How to utilize the CVTDQ2PD instruction?

[Brian Budge]

On Fri, Sep 7, 2012 at 7:57 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Fri, 7 Sep 2012, Ian Lance Taylor wrote:
>
>> On Fri, Sep 7, 2012 at 5:05 AM, Henrik Mannerström
>> <henrik.mannerstrom@gmail.com> wrote:
>>>
>>>
>>> If I understand the Intel reference correctly, the CVTDQ2PD should allow
>>> me to convert two 64 bit integers into two 64 bit doubles in one blow.
>>> How should I write this in gcc C/C++?
>>>
>>> Program 1 uses casting to illustrate the point, it outputs 0 and 1.
>>> Program 2 was my naive attempt to use vectorization, it outputs 0 and
>>> 4.94066e-324, which is _not_ what I intended.
>>
>>
>>>   a.vd = __v2df(a.vi);
>>
>>
>> This is just a type cast of the vector as a whole.  It doesn't convert
>> the individual vector elements.
>
>
> I always found it strange that vectors act like values everywhere, except
> for casts where they act like pointers. I understand this is convenient to
> allow calling intrinsics with vector types that don't exactly match, but it
> steals a good syntax that gcc's vector extensions can't use for true
> conversions.
>
> --
> Marc Glisse

I'm also a fan of explicit "alias" assignment ala _mm_cast_* functions.

Just my 2 cents :)

  Brian