Re: [eigen] Map: introduce "AlignedPointer" flag?

[Rohit Garg <rpg.314@xxxxxxxxx>]

Thanks, this will help.

On Fri, Oct 23, 2009 at 5:57 PM, Gael Guennebaud
<gael.guennebaud@xxxxxxxxx> wrote:
>
> ok, I committed  that change, but I simply reused the Unaligned / Aligned
> constants, e.g:
>
> EIGEN_ALIGN16 float f[4];
> Map<Vector4f,Aligned> mv(v);
>
> gael.
>
> On Thu, Oct 22, 2009 at 5:50 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
> wrote:
>>
>> 2009/10/22 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>> >
>> >
>> > On Wed, Oct 21, 2009 at 4:10 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
>> > wrote:
>> >>
>> >> This is for Gael. Currently in Map, what's the precise meaning of
>> >> "ForceAligned" ? Is it:
>> >>
>> >>  " If you use packet access, then use aligned packet access " ?
>> >>
>> >> If yes, is it correct to say that at the moment we don't offer any way
>> >> to tell that the pointer passed to Map is aligned, and that therefore
>> >> we may enable packet access also e.g. for good fixed sizes? I remember
>> >> you saying something like "we need a FirstAligned flag", were you
>> >> talking about that? What do you think about calling that
>> >> AlignedPointer and renaming ForceAligned to ForceAlignedAccess ?
>> >> Finally, can you remind me what the use case for ForceAligned is, if
>> >> we have AlignedPointer?
>> >
>> > yes this issue was on my todo list as well.
>>
>> oh, if it is, i'll gladly let you do it! You are more informed than me
>> on this class since you basically rewrote it.
>>
>> > So basically we could add an
>> > AlignedPointer option to Map setting the AlignedBit flag. That's it I
>> > think.
>>
>> I think you also need to carefully set PacketAccessBit to something
>> like "If AlignedBit AND (either dyn-size OR fixed size multiple of
>> 16)".
>>
>> >
>> > About ForceAligned, it is only used internally, and its purpose is to
>> > avoid
>> > unligned loads in such cases:
>> >
>> > a += b;
>> >
>> > where a is a Map or a Block object.
>> >
>> > Indeed, since a += b; is transformed to "a = a + b;" and that in assign
>> > we
>> > make sure that stores to "a" will be aligned, we know that loads from
>> > "a"
>> > will be aligned too but we need a way to enforce that. The solution is
>> > to
>> > reimplement operator += in MapBase (inherited by Block and Map) like
>> > this:
>> >
>> > *this = *this_cast_to_the_same_type_but_with_the_ForceAligned_option +
>> > other;
>> >
>> > I agree that's probably more useful for Block than for Map, but since
>> > they
>> > share the same code...
>>
>> I see! thanks for the explanation.
>>
>> Benoit
>>
>>
>> >
>> > I'm also ok to rename it to ForceAlignedAccess.
>> >
>> > gael
>> >
>> >
>> >>
>> >> If you tell me all that I can try to get that done ;)
>> >>
>> >> Benoit
>> >>
>> >
>>
>>
>
>
>
>



-- 
Rohit Garg

http://rpg-314.blogspot.com/

Senior Undergraduate
Department of Physics
Indian Institute of Technology
Bombay