Re: [Patch] bitmap_allocator.

[Dhruv Matani <dhruvbird at gmx dot net>]

A quick question:

Would you mind if I changed the entries in the linker.map.gnu for the
alignment patch? That's because a few non-inline function signatures,
and static signatures need to change.

-Dhruv.


On Thu, 2004-10-14 at 14:29, Paolo Carlini wrote:
> Dhruv Matani wrote:
> 
> >Now, IMO, the problem might be starting at the place when I start
> >accessing bit-maps! That's because half of them will have only 4-byte
> >alignment! If this in fact is the problem, then how does the compiler
> >arrays of unsigned ints for 64-bit machines? We can then adopt the same
> >strategy!
> >  
> >
> Is it clear that the alignment requirements depend on the type? If you 
> align 8
> on 32-bit and 16 on 64-bit you are *always* safe, if you align less, you 
> are safe
> sometimes, not safe other times, depending on the type. For instance 
> when the
> allocator is called by standard containers, such as std::list, std::map, 
> std::set, and
> so on, you need space for relatively big structs, that in general have 
> higher
> alignment requirements...
> 
> Another thing: it looks like in your allocator, all the various 
> quantities are
> unsigned ints. Besides alignment issues, this is *not* ok on 64-bit machines
> because everything becomes severely limited!! Do you know that linux on 
> x86_64,
> an archs becoming rather popular, already supports 512 G of virtual address
> space per process and is being extended very soon to 47 bits (128TB)? 
> Doesn't
> make sense to use unsigned ints on modern 64-bit machines.
> 
> >Yes, currently I use the __builting_ctz instruction, which is working
> >for 32-bit types. Now, I need to change that to __builtin_ctzl. But is
> >it guaranteed that it will work for 8-byte types, and also will
> >sizeof(long) == 8-bytes on 32-bit and 64-bit machines?
> >  
> >
> Of course sizeof(long) is 4 on 32-bit machines and that's ok, because, 
> as I already
> wrote, 2 * sizeof(long) gives you the optimal alignment everywhere. And 
> of course
> a builtin for long works for long, whatever size it has on that target.
> 
> >Should I assume that 8-byte alignment is sufficient?
> >  
> >
> As I already told you 8 is the bare minimum you should aim to, but that must
> be configurable. Please try to follow my suggestion: use *everywhere* longs
> for the memory and the bookeeping quantities, and possibly pairs of them for
> the memory in case the user wants that.
> 
> Paolo.
-- 
        -Dhruv Matani.
http://www.geocities.com/dhruvbird/

The price of freedom is responsibility, but it's a bargain, because
freedom is priceless. ~ Hugh Downs