[Bug middle-end/106081] missed vectorization

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106081

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
So what's interesting is that we now get as of r14-2117-gdd86a5a69cbda4 the
following.  The odd thing is that we fail to eliminate the load permutation
{ 3 2 1 0 } even though this is a reduction group.

I _suppose_ the reason is the { 0 0 0 0 } load permutation (the "splat")
which we don't "support".  In vect_optimize_slp_pass::start_choosing_layouts
there's

      if (SLP_TREE_LOAD_PERMUTATION (node).exists ())
        {
          /* If splitting out a SLP_TREE_LANE_PERMUTATION can make the node
             unpermuted, record a layout that reverses this permutation.

             We would need more work to cope with loads that are internally
             permuted and also have inputs (such as masks for
             IFN_MASK_LOADs).  */
          gcc_assert (partition.layout == 0 && !m_slpg->vertices[node_i].succ);
          if (!STMT_VINFO_GROUPED_ACCESS (dr_stmt))
            continue;

which means we'll keep the permute there (well, that's OK - any permute
of the permute will retain it ...).  I suspect this prevents the optimization
here.  Massaging start_choosing_layouts to allow a splat on element zero
for a non-grouped access breaks things as we try to move that permute.
So I guess this needs a new kind of layout constraint?  The permute
can absorb any permute but we cannot "move" it.

Richard?


t.c:14:18: note:   === scheduling SLP instances ===
t.c:14:18: note:   Vectorizing SLP tree:
t.c:14:18: note:   node 0x4304170 (max_nunits=16, refcnt=2) vector(4) double
t.c:14:18: note:   op template: _21 = _20 + results$d_60;
t.c:14:18: note:        stmt 0 _21 = _20 + results$d_60;
t.c:14:18: note:        stmt 1 _17 = _16 + results$c_58;
t.c:14:18: note:        stmt 2 _13 = _12 + results$b_56;
t.c:14:18: note:        stmt 3 _9 = _8 + results$a_54;
t.c:14:18: note:        children 0x43041f8 0x4304418
t.c:14:18: note:   node 0x43041f8 (max_nunits=16, refcnt=1) vector(4) double
t.c:14:18: note:   op template: _20 = _1 * _19;
t.c:14:18: note:        stmt 0 _20 = _1 * _19;
t.c:14:18: note:        stmt 1 _16 = _1 * _15;
t.c:14:18: note:        stmt 2 _12 = _1 * _11;
t.c:14:18: note:        stmt 3 _8 = _1 * _7;
t.c:14:18: note:        children 0x4304280 0x4304308
t.c:14:18: note:   node 0x4304280 (max_nunits=4, refcnt=1) vector(4) double
t.c:14:18: note:   op template: _1 = *k_50;
t.c:14:18: note:        stmt 0 _1 = *k_50;
t.c:14:18: note:        stmt 1 _1 = *k_50;
t.c:14:18: note:        stmt 2 _1 = *k_50;
t.c:14:18: note:        stmt 3 _1 = *k_50;
t.c:14:18: note:        load permutation { 0 0 0 0 }
t.c:14:18: note:   node 0x4304308 (max_nunits=16, refcnt=1) vector(4) double
t.c:14:18: note:   op template: _19 = (double) _18;
t.c:14:18: note:        stmt 0 _19 = (double) _18;
t.c:14:18: note:        stmt 1 _15 = (double) _14;
t.c:14:18: note:        stmt 2 _11 = (double) _10;
t.c:14:18: note:        stmt 3 _7 = (double) _6;
t.c:14:18: note:        children 0x4304390
t.c:14:18: note:   node 0x4304390 (max_nunits=16, refcnt=1) vector(16) short
int
t.c:14:18: note:   op template: _18 = _5->d;
t.c:14:18: note:        stmt 0 _18 = _5->d;
t.c:14:18: note:        stmt 1 _14 = _5->c;
t.c:14:18: note:        stmt 2 _10 = _5->b;
t.c:14:18: note:        stmt 3 _6 = _5->a;
t.c:14:18: note:        load permutation { 3 2 1 0 }
t.c:14:18: note:   node 0x4304418 (max_nunits=4, refcnt=1) vector(4) double
t.c:14:18: note:   op template: results$d_60 = PHI <_21(5), 0.0(6)>
t.c:14:18: note:        stmt 0 results$d_60 = PHI <_21(5), 0.0(6)>
t.c:14:18: note:        stmt 1 results$c_58 = PHI <_17(5), 0.0(6)>
t.c:14:18: note:        stmt 2 results$b_56 = PHI <_13(5), 0.0(6)>
t.c:14:18: note:        stmt 3 results$a_54 = PHI <_9(5), 0.0(6)>
t.c:14:18: note:        children 0x4304170 (nil)