https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68707
--- Comment #20 from alalaw01 at gcc dot gnu.org --- > Would be nice to have a reduced testcase for this one. Working on it. Sadly it's fortran :( The SLP tree that gets cancelled, is quite big (and quite untreelike, if we could see that - a large portion, 7 nodes, is repeated but with the 2 stmts in each SLP node reversed). "Decided to SLP 2 instances" indeed becomes "Decided to SLP 1 instances", with Unrolling factor 2 both times. In the case where the SLP gets cancelled, several more stmts that would have featured in that tree are marked hybrid. The 'vector inside of loop cost' increases from 180 (with SLP) to 308 (if cancelled), but minimum iters for profitability stays at 3. However, the SLP-cancelled case, outputs a whole extra section note: === scheduling SLP instances === ... note: ------>vectorizing SLP node starting from: (one of the loads in the cancelled tree) * 4 ... note: vectorizing stmts using SLP. (Tho I suspect that's a red herring.) Whereas later the non-cancelled case, clearly has an extra 'note: add new stmt: MEM[...] = STORE_LANES'...sounding as if perhaps the SLP finds it can use ST2 opportunistically (??).
