Re: [PATCH v2 1/2] Allow vec_duplicate_optab to fail

[Richard Sandiford via Gcc-patches]

"H.J. Lu" <hjl.to...@gmail.com> writes:
> Update vec_duplicate to allow to fail so that backend can only allow
> broadcasting an integer constant to a vector when broadcast instruction
> is available.

I'm not sure why we need this to fail though.  Once the optab is defined
for target X, the optab should handle all duplicates for target X,
even if there are different strategies it can use.

AIUI the case you want to make conditional is the constant case.
I guess the first question is: why don't we simplify those CONSTRUCTORs
to VECTOR_CSTs in gimple?  I'm surprised we still see the constant case
as a constructor here.

If we can't rely on that happening, then would it work to change:

        /* Try using vec_duplicate_optab for uniform vectors.  */
        if (!TREE_SIDE_EFFECTS (exp)
            && VECTOR_MODE_P (mode)
            && eltmode == GET_MODE_INNER (mode)
            && ((icode = optab_handler (vec_duplicate_optab, mode))
                != CODE_FOR_nothing)
            && (elt = uniform_vector_p (exp)))

to something like:

        /* Try using vec_duplicate_optab for uniform vectors.  */
        if (!TREE_SIDE_EFFECTS (exp)
            && VECTOR_MODE_P (mode)
            && eltmode == GET_MODE_INNER (mode)
            && (elt = uniform_vector_p (exp)))
          {
            if (TREE_CODE (elt) == INTEGER_CST
                || TREE_CODE (elt) == POLY_INT_CST
                || TREE_CODE (elt) == REAL_CST
                || TREE_CODE (elt) == FIXED_CST)
              {
                rtx src = gen_const_vec_duplicate (mode, expand_normal (node));
                emit_move_insn (target, src);
                break;
              }
            …
          }

Thanks,
Richard