[Mesa-dev] [RFC PATCH 00/16] A new IR for Mesa

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://video.fosdem.org/2014/H1301_Cornil/Saturday/Three_Years_Experience_with_a_Treelike_Shader_IR.webm 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. The first two patches are preperatory patches that I already sent to the list, but I'm re-sending them as part of the series as they haven't landed yet. Right now, the series only has code to convert GLSL IR to NIR, but no way to actually hook it up to a backend in order to generate code from it, and it also doesn't do anything with the SSA part of the IR. I have a branch on my Github that does the conversion to SSA and a few simple SSA-based optimizations, which hasn't been tested as much (since I haven't written a pass to get out of SSA or a backend that uses SSA): https://github.com/cwabbott0/mesa/tree/nir and an experimental backend for i965 fs that I hope to combine with Matt's SSA work; right now, there are only a few piglit regressions and most of them are because of the hacky way I changed boolean true to be 0xFFFFFF instead of 1 (with Matt's series to do the same thing in a better way, they should go away) or because of unimplemented features (atomics and some system values): https://github.com/cwabbott0/mesa/tree/nir-i965-fs NIR has been what I've worked on for my entire summer internship at Intel, and before I go off to my freshman year at college, I'd like to thank the other Intel folks for the knowledge they've given me and the many interesting discussions that made this go from an idea to a reality - I'll miss you guys! Connor Connor Abbott (16): exec_list: add a list_foreach_typed_reverse() macro glsl/linker: pass through the is_intrinsic flag nir: add initial README nir: add a simple C wrapper around glsl_types.h nir: add the core datastructures nir: add core helper functions nir: add a printer nir: add a validation pass nir: add a glsl-to-nir pass nir: add a pass to lower variables for scalar backends nir: keep track of the number of input, output, and uniform slots nir: add a pass to remove unused variables nir: add a pass to lower sampler instructions nir: add a pass to lower system value reads nir: add a pass to lower atomics nir: add an optimization to turn global registers into local registers src/glsl/Makefile.sources | 18 +- src/glsl/link_functions.cpp | 2 + src/glsl/list.h | 6 + src/glsl/nir/README | 118 ++ src/glsl/nir/glsl_to_nir.cpp | 1759 +++++++++++++++++++++++++++++ src/glsl/nir/glsl_to_nir.h | 40 + src/glsl/nir/nir.c | 1717 ++++++++++++++++++++++++++++ src/glsl/nir/nir.h | 1270 +++++++++++++++++++++ src/glsl/nir/nir_intrinsics.c | 49 + src/glsl/nir/nir_intrinsics.h | 158 +++ src/glsl/nir/nir_lower_atomics.c | 127 +++ src/glsl/nir/nir_lower_samplers.cpp | 170 +++ src/glsl/nir/nir_lower_system_values.c | 106 ++ src/glsl/nir/nir_lower_variables_scalar.c | 1243 ++++++++++++++++++++ src/glsl/nir/nir_opcodes.c | 46 + src/glsl/nir/nir_opcodes.h | 346 ++++++ src/glsl/nir/nir_opt_global_to_local.c | 103 ++ src/glsl/nir/nir_print.c | 916 +++++++++++++++ src/glsl/nir/nir_remove_dead_variables.c | 138 +++ src/glsl/nir/nir_types.cpp | 155 +++ src/glsl/nir/nir_types.h | 78 ++ src/glsl/nir/nir_validate.c | 798 +++++++++++++ 22 files changed, 9362 insertions(+), 1 deletion(-) create mode 100644 src/glsl/nir/README create mode 100644 src/glsl/nir/glsl_to_nir.cpp create mode 100644 src/glsl/nir/glsl_to_nir.h create mode 100644 src/glsl/nir/nir.c create mode 100644 src/glsl/nir/nir.h create mode 100644 src/glsl/nir/nir_intrinsics.c create mode 100644 src/glsl/nir/nir_intrinsics.h create mode 100644 src/glsl/nir/nir_lower_atomics.c create mode 100644 src/glsl/nir/nir_lower_samplers.cpp create mode 100644 src/glsl/nir/nir_lower_system_values.c create mode 100644 src/glsl/nir/nir_lower_variables_scalar.c create mode 100644 src/glsl/nir/nir_opcodes.c create mode 100644 src/glsl/nir/nir_opcodes.h create mode 100644 src/glsl/nir/nir_opt_global_to_local.c create mode 100644 src/glsl/nir/nir_print.c create mode 100644 src/glsl/nir/nir_remove_dead_variables.c create mode 100644 src/glsl/nir/nir_types.cpp create mode 100644 src/glsl/nir/nir_types.h create mode 100644 src/glsl/nir/nir_validate.c -- 1.9.3