https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61634
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> --- The issue seems that we fail to detect hybrid SLP here because vect_detect_hybrid_slp_stmts walks immediate uses of SLP defs but those do not reach pattern stmts but only original ones. In this case we have _11 = *prephitmp_46; _12 = _11 >> 2; f_13 = (int) _12; *prephitmp_46 = _12; _17 = f_13 * f_13; where the SLP def _12 is used in f_13 = (int) _12; but that stmt is marked as vect_unused_in_scope because it feeds a stmt that was pattern detected as widening multiplication. But we never visit that stmt, as its immediate uses are not accounted for (pattern stmts never get updated). That makes the vect_detect_hybrid_slp_stmts functions non-functional for non-trivial patterns. We can "fix" this by marking the stmts in some way and make sure to account for them in the loop. For the testcase forcing hybrid SLP by making !STMT_VINFO_RELEVANT stmts force it fixes it. I think that the best thing to do is to re-write hybrid SLP detection in terms of walking all stmts in the loop, similar to vect_mark_stmts_to_be_vectorized and mark pure SLP stmts as hybrid. The flag then still has to be propagated to the SLP tree children of such node. I have a prototype but that still needs quite some TLC.
