Sigh, I knew I should have waited until the morning to proof-read
and send this.
Richard Sandiford <richard.sandif...@arm.com> writes:
> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> index 40767736389..00fce4945a7 100644
> --- a/gcc/fold-const.cc
> +++ b/gcc/fold-const.cc
> @@ -10743,27 +10743,37 @@ fold_vec_perm_cst (tree type, tree arg0, tree arg1,
> const vec_perm_indices &sel,
> unsigned res_npatterns, res_nelts_per_pattern;
> unsigned HOST_WIDE_INT res_nelts;
>
> - /* (1) If SEL is a suitable mask as determined by
> - valid_mask_for_fold_vec_perm_cst_p, then:
> - res_npatterns = max of npatterns between ARG0, ARG1, and SEL
> - res_nelts_per_pattern = max of nelts_per_pattern between
> - ARG0, ARG1 and SEL.
> - (2) If SEL is not a suitable mask, and TYPE is VLS then:
> - res_npatterns = nelts in result vector.
> - res_nelts_per_pattern = 1.
> - This exception is made so that VLS ARG0, ARG1 and SEL work as before.
> */
> - if (valid_mask_for_fold_vec_perm_cst_p (arg0, arg1, sel, reason))
> - {
> - res_npatterns
> - = std::max (VECTOR_CST_NPATTERNS (arg0),
> - std::max (VECTOR_CST_NPATTERNS (arg1),
> - sel.encoding ().npatterns ()));
> + /* First try to implement the fold in a VLA-friendly way.
> +
> + (1) If the selector is simply a duplication of N elements, the
> + result is likewise a duplication of N elements.
> +
> + (2) If the selector is N elements followed by a duplication
> + of N elements, the result is too.
>
> - res_nelts_per_pattern
> - = std::max (VECTOR_CST_NELTS_PER_PATTERN (arg0),
> - std::max (VECTOR_CST_NELTS_PER_PATTERN (arg1),
> - sel.encoding ().nelts_per_pattern ()));
> + (3) If the selector is N elements followed by an interleaving
> + of N linear series, the situation is more complex.
>
> + valid_mask_for_fold_vec_perm_cst_p detects whether we
> + can handle this case. If we can, then each of the N linear
> + series either (a) selects the same element each time or
> + (b) selects a linear series from one of the input patterns.
> +
> + If (b) holds for one of the linear series, the result
> + will contain a linear series, and so the result will have
> + the same shape as the selector. If (a) holds for all of
> + the lienar series, the result will be the same as (2) above.
linear
> +
> + (b) can only hold if one of the inputs pattern has a
input patterns
Sorry for the typos.
Richard
> + stepped encoding. */
> + if (valid_mask_for_fold_vec_perm_cst_p (arg0, arg1, sel, reason))
> + {
> + res_npatterns = sel.encoding ().npatterns ();
> + res_nelts_per_pattern = sel.encoding ().nelts_per_pattern ();
> + if (res_nelts_per_pattern == 3
> + && VECTOR_CST_NELTS_PER_PATTERN (arg0) < 3
> + && VECTOR_CST_NELTS_PER_PATTERN (arg1) < 3)
> + res_nelts_per_pattern = 2;
> res_nelts = res_npatterns * res_nelts_per_pattern;
> }
> else if (TYPE_VECTOR_SUBPARTS (type).is_constant (&res_nelts))
