https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118468
Bug ID: 118468
Summary: vectorizer: extra phi blocks vectorization of if
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: andi-gcc at firstfloor dot org
Target Milestone: ---
This is forked from PR116126 to handle another early exit problem
const unsigned char *search_line_fast2 (const unsigned char *s, const unsigned
char *end)
{
while (s < end) {
if (*s == '\n' || *s == '\r'
#ifdef MORE
|| *s == '\\' || *s == '?'
#endif
)
return s;
s++;
}
return s;
}
When build
cc1 -O3 -mavx10.2 search-line-fast-short.c -quiet -fopt-info-all -o x.s
-fdump-tree-vect
it vectorizes but with -DMORE=1 it gives:
search-line-fast-short.c:3:18: missed: couldn't vectorize loop
search-line-fast-short.c:3:18: missed: not vectorized: unsupported control flow
in loop.
tree-vect-loop.c:
/* Check if we have any control flow that doesn't leave the loop. */
basic_block *bbs = get_loop_body (loop);
for (unsigned i = 0; i < loop->num_nodes; i++)
if (EDGE_COUNT (bbs[i]->succs) != 1
&& (EDGE_COUNT (bbs[i]->succs) != 2
|| !loop_exits_from_bb_p (bbs[i]->loop_father, bbs[i])))
{
free (bbs);
return opt_result::failure_at (vect_location,
"not vectorized:"
" unsupported control flow in loop.\n");
tree-if-conv converts the if, but then generates:
_13 = _5 | prephitmp_26;
if (_13 != 0)
goto <bb 12>; [8.03%]
else
goto <bb 6>; [91.97%]
(still inside loop)
<bb 12> [local count: 83800317]:
# s_24 = PHI <s_15(5)>
goto <bb 7>; [100.00%] -> leaving loop
So the empty basic block with the extra PHI prevents vectorization.
The question is what to do about it. Either if-conv could avoid it, or the
vectorizer could handle this case.
