forwprop: Tweak choice of VEC_PERM_EXPR filler [PR92822]

forwprop: Tweak choice of VEC_PERM_EXPR filler [PR92822]

For the 2s failures in the PR, we have a V4SF VEC_PERM_EXPR in
which the first two elements are duplicates of one element and
the other two are don't-care:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, ?, ? }>;

The heuristic was to extend this with a blend:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, 2, 3 }>;

but it seems better to extend a partial duplicate to a full duplicate:

    v4sf_b = VEC_PERM_EXPR <v4sf_a, v4sf_a, { 1, 1, 1, 1 }>;

Obviously this is still just a heuristic though.

I wondered whether to restrict this to two elements or more
but couldn't find any examples in which it made a difference.
Either way should be fine for the purposes of fixing this PR.

2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92822
	* tree-ssa-forwprop.c (simplify_vector_constructor): When filling
	out the don't-care elements of a vector whose significant elements
	are duplicates, make the don't-care elements duplicates too.