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forwprop: Tweak choice of VEC_PERM_EXPR filler [PR92822]

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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.
This commit is contained in:
Richard Sandiford 2020-01-25 17:18:51 +00:00
parent 6cedf5d8e2
commit 1ee3b380df
2 changed files with 23 additions and 6 deletions
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7
gcc/ChangeLog
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@ -1,3 +1,10 @@
2020-01-28 Richard Sandiford <richard.sandiford@arm.com>
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.
2020-01-28 Richard Sandiford <richard.sandiford@arm.com>
PR tree-optimization/93434

22
gcc/tree-ssa-forwprop.c
View File

@ -2455,16 +2455,26 @@ simplify_vector_constructor (gimple_stmt_iterator *gsi)
it and its source indexes to make the permutation supported.
For now it mimics a blend. */
vec_perm_builder sel (refnelts, refnelts, 1);
bool all_same_p = true;
for (i = 0; i < elts.length (); ++i)
sel.quick_push (elts[i].second + elts[i].first * refnelts);
{
sel.quick_push (elts[i].second + elts[i].first * refnelts);
all_same_p &= known_eq (sel[i], sel[0]);
}
/* And fill the tail with "something". It's really don't care,
and ideally we'd allow VEC_PERM to have a smaller destination
vector. As heuristic try to preserve a uniform orig[0] which
facilitates later pattern-matching VEC_PERM_EXPR to a
BIT_INSERT_EXPR. */
vector. As a heuristic:
(a) if what we have so far duplicates a single element, make the
tail do the same
(b) otherwise preserve a uniform orig[0]. This facilitates
later pattern-matching of VEC_PERM_EXPR to a BIT_INSERT_EXPR. */
for (; i < refnelts; ++i)
sel.quick_push ((elts[0].second == 0 && elts[0].first == 0
? 0 : refnelts) + i);
sel.quick_push (all_same_p
? sel[0]
: (elts[0].second == 0 && elts[0].first == 0
? 0 : refnelts) + i);
vec_perm_indices indices (sel, orig[1] ? 2 : 1, refnelts);
if (!can_vec_perm_const_p (TYPE_MODE (perm_type), indices))
return false;