On Wed, Mar 14, 2018 at 4:58 PM, Connor Abbott <cwabbo...@gmail.com> wrote: > FWIW, the way I imagined doing this was something like: > > 1. Add nir_deref_instr and nir_deref_type_pointer. At this point, just > make everything assert if the base deref isn't a nir_deref_var. This > will be a bit of a flag-day, but also very mechanical. It'll also help > us catch cases where we don't handle new-style derefs later. > 2. Add a pass to flatten nir_deref_type_pointer into > nir_deref_type_var if possible (i.e. if there's a clear chain up to > the base variable without any phi nodes or whatever). This should > always be possible for GLSL, as well as SPIR-V unless > KHR_variable_pointers is enabled. We'll use this to avoid too much > churn in drivers, passes that haven't been updated, etc. We might also > want a pass to do the opposite, for converting passes where we don't > want to have codepaths for both forms at once.
btw, does it seem reasonable to assert that deref instruction src's are *always* in SSA form? That seems reasonable to me since they will be mostly lowered away before the driver sees them (and I think makes some of the operation on them easier), and I can't think of any way for them *not* to be SSA (since they aren't real instructions). If so, my rough thoughts are a deref instruction chain (formed by ssa links to previous deref instruction) either start w/ nir_deref_instr_pointer or nir_deref_instruction_var instructions at the head of the list (to start, I guess you could ignore adding the nir_deref_instr_pointer instruction and I could add that for clover/spirv work). Followed by N links of struct/array deref_link instructions that have two ssa src's (one that is previous deref instruction and one that is array or struct member offset) > 3. Modify nir_lower_io to handle new-style derefs, especially for > shared variables (i.e. KHR_variable_pointers for anv). We might have > to modify a few other passes, too. > 4. Add the required deref lowering passes to all drivers. > 5. Rewrite glsl_to_nir and spirv_to_nir to emit the new-style derefs. > At the very least, we should be using this to implement the shared > variable bits of KHR_variable_pointers. If we add stride/offset > annotations to nir_deref_instr for UBO's and SSBO's, then we might > also be able to get rid of the vtn_deref stuff entirely (although I'm > not sure if that should be a goal right now). I think I might try to prototype something where we convert vtn over to new-style deref instructions, plus a pass to lower to old style deref chains. It partly comes down to how quickly I can finish a couple other things, and how much I can't sleep on a long-ass flight. (I guess even if throw-away, if it gives some idea of what to do or what not to do it might be useful?) Anyways, as far as decoupling this from backend drivers, I think a nir_intr_get_var(intr, n) instruction to replace open coded intr->variables[0]->var could go a long way. (In the new world this would follow ssa links to previous deref instruction to find the nir_deref_instruction_var.) I'll try typing this up in a few minutes. > At this point, we can fix things up and move everything else over to > new-style derefs at our leisure. Also, it should now be pretty > straightforward to add support for shared variable pointers to radv > without lowering everything to offsets up-front, which is nice. > > Connor > > > On Wed, Mar 14, 2018 at 2:32 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: >> All, >> >> Connor and I along with several others have been discussing for a while >> changing the way NIR dereferences work. In particular, adding a new >> nir_deref_instr type where the first one in the chain takes a variable and >> is followed by a series of instructions which take another deref instruction >> and do an array or structure dereference on it. >> >> Much of the motivation for this is some of the upcoming SPIR-V stuff where >> we have more real pointers and deref chains don't really work anymore. It >> will also allow for things such as CSE of common derefs which could make >> analysis easier. This is similar to what LLVM does and it's working very >> well for them. >> >> The reason for this e-mail is that this is going to be a flag-day change. >> We've been talking about it for a while but this is going to be a major and >> fairly painful change in the short term so no one has actually done it. >> It's time we finally just suck it up and make it happen. While we will try >> to make the change as incrementally and reviewably as possible but there is >> a real limit as to what is possible here. My plan is to start cracking away >> at this on Monday and hopefully have something working for i965/anv by the >> end of the week or maybe some time the week after. If anyone has something >> to say in opposition, please speak up now and not after I've spent a week >> straight frantically hacking on NIR. >> >> I would like everyone to be respectful of the fact that this will be a major >> change and very painful to rebase. If you've got outstanding NIR, GLSL, or >> SPIR-V work that is likely to conflict with this, please try to land it >> before Monday so that we can avoid rebase conflicts. If you have interest >> in reviewing this, please try to be responsive so that we can get it >> reviewed and landed before it becomes too painful. I'll try to send out >> some preview patches as I go so that the data structures themselves can get >> some review before the rest of the changes have been made. >> >> I'm also asking for help from Rob, Bas, and Eric if there are changes needed >> in any of their drivers. I suspect the impact on back-end drivers will be >> low because most of them don't use derefs directly, but it would be good of >> people were on hand to help catch bugs if nothing else. >> I'll be at linaro conf next week.. I'll be responsive, but with some latency. But I expect I should be able to test/fix back-end issues on the freedreno/ir3 side of things, if not in real time, at least fast enough to not be a problem. (Sorry timing is slightly non-ideal, but I don't think I should be holding anything up, and I really like the deref instruction approach) BR, -R >> Thanks, >> >> --Jason Ekstrand >> _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
