Re: moving data between x87 and xmm registers

["Frédéric Bastien" <nouiz at nouiz dot org>]

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
>