https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98291
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |53947
Last reconfirmed| |2021-01-04
Status|UNCONFIRMED |ASSIGNED
Ever confirmed|0 |1
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is a missed optimization in SLP reduction vectorization. We're
detecting the optimal vectorization opportunity but:
x.c:6:36: note: ==> examining statement: d2_30 = PHI <d2_25(7), 0.0(6)>
x.c:6:36: note: vect_is_simple_use: operand _11 * _14, type of def: internal
x.c:6:36: note: vect_is_simple_use: operand d2_30 = PHI <d2_25(7), 0.0(6)>,
type of def: reduction
x.c:6:36: missed: reduc op not supported by target.
x.c:6:36: missed: in-order unchained SLP reductions not supported.
x.c:1:8: missed: not vectorized: relevant stmt not supported: d2_30 = PHI
<d2_25(7), 0.0(6)>
x.c:6:36: note: removing SLP instance operations starting from: d1_24 = _7 +
d1_28;
x.c:6:36: missed: unsupported SLP instances
x.c:6:36: note: re-trying with SLP disabled
and end up vectorizing two reductions on interleaved data (ugh).
The fact we fail to notice is that there's no reduction needed in the
epilogue (VF == 1) and the reduction is vectorized in-order already.
if (reduction_type == FOLD_LEFT_REDUCTION
&& slp_node
&& !REDUC_GROUP_FIRST_ELEMENT (stmt_info))
{
/* We cannot use in-order reductions in this case because there is
an implicit reassociation of the operations involved. */
if (dump_enabled_p ())
dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
"in-order unchained SLP reductions not supported.\n");
return false;
If we allow VF == 1 here we end up with odd (wrong?) code so I guess we
shouldn't classify a VF == 1 SLP reduction as FOLD_LEFT_REDUCTION in the
first place but IIRC this classification is done before the VF is fixed.
Mine.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
