yelite commented on code in PR #79: URL: https://github.com/apache/tvm-rfcs/pull/79#discussion_r915195754
########## rfcs/0079-tvmscript-metaprogramming.md: ########## @@ -0,0 +1,398 @@ +- Feature Name: tvmscript-metaprogramming +- Start Date: 2022-06-16 +- RFC PR: [apache/tvm-rfcs#79](https://github.com/apache/tvm-rfcs/pull/79) +- GitHub Issue: [apache/tvm#0000](https://github.com/apache/tvm/issues/0000) +- Co-Authors: Yaxing Cai ([**@cyx-6**](https://github.com/cyx-6), main implementation), Lite Ye + ([**@yelite**](https://github.com/yelite)), Yong Wu + ([**@yongwww**](https://github.com/yongwww)), Yuchen Jin + ([**@YuchenJin**](https://github.com/YuchenJin)), Eric Lunderberg + ([**@Lunderberg**](https://github.com/Lunderberg)), Masahiro Masuda + ([**@masahi**](https://github.com/masahi)), Junru Shao + ([**@junrushao1994**](https://github.com/junrushao1994), main designer) + +# Summary +[summary]: #summary + +This RFC proposes a new TVMScript parser infrastructure, supporting extensive +metaprogramming and syntactic sugars. The new infrastructure is IR-agnostic, +treating TIR just as one of dialects. Additionally, the new infrastructure will +provide better tooling around Python ecosystem (pylint, mypy, etc.). + +# Motivation +[motivation]: #motivation + +**What is TVMScript**. +Check [Blitz Course to TensorIR](https://tvm.apache.org/docs/tutorial/tensor_ir_blitz_course.html) and +[TVMScript Unified Printer RFC](https://github.com/apache/tvm-rfcs/pull/74/files#diff-6965a40ad8df7618ae68e11c88f924542a506c74a931cc3011ae9f99989b5f51R20-R26) +for an introduction into TVMScript. + +**What is metaprogramming.** In the context of TVMScript, metaprogramming means +a programmable way to control IR generation. For example, in +https://github.com/apache/tvm/pull/11097, a metaprogramming feature was added +to the TVMScript parser, allows users to programmably control the shapes of the +input buffers of a `PrimFunc`. + +### Limitation of current design + +The current parser lacks capability on generic metaprogramming that allows user +to have more control on IR construction. This makes it challenging to support +operators like NMS (non-maximum suppression, which is crucial to object +detection model). There is an implementation of NMS at +[python/tvm/topi/cuda/nms.py#L367-L386](https://github.com/apache/tvm/blob/d0650bad66d0ff89a01347537021bc442a98c223/python/tvm/topi/cuda/nms.py#L367-L386). +The implementation of NMS-like operators requires rank-polymorphism and the +ability to interleave host program with TVMScript, which is difficult to be +implemented under the current design. + +TVMScript also needs reasonable support on Python tooling. Currently it doesn’t +play nicely with pylint and mypy. For example, +[test_meta_schedule_postproc_rewrite_tensorize.py](https://github.com/apache/tvm/blob/d0650bad66d0ff89a01347537021bc442a98c223/tests/python/unittest/test_meta_schedule_postproc_rewrite_tensorize.py) +has 100+ warnings from pylint within only 500 hundred lines of code. This +creates confusion to the user and leaves an impression that TVMScript isn’t a +mature product and not production-ready. Even though it’s something that can be +incrementally improved under the current design, we believe it’s easier to get +an ideal result if we have a design with the tooling support in mind. + +The current design also lacks of unified approach for different IRs. At +[https://github.com/tlc-pack/relax/tree/relax/python/tvm/script/relax](https://github.com/tlc-pack/relax/tree/relax/python/tvm/script/relax), +a mature implementation of TVMScript parser is maintained for Relax. But it’s +hard to extend if we want to support more IRs for TVM unity. + +To conclude, with this RFC, we want to: + +1. Add more metaprogramming features to TVMScript, making it easier for TVM + developers to write complicated operators. +2. Improve tooling and documentation of TVMScript, reducing the friction for an + average machine learning practitioner to use TVMScript. +3. Modularize and infrastructuralize the TVMScript parser, lowering the cost to + implement parser for new IR. + + +# Guide-level explanation +[guide-level-explanation]: #guide-level-explanation + +## Metaprogramming features to support + +### (F1) Template Metaprogramming + +Users should be able to use variables from outer scope in the TVMScript +function/class. The parsed result should be identical to function/class with +the variable replaced by its value. For instance, + +```python +@T.prim_func +def matmul( + A: T.Buffer[(128, 128)], +) -> None: + ... + +def gen_matmul(n, m) -> None: + @T.prim_func + def f(A: T.Buffer[(n, m)]): + ... + return f + +f = matmul(n=128, m=128) # `f` should be identical to `matmul` +``` + +This is already partially supported by https://github.com/apache/tvm/pull/11097 +for using `PrimExpr` captured by outer function. With the new parser, we want +to support this feature in more places and with more variable types. + +### (F2) Rank-polymorphism + +Users should be able to write a single function to handle different ranks of +input buffers (different numbers of dimensions). For example, user should be +able to write a generic function to do broadcast add, + +```python +def broadcast_add(a, b, c): + @T.prim_func + def f( + A: T.BufferFrom(a), + B: T.BufferFrom(b), + C: T.BufferFrom(c), + ) -> None: + for i, i_a, i_b in T.some_broadcast_method(A.shape, B.shape): + with T.block(): + C[*i] = A[*i_a] + B[*i_b] + +broadcast_add( + a = Buffer((128, 1), "float32"), + b = Buffer((1, 128), "float32"), + c = Buffer((128, 128), "float32"), +) +``` + +### (F3) Sugar: TE Compute in TIR + +Users should be able to replace boilerplate code with a function call, which’s +expanded to large chunk of code during parsing. For example, we may want to use +TE’s compute-like syntax to replace nested loop, + +```python +@T.prim_func +def te_compute_sugar( + A: T.Buffer[(128, 128)], + B: T.Buffer[(128, 128)], +) -> None: + ... + C = T.compute((128, 128), lambda i, j: A[i, j] + B[i, j]) + ... + +## expands to ====> + +@T.prim_func +def te_compute_expanded( + A: T.Buffer[(128, 128)], + B: T.Buffer[(128, 128)], +) -> None: + ... + for i in range(128): + for j in range(128): + with T.block("..."): + C[i, j] = A[i, j] + B[i, j] + ... +``` + +### (F4) Interleave host program and TVMScript program to customize metaprogramming + +As an escape hatch from writing code to be parsed by the TVMScript +parser, users should be able to write imperative code to construct IR nodes +directly and embed it inside regular TVMScript. Those code will be evaluated +by the Python interpreter when parsing. This gives users the ultimate tool when +TVMScript isn’t expressible enough for their use cases. For example, at +[python/tvm/topi/vision/nms.py#L380-L431](https://github.com/apache/tvm/blob/3cb4597ed48360e3f3d80161d1c03f833072d28e/python/tvm/topi/vision/nms.py#L380-L431), +there are blocks of repetitive code on computing the coordinates of the four +corners of bounding box. This can be simplified as: + +```python +# Before, without IRBuilder interleaving +@T.prim_func +def nms(...): + ... + for i in range(batch_size): + ... + a_l = min( + output[batch_idx, box_a_idx, box_start_idx], + output[batch_idx, box_a_idx, box_start_idx + 2], + ) + a_t = min( + output[batch_idx, box_a_idx, box_start_idx + 1], + output[batch_idx, box_a_idx, box_start_idx + 3], + ) + a_r = max( + output[batch_idx, box_a_idx, box_start_idx], + output[batch_idx, box_a_idx, box_start_idx + 2], + ) + a_b = max( + output[batch_idx, box_a_idx, box_start_idx + 1], + output[batch_idx, box_a_idx, box_start_idx + 3], + ) + ... + for k in range(j): + check_iou = ... + ... + if check_iou > 0: + # b_l: left, b_t: top, b_r: right, b_b: bottom + b_l = min( + output[batch_idx, box_b_idx, box_start_idx], + output[batch_idx, box_b_idx, box_start_idx + 2], + ) + b_t = min( + output[batch_idx, box_b_idx, box_start_idx + 1], + output[batch_idx, box_b_idx, box_start_idx + 3], + ) + b_r = max( + output[batch_idx, box_b_idx, box_start_idx], + output[batch_idx, box_b_idx, box_start_idx + 2], + ) + b_b = max( + output[batch_idx, box_b_idx, box_start_idx + 1], + output[batch_idx, box_b_idx, box_start_idx + 3], + ) + ... + +# With IRBuilder interleaving: + +from tvm.script import tir as T + +def get_box_coordinates(output, batch_idx, box_idx, box_start_idx): + """a method executed by python interpreter""" + box_l = T.min( + output[batch_idx, box_idx, box_start_idx], + output[batch_idx, box_idx, box_start_idx + 2], + ) # type(box_l) is PrimExpr + ... # Repeat for other coordinates + return box_l, box_t, box_r, box_b + +@T.prim_func(capture=[get_box_coordinates]) +def nms(...): + ... + for i in range(batch_size): + ... + a_l, a_t, a_r, a_b = get_box_coordinates(output, batch_idx, box_a_idx, box_start_idx) + ... + for k in range(j): + check_iou = ... + ... + if check_iou > 0: + b_l, b_t, b_r, b_b = get_box_coordinates(output, batch_idx, box_b_idx, box_start_idx) + ... +``` + +# Reference-level explanation +[reference-level-explanation]: #reference-level-explanation + +## IRBuilder as Core + +As the foundation of IR construction, we will provide a set of APIs called +IRBuilder to let user construct IR imperatively. IRBuilder will be used by the +parser, as well as by users directly as described in the feature F4. IRBuilder +allows user to write code in a style that’s similar to TVMScript, while it’s +being executed as host program. For example, + +```python +from tvm.script.builder import Builder, def_, def_many +from tvm.script import tir as T + +with Builder() as b: + with T.prim_func(): + T.func_name("main") + buffer_a = T.Buffer((128, 128, 128), "float32") + buffer_b = T.Buffer((128, 128, 128), "float32") + arg_a = T.arg("A", buffer_a) + arg_b = T.arg("B", buffer_b) + with T.grid(128, 128, 128) as (i, j, k): + def_many(["i", "j", "k"], [i, j, k]) + with T.block(name="block"): + vi = def_("vi", T.axis.spatial(128, i)) + vj = def_("vj", T.axis.spatial(128, j)) + vk = def_("vk", T.axis.reduce(128, k)) + +f = b.get() # f is a PrimFunc +``` + +produces similar result to + +```python +@T.prim_func +def main( + A: T.Buffer(shape=(128, 128, 128), dtype="float32"), + B: T.Buffer(shape=(128, 128, 128), dtype="float32"), +) -> None: + for i, j, k in T.grid(128, 128, 128): + with T.block("block"): + vi = T.axis.S(128, i) + vj = T.axis.S(128, j) + vk = T.axis.R(128, k) +``` + +The implementation of IRBuilder will be in C++ so that it can be used in an +environment without Python. Python binding will be created to expose IRBuilder +to TVMScript parser. + +## Parse-time evaluation + +To support metaprogramming, the TVMScript parser needs to evaluate the +expression using Python interpreter. All metaprogramming +features we discussed above can be implemented through this parse-time +evaluation. Using the same `gen_matmul` example from F1, + +```python +def gen_matmul(n, m) -> None: + @T.prim_func + def f(A: T.Buffer[(n, m)]): + ... + return f +``` + +What parser does here is to: + +1. Collect the environment inside `gen_matmul`, getting a dictionary + 1. All primitive types will be captured automatically, while advanced types + (like function) needs to be explicitly declared in the decorator to be + captured (for example, `@T.prim_func(capture=[get_box_coordinates])`) +2. Call the corresponding function from IRBuilder, as the parser visits the AST + of function `f`. + 1. When visiting the function argument, call `eval` on its type annotation, + with the environment captured in the first step. + 2. `T.Buffer[(n, m)]` gets evaluated to a value with type `tir.Buffer`. + 3. Call the IRBuilder API `T.arg("A", buffer)` to add an arg to the function + that’s being constructed + +Another example, + +```python +for *i in T.grid(*A.shape): + ... +``` + +The parser will: + +1. Evaluate the expression `T.grid(*A.shape)` by the step described above. + `T.grid` returns a value that is nearly equivalent to `List[Var]`. +2. Call `exec` on a specially constructed statement `*i = __tvm_rhs_var__`, + with `locals` that maps `__tvm_rhs_var__` to the value evaluated in step 1. +3. Collect the value of `i` from the `locals` dictionary +4. Call the IRBuilder API `def_many(["i"], [i])` + +By running `eval` and `exec` provided by the Python interpreter, we can implement +language features which are difficult to implement manually, and also make sure +TVMScript has the same semantics on expression compared to regular Python code. + +## Parser Registration + +The logic of how to process a particular Python syntax node is registered +through decorator. For example, + +```python +@dispatch.register(token="tir", type_name="For") +def visit_for(self: Parser, node: doc.For) -> None: Review Comment: The parser is a thin wrapper of the IRBuilder API and doesn't directly construct IR graph. IRBuilder uses thread local store for builder state so there is no need to return things from visit methods. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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