointer to any address it wants on an
x86
>and still conform to the restriction.
Yes and I think it returns 4 byte aligned objects for performance reasons.
(This is more efficient than storing double/int on 1/2 byte boundaries.)

>
>On a Sparc or an Alpha, objects must to be aligned to the sizeof the type.
>So shorts are 2 byte aligned, ints 4 byte, and doubles 8 byte. Since malloc
>has no idea what you are allocating it has to return an 8 byte aligned
pointer.
>
>But all this is a bit of a red herring. The original bug if I recall was
>caused by the caller assuming that the return from palloc was 8 byte
aligned.
>Since the caller "knew" that the pointer was 8 byte aligned it then went on
>to do some "fancy" pointer arithmetic to arrive at an offset. This is what
>is broken, not the Linux malloc().
Yes, I think so too.
>
>That said, I have a plan in mind to get a nice performance boost for
palloc().
>As a side effect, I will guarantee 8 byte alignment of pointers returned
>from palloc().
>
Does this imply that more memory will be used that strictly necesary under
linux?

With regards from Maurice.




Sun, 22 Mar 1998 10:42:30 +0100
"Maurice Gittens" <mgittens@gits.nl>
Re: [HACKERS] patch for memory overrun on Linux(i386)
> >That said, I have a plan in mind to get a nice performance boost for
> palloc().
> >As a side effect, I will guarantee 8 byte alignment of pointers returned
> >from palloc().
> >
> Does this imply that more memory will be used that strictly necesary under
> linux?
> 
> With regards from Maurice.

Actually, perhaps less memory, because malloc memory is not aligned on
double boundaries, but this is all still just conjecture.


-- 
Bruce Momjian                          |  830 Blythe Avenue
maillist@candle.pha.pa.us              |  Drexel Hill, Pennsylvania 19026
  +  If your life is a hard drive,     |  (610) 353-9879(w)
  +  Christ can be your backup.        |  (610) 853-3000(h)

Sun, 22 Mar 1998 15:00:56 -0500 (EST)
Bruce Momjian <maillist@candle.pha.pa.us>
Re: [HACKERS] patch for memory overrun on Linux(i386)
> 
> > >That said, I have a plan in mind to get a nice performance boost for
> > palloc().
> > >As a side effect, I will guarantee 8 byte alignment of pointers returned
> > >from palloc().
> > >
> > Does this imply that more memory will be used that strictly necesary under
> > linux?
> > 
> > With regards from Maurice.
> 
> Actually, perhaps less memory, because malloc memory is not aligned on
> double boundaries, but this is all still just conjecture.


Ok, you've convinced me. Sheesh...

Change the description to "nice performance boost and use less memory".

;-)

-dg
 
David Gould            dg@illustra.com           510.628.3783 or 510.305.9468 
Informix Software  (No, really)         300 Lakeside Drive  Oakland, CA 94612
 - I realize now that irony has no place in business communications.


Sun, 22 Mar 1998 14:28:07 -0800 (PST)
dg@illustra.com (David Gould)
Re: [HACKERS] patch for memory overrun on Linux(i386)