https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101842
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Blocks| |53947
Last reconfirmed| |2021-08-10
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> ---
The issue is that there's no symbolic expression to compute the number of
iterations since the number of iterations depends on data computed inside the
loop. We require a symbolic number of iterations in various places since
we're using a canonical IV for loop control. There's also the dynamic cost
check which depends on the number of vector iterations - I suppose for this
kind of loop we'd have to statically assert the vectorization is always
profitable.
But confirmed, we can't vectorize this loop. But we should vectorize the
basic-block eventually. We currently don't because the reduction handling
has the mixed +- case not implemented yet and we see
_41 = powmult_3 + powmult_5;
_42 = powmult_7 + _41;
_43 = powmult_9 + _42;
d_25 = d_35 - _43;
we detect this as reduction of 5 lanes and fail to see the opportunity to
reduce the 4 lanes with PLUS and then do the final minus with the remaining
(unvectorized) scalar.
diff --git a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
index f9ca24415a2..33b21c8c247 100644
--- a/gcc/tree-vect-slp.c
+++ b/gcc/tree-vect-slp.c
@@ -5666,10 +5666,12 @@ vect_slp_check_for_constructors (bb_vec_info bb_vinfo)
{
if (chain[i].dt != vect_internal_def)
invalid_cst = true;
- else if (chain[i].code != code)
- invalid_op = true;
else
- valid_lanes++;
+ {
+ valid_lanes++;
+ if (chain[i].code != code)
+ invalid_op = true;
+ }
}
if (!invalid_op && !invalid_cst)
{
then properly prints:
t.c:4:27: optimized: BB reduction missed with 5 lanes
The one different op lane could be handled similar as to the yet unsupported
constant - we need to record this operand and apply the it to the reduction
int the epilogue.
Let me try sth.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
