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@xxxxxxxxx> 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 >
