Re: moving data between x87 and xmm registers

["Gautam Sewani" <gautamcool88 at gmail dot com>]

That is very bad news indeed :-( .
Can anyone confirm this with some testing? (I am using a Core duo, and
don't have access to Core 2 Duo.)
Regards
Gautam
On Thu, Jun 5, 2008 at 7:26 PM, Frédéric Bastien <nouiz@nouiz.org> wrote:
> Hi,
>
> With processor before core2 from intel, their was a bottleneck in the
> CPU that make all sse instruction being split in two. So as you have
> only two double in a sse instruction and if you have a processor with
> such a bottleneck, I see only 1 way to have a speed up. Use float
> instead of double. I know, this is not always an option. To my
> knowledge prescott cpu have this bottleneck.
>
> Frederic Bastien
>
> On Thu, Jun 5, 2008 at 2:39 AM, Gautam Sewani <gautamcool88@gmail.com> wrote:
>> Hi,
>> I am trying to vectorize a piece of code using SSE 2 intrinsics (the
>> one's in emmintrin.h).I am using double precision floating point
>> arithmetic.The running times I obtained were very similar with and
>> without the vectorization. I suspect the reason for this is that in
>> the vectorized code, I am storing the contents of a packed xmm
>> register (represented by an __m128d variable) into a double array.
>>
>> Looking into the assembly code generated, I saw that for this, the
>> contents of the xmm register were first saved to a memory location and
>> then loaded into the x87 FPU stack. Apparently there is no direct way
>> to transfer data between x87 and xmm registers. One way to eliminate
>> this would be to use xmm registers for all floating point
>> calculations. But inspite of using -march=prescott and -mfpmath=sse,
>> x87 instructions like fld and fstp are still used. Is there any to
>> force GCC to use only the xmm registers for all floating point
>> calculations?(I tried using the -mno-80387 option but I am getting
>> lots of weird linker errors with that). Or is there anyway to move
>> data between x87 and xmm registers without using memory as an
>> intermediary ?
>>
>> Regards
>> Gautam
>>
>