On Tue, Jun 9, 2015 at 11:11 AM, Neil Roberts <n...@linux.intel.com> wrote: > Jason Ekstrand <ja...@jlekstrand.net> writes: > >> The only place when the fact that the MRFs are virtual matters is in >> register allocation. Implied MRF writes are taken into account in >> setup_mrf_hack_interference. We figure out what MRFs are used and >> then mark them as conflicting with *all* of the VGRFs. We also force >> them to be placed in the correct physical register using >> ra_set_node_reg. The register allocator is then smart enough to not >> place any other VGRF values into one of those registers. The problem >> is that we were also forcing the message for the final FB write and we >> were forcing it to overlap. > > Ok, that makes sense. Thanks for the explanation. > >>> I was thinking it's a bit risky to just use a register that is lower >>> than the lowest used MRF register because it could be possible for a >>> message to use a really low register number and then this would end up >>> using a negative register number. However in practice on Gen7+ it looks >>> like there's hardly anything left that is using the fake MRF and we're >>> unlikely to add more so it probably doesn't really matter. >> >> The maximum length of a message is 16 and we have at most 16 virtual >> MRFs so the message will start no earlier than g96. I think it's fine >> as long as it's in the upper half or something like that. > > Page 1039 of the Broadwell PRM says: > > “The send with EOT should use register space R112-R127 for <src>. This > is to enable loading of a new thread into the same slot while the > message with EOT for current thread is pending dispatch”
Experimental evidence suggests that this is a bit conservative. Back when I first started using GRFs for sends, I did some experiments and anywhere towards the top seems to work ok. > So 96 wouldn't work. I guess that means if anything is using a base_mrf > of 7 or below then it could potentially break. At a glance I can't find > anything that would do that on Gen7+, but maybe it's a bit risky. The other thing to remember is that the MRF hack is *only* used for spilling on GEN7+ as of when we started using GRFs for fb writes. Therefore, the most MRFs we can possibly have is 3. It's still possible to go outside 112-127, but it's pretty hard at that point. Basically, you would have to have a 15 or 16 register message with spilling. I don't know if that's possible today. --Jason _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
