[RFC] vector subscripts/BIT_FIELD_REF in Big Endian.

[Tejas Belagod <tbelagod at arm dot com>]

Hi,

I'm looking for some help understanding how BIT_FIELD_REFs work with big-endian.

Vector subscripts in this example:

#define vector __attribute__((vector_size(sizeof(int)*4) ))

typedef int vec vector;

int foo(vec a)
{
  return a[0];
}

gets lowered into array accesses by c-typeck.c

;; Function foo (null)
{
  return *(int *) &a;
}

and gets gimplified into BIT_FIELD_REFs a bit later.

foo (vec a)
{
  int _2;

  <bb 2>:
  _2 = BIT_FIELD_REF <a_3(D), 32, 0>;
  return _2;

}

What's interesting to me here is the bitpos - does this not need 
BYTES_BIG_ENDIAN correction? This seems to be inconsistenct with what happens 
with reduction operations in the autovectorizer where the scalar result in the 
reduction epilogue gets extracted with a BIT_FIELD_REF but the bitpos there is 
corrected for BIG_ENDIAN.

... from tree-vect-loop.c:vect_create_epilog_for_reduction ()

  /* 2.4  Extract the final scalar result.  Create:
          s_out3 = extract_field <v_out2, bitpos>  */

  if (extract_scalar_result)
    {
      tree rhs;

      if (dump_enabled_p ())
        dump_printf_loc (MSG_NOTE, vect_location,
			 "extract scalar result");

      if (BYTES_BIG_ENDIAN)
        bitpos = size_binop (MULT_EXPR,
                             bitsize_int (TYPE_VECTOR_SUBPARTS (vectype) - 1),
                             TYPE_SIZE (scalar_type));
      else
        bitpos = bitsize_zero_node;


For eg:

int foo(int * a)
{
  int i, sum = 0;

  for (i=0;i<16;i++)
   sum += a[i];

  return sum;
}

gets autovectorized into:

...
  vect_sum_9.17_74 = [reduc_plus_expr] vect_sum_9.15_73;
  stmp_sum_9.16_75 = BIT_FIELD_REF <vect_sum_9.17_74, 32, 96>;
  sum_76 = stmp_sum_9.16_75 + sum_47;

the BIT_FIELD_REF here seems to have been corrected for BYTES_BIG_ENDIAN

If vec_extract is defined in the back-end, how does one figure out if the 
BIT_FIELD_REF is a product of the gimplifier's indirect ref folding or the 
vectorizer's bit-field extraction and apply the appropriate correction in 
vec_extract's expansion? Or am I missing something that corrects BIT_FIELD_REFs 
between the gimplifier and the RTL expander?

Thanks,
Tejas.