I did some experiments overnight. Running before combine with your
patch to insert the SIGN_EXTEND node seems best to me.
The key realization is that combine will do the bit tracking and will
often find that the SIGN_EXTEND is not necessary and will remove it.
Thus we get the two instruction sequence (ctz+bclr) instead of
(ctz+bclr+sext).
WRT nonzero_bits. They are tracked outside combine, combine just
tends to do a better job. While the current nonzero bits tracking for
REGs outside combine is minimal, there's no strong reason not to use
it. Also note that we *could* do more tracking if we found it
useful. We could register a hook similar to how combine does. It may
not matter for this specific case, but it would have the potential to
help elsewhere. Point being it's an optimization that could
potentially allow us to avoid the SIGN_EXTEND in some cases. But I
don't mind deferring this.
Jeff
Thanks for the detailed analysis.
One new data point: a colleague tried this patch and, while it does fix the
DPDK hang, they are also hitting a desktop that fails to start which is not
fixed by it. So there is likely more to this than the case I have here.
They would like to take over and send a complete fix. I have forwarded your
comments to them, and they will post a new series rather than a v3 of this
one.
Sun Yuechi
