arch-specific template code

[Ulrich Drepper]

On Tue, Aug 28, 2012 at 8:33 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> Actually, it looks to me like most of it can be rewritten using gcc's vector
> extensions. _mm_mul_pd(a,b) is just a*b, a[0] gives the first element,

It does?  How?

If I use

typedef double v2df __attribute__ ((vector_size (16)));
v2df a;

then I cannot use a[0].  gcc has an arch-specific builtin
__builtin_ia32_vec_ext_v2df().  There is nothing else I have found
which can take its place.  The horizontal add you already noticed.
What is also missing is control over memory accesses.  I.e., loads and
stores from scalar pointers that are possibly unaligned.

There are big gaps in the vector extensions.  In other functions sqrt
is needed, masking is required, etc.  None of this works (at least
with the same efficiency) as using the Intel intrinsics.  Waiting for
the compiler to catch up isn't good IMO.  As you have seen in the
code, it should be easy enough for the SSE code to be converted to use
the gcc vector extensions if and when they catch up.