[RFC, vectorizer] Allow single element vector types for vector reduction operations

[Jon Beniston]

Hi,

I have an out-of-tree GCC port and it is struggling supporting
auto-vectorization on some dot product instructions.  For example, I have an
instruction that takes three operands which are all 32-bit general
registers. The second and third operands will be treated as V2HI then do dot
product, and then generate an SI result which is then added to the first
operand which is SI as well.

I do see there is dot product recognizer in tree-vect-patters.c, however, I
found the following testcase still can't be auto-vectorized on my port which
has implemented all necessary dot product standard patterns.  This testcase
can't be auto-vectorized on other targets that have similar V2HI dot product
instructions as well, for example ARC.

=== test.c ===
#define K 4
#define M 4
#define N 256
int in[N*K][M];
int out[K];
int coeff[N][M];
void
foo (void)
{
  int i, j, k;
  int sum;
  for (k = 0; k < K; k++)
    {
      sum = 0;
      for (j = 0; j < M; j++)
        for (i = 0; i < N; i++)
          sum += in[i+k][j] * coeff[i][j];
      out[k] = sum;
    }
}
===
  The reason that auto-vectorizer doesn't work seems to be that GCC doesn't
support single-element vector types in get_vectype_for_scalar_type_and_size.
tree-vect-stmts.c: get_vectype_for_scalar_type_and_size
  ...
  if (nunits <= 1)
    return NULL_TREE;

So, I am thinking this actually should be relaxed to support more cases.  At
least on vector reduction operations which normally will have scalar result
with wider types than the element type of input operands.

I have tried to make the auto-vectorizer work for my V2HI dot product case,
with the patch attached. Is this the correct approach?

Cheers,
Jon

gcc/
2017-08-27  Jon Beniston <jon@beniston.com>

        * tree-vectorizer.h (get_vectype_for_scalar_type): New optional
        parameter declaration.
        * tree-vect-stmts.c (get_vectype_for_scalar_type_and_size): Add new
        optional parameter "reduct_p".  Support single element vector types
        if it is true.
        (get_vectype_for_scalar_type): Add new parameter "reduct_p".
        * tree-vect-patterns.c (vect_pattern_recog_1): Pass new parameter
        "reduct_p".
        * tree-vect-loop.c (vect_determine_vectorization_factor): Likewise.
        (vect_model_reduction_cost): Likewise.
        (get_initial_def_for_induction): Likewise.
        (vect_create_epilog_for_reduction): Likewise.

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