[eigen] Re: sse4 and integer multiplication

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

oooh.... it was able to optimize integer multiplications by 3/5/9 as
LEA (load effective address) instructions ??!!

This reminds me very vague memories...

Benoit

2009/11/24 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Hi,
>
> i just added SSE4 integer mul support. It is an improvement over the
> current vectorized integer multiplication where SSE4 is available, but
> i am puzzled: here is my benchmark:
>
>
>
> #include <Eigen/Dense>
> using namespace Eigen;
> using namespace std;
>
> EIGEN_DONT_INLINE void foo()
> {
>  // i was wondering if the cpu could be clever enough to
>  // optimize when the ints are 0 or 1; it's not so easy to
>  // ensure that we don't end up with only 0 and 1...
>
>  Vector4i v(5,-7,11,13);
>  Vector4i w(9,3,-5,-7);
>
>  for(int i = 0; i<100000000; i++)
>  {
>    EIGEN_ASM_COMMENT("begin");
>    v = v.cwise()*v;
>    v = v.cwise()*w;
>    EIGEN_ASM_COMMENT("end");
>  }
>  cout << v << endl;
> }
>
> int main()
> {
>  foo();
> }
>
>
>
> OK so i'm puzzled because the fastest is... with no vectorization at all.
>
> No vectorization:   0.57 sec
> With SSE4.1:      0.81 sec
> With SSE2:         1.21 sec
>
> So i did what i usually do in such circumstances: dump the assembly
> and go whine until daddy Gael takes care of me.
>
> Without vec:
>
>        imull   %edx, %edx
>        imull   %eax, %eax
>        leal    0(,%rdx,8), %edi
>        imull   %ebx, %ebx
>        leal    (%rax,%rax,4), %eax
>        imull   %ecx, %ecx
>        subl    %edi, %edx
>        negl    %eax
>        leal    (%rbx,%rbx,8), %ebx
>        leal    (%rcx,%rcx,2), %ecx
>
> With SSE4.1:
>
>        movdqa  %xmm1, %xmm0
>        pmulld  %xmm1, %xmm0
>        pmulld  (%rdx), %xmm0
>        movdqa  %xmm0, %xmm1
>        movdqa  %xmm0, (%rbp)
>
> Cheers,
> Benoit
>