best data extraction for SSE2 emulation?

[David Mathog]

In my software SSE2 emulation I am currently using this sort of approach
to extract data fields out of __m128i and __m128d vectors:

#define EMM_SINT4(a)   ((int *)&(a))

static __inline __m128i __attribute__((__always_inline__))
_mm_slli_epi32 (__m128i __A, int __B)
{
  __v4si __tmp = {
    EMM_SINT4(__A)[0] << __B,
    EMM_SINT4(__A)[1] << __B,
    EMM_SINT4(__A)[2] << __B,
    EMM_SINT4(__A)[3] << __B};
  return (__m128i)__tmp;
}

This works fine when testing one _mm function at a time, but does not
work reliably in real programs unless -O0 is used.  I think at least
part of the problem is that once the function is inlined the parameter
__A is in some cases a  register variable, and the pointer method is not
valid there.  To get around that I'm think of introducing an explicit
local variable, like this:

static __inline __m128i __attribute__((__always_inline__))
_mm_slli_epi32 (__m128i __A, int __B)
{
__m128i A = __A;
  __v4si __tmp = {
    EMM_SINT4(A)[0] << __B,
    EMM_SINT4(A)[1] << __B,
    EMM_SINT4(A)[2] << __B,
    EMM_SINT4(A)[3] << __B};
  return (__m128i)__tmp;
}

I'm not sure that will work all the time either.  The only other
approach I an aware of would be something like this:

#typedef union {
  __m128i           vi;
  __m128d           vd;
  int               s[4];
  unsigned int      us[4];
  /* etc. for other types */
} emm_universal ;

#define EMM_SINT4(a)   (a).s

static __inline __m128i __attribute__((__always_inline__))
_mm_slli_epi32 (__m128i __A, int __B)
{
emm_universal A;
  A.vi = __A;
 __v4si __tmp = {
    EMM_SINT4(A)[0] << __B,
    EMM_SINT4(A)[1] << __B,
    EMM_SINT4(A)[2] << __B,
    EMM_SINT4(A)[3] << __B};
  return (__m128i)__tmp;
}

The union approach seems to be just a different a way to spin the
pointer operations.  For gcc in particular, is one approach or the other
to be preferred and why?

Thanks,

David Mathog
mat...@caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech