On Tue, 30 May 2023, Richard Sandiford wrote:
> Richard Biener <rguent...@suse.de> writes:
> >> But how easy would it be to extend SCEV analysis, via a pattern match?
> >> The evolution of the IV phi wrt the inner loop is still a normal SCEV.
> >
> > No, the IV isn't a normal SCEV, the final value is different.
>
> Which part of the IV though?
The relevant IV (for niter analysis) is the one used in the loop
exit test and that currently isn't a SCEV. The IV used in the
*_len operations isn't either (and that's not going to change,
obviously).
> Won't all executions of the latch edge
> decrement the IV phi (and specifically the phi) by VF (and only VF)?
But currently there's no decrement by invariant VF but only
by MIN (VF, remain), that's what I suggested to address to
make the loop exit condition analyzable (as said, in theory
we can try pattern matching the analysis of the exit test in
niter analysis).
> So if we analyse the IV phi wrt the inner loop, the IV phi is simply
> { initial, -, VF }.
>
> I agree "IV phi - step" isn't a SCEV, but that doesn't seem fatal.
Right. Fatal is the non-SCEV in the exit test which makes most
followup loop optimizations fail to consider the loop because the
number of iterations cannot be determined.
Richard.
