Am 16.08.2014 02:12, schrieb Connor Abbott: > I know what you might be thinking right now. "Wait, *another* IR? Don't > we already have like 5 of those, not counting all the driver-specific > ones? Isn't this stuff complicated enough already?" Well, there are some > pretty good reasons to start afresh (again...). In the years we've been > using GLSL IR, we've come to realize that, in fact, it's not what we > want *at all* to do optimizations on. Ian has done a talk at FOSDEM that > highlights some of the problems they've run into: > > https://urldefense.proofpoint.com/v1/url?u=https://video.fosdem.org/2014/H1301_Cornil/Saturday/Three_Years_Experience_with_a_Treelike_Shader_IR.webm&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=F4msKE2WxRzA%2BwN%2B25muztFm5TSPwE8HKJfWfR2NgfY%3D%0A&m=iXhCeAYmidPDc1lFo757Cc9V0PvWAN4n3X%2Fw%2B%2F7Lx%2Fs%3D%0A&s=f103fb26bf53eee64318a490517d1ee9ab88ecd29fcdbe49d54b5a27e7581c2e > > But here's the summary: > > * GLSL IR is way too much of a memory hog, since it has to make a new > variable for each temporary the compiler creates and then each time you > want to dereference that temporary you need to create an > ir_dereference_variable that points to it which is also very > cache-unfriendly ("downright cache-mean!"). > > * The expression trees were originally added so that we could do > pattern matching to automatically optimize things, but this turned out > to be both very difficult to do and not very helpful. Instead, all it > does is add more complexity to the IR without much benefit - with SSA or > having proper use-def chains, we could get back what the trees give us > while also being able to do lots more optimizations. > > * We don't have the concept of basic blocks in GLSL IR, which makes a > lot of optimizations harder because they were originally designed with > basic blocks in mind - take, for example, my SSA series. I had to map a > whole lot of concepts that were based on the control flow graph to this > tree of statements that GLSL IR uses, and the end result wound up > looking nothing at all like the original paper. This problem gets even > worse for things like e.g. Global Code Motion that depend upon having > the dominance tree. > > I originally wanted to modify GLSL IR to fix these problems by adding > new instruction types that would address these issues and then > converting back and forth between the old and the new form, but I > realized that fixing all the problems would basically mean a complete > rewrite - and if that's the case, then why don't we start from scratch? > So I took Ken's suggestions and started designing, and then at Intel > over the summer started implementing, a completely new IR which I call > NIR that's at a lower level than GLSL IR, but still high-level enough to > be mostly device-independant (different drivers may have different > passes and different ways of lowering e.g. matrix multiplies) so that > we can do generic optimizations on it. Having support for SSA from the > beginning was also a must, because lots of optimisations that we really > want for cleaning up DX9-translated games are either a lot easier in or > made possible by SSA. I also made the decision for it to be typeless, > because that's what the cool kids are all doing :) and for a > lower-level, flat IR it seemed like the thing to do (it could have gone > either way, though). So the key design points of NIR (pronounced either > like "near" as in "NIR is near!" or to rhyme with "burr") are: > > * It's flat (no expression trees) > > * It's typeless > > * Modifiers (abs, negate, saturate), swizzles, and write masks are part > of ALU instructions > > * It includes enough GLSL-like things (variables that you can load from > or store to, function calls) to be hardware-agnostic (although we don't > have a way to represent matrix multiplies right now, but that could > easily be added) to be able to do optimizations at a high level, while > having lowering passes that convert variables to registers and > input/output/uniform loads/stores that will open up more opportunities > for optimization and save memory while being more hardware-specific. > > * Control flow consists of a tree of if statements and loops, like in > GLSL IR, except the leaves of the tree are now basic blocks instead of > instructions. Also, each basic block keeps track of its successors and > predecessors, so the control flow graph is explicit in the IR. > > * SSA is natively supported, and SSA uses point directly to the SSA > definition, which means that the use-def chains are always there, and > def-use chains are kept by tracking the set of all uses for each > definition. > > * It's written in C. > > (see the README in patch 3 and nir.h in patch 4 for more details) > > Some things that are missing or could be improved: > > * There's currently no alias tracking for inputs, outputs, and uniforms. > This is especially important for uniforms because we don't pack them > like we pack inputs and outputs. > > * We need a way to represent matrix multiplies so that we can do > matrix-flipping optimizations in NIR (currently GLSL IR does this for > us). > > * I'm not entirely happy about how we represent loads and stores in the > IR. Right now, they're intrinsics, but that means we need a different > intrinsic for each size and combination of arguments (indirect vs. not > indirect, etc.) and we might run into a combinatorial explosion problem > in the future, so we might need to make separate load/store instructions > like what I did for textures. > > * Right now, we only have a pass that lowers variables for scalar > backends. We need to write a similar pass for vector backends that uses > std140 packing or something similar, as well as porting > lower_ubo_reference to NIR and changing it to output offsets in the > hardware-native units instead of bytes. > > * We'll need to write a pass that splits up vector expressions for > scalar backends.
Interesting. I think conceptually this makes sense (I'm far from an expert in that area though), though I wonder if we actually even should have our own IR? GL NG will specify a common shading language intermediate representation, and I suspect there'd be benefits if we'd just use that? Obviously I don't have any idea how that's going to look like but maybe it will be just like SPIR (which is llvm ir essentially)? Granted a lot of stuff in your isa is conceptually similar (such as being based around basic blocks). Roland _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev