Add Target Pre-processing RFC

rfcs/0071-target-json-parser.md

@@ -0,0 +1,172 @@
+- Feature Name: target-json-preprocessor
+- Start Date: 2022-04-04
+- RFC PR: [apache/tvm-rfcs#0071](https://github.com/apache/tvm-rfcs/pull/71)
+- GitHub Issue: [apache/tvm#0000](https://github.com/apache/tvm/issues/0000)
+
+# Summary
+[summary]: #summary
+Extend the existing `TargetKind` `preprocessor` to allow preprocessing of the entire `Target` JSON representation rather than just `attrs`.
+
+# Motivation
+[motivation]: #motivation
+
+Taking an example `Target` in JSON form:
+
+```js
+{
+    "id": "cuda",
+    "tag": "nvidia/tx2-cudnn",
+    "keys": ["cuda", "gpu"],
+    "libs": ["cudnn"],
+    "target_host": {
+        "id": "llvm",
+        "system_lib": True,
+        "mtriple": "aarch64-linux-gnu",
+        "mattr": "+neon"
+    }
+}
+```
+
+We can see that there are additional fields which are of interest to TVM, note-ably `keys` and `libs` which we currently do not apply parsing to on `Target` instantiation. Extending the `TargetKind` `preprocessor` beyond `attrs` enables to customise parsing of the entire `Target`, enabling the values passed by the user to be used to infer other properties used during compilation.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Alongside the existing `set_attrs_preprocessor` method on `TargetKind`, there will be an alternative `set_target_parser` method, this new parser will take precedence over the `attrs` preprocessor if present:
+
+```c++
+TVM_REGISTER_TARGET_KIND("target", kDLCPU)
+    .set_target_parser(TargetParser);
+```
+
+The canonical JSON form of `Target` will be passed to the new `Target` parser and the parser will return the transformed variant in JSON form for further steps:
+
+```c++
+using TargetJSON = Map<String, ObjectRef>;
+TargetJSON TargetParser(TargetJSON target) {
+    // ... transforms ...
+    return target;
+}
+```
+
+The parser will have to be capable of handling the diversity of types of `Target` in TVM, therefore the underlying mechanism of the parser is left as an implementation detail. Using the example of pre-processing the `keys` attribute (used for detecting appropriate schedules), it can be seen how this can apply to various `Target`s. 
+
+## TVM Target's Directly Mapping to a Backend's Target
+Take the example of pre-processing `keys` (in this case using the `cuda` `Target`):
+```c++
+using TargetJSON = Map<String, ObjectRef>;
+
+TargetJSON CUDAParser(TargetJSON target) {
+    if (IsSuper(target)) {
+        target["keys"].push_back("super_cuda");
+    }
+}
+
+TVM_REGISTER_TARGET_KIND("cuda", kDLGPU)
+    .set_target_parser(CUDAParser);
+```
+
+This takes the `attrs` from `Target` and maps them to relevant `keys` for use when selecting schedules:
+
+```c++
+Target my_target("cuda -msuper");
+my_target->keys; // ["cuda", "gpu", "super_cuda"] <-- "cpu" and "cuda" are taken from default keys - "super_cuda" is added
+```
+
+## TVM Target's Mapping to a Backend with Multiple Target's
+The previous example would work for `Target`s which map to a specific architecture, such as `cuda`. To parse a `Target` which has a number of its own targets, such as `llvm`, the parser can be broken down within the parent parser:
+
+```c++
+using TargetJSON = Map<String, ObjectRef>;
+
+TargetJSON AArch64TargetParser(TargetJSON target) {
+    target["keys"].push_back("arm_cpu");
+    return target;
+}
+
+TargetJSON x86TargetParser(TargetJSON target) {
+    target["keys"].push_back("x86_64");
+    return target;
+}
+
+TargetJSON CPUTargetParser(TargetJSON target) {
+    if (IsAArch64Target(target)) {
+        return AArch64TargetParser(target);
+    }
+    if (IsX86Target(target)) {
+        return x86TargetParser(target);
+    }
+    return target;
+}
+
+TVM_REGISTER_TARGET_KIND("llvm", kDLCPU)
+    .set_target_parser(CPUTargetParser);
+```
+
+This has the additional advantage that if there are standard arguments, such as `mcpu`, `mattr` and `march`, the parser can be re-used in both `Target`s - for example the `c` `Target` can re-use the above `llvm` `Target` parser:
+
+```c++
+TVM_REGISTER_TARGET_KIND("c", kDLCPU)
+    .set_target_parser(CPUTargetParser);
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Currently, there is a single `preprocessor` which takes an input of `attrs` and expects the same `attrs` returned with pre-processing applied:
+
+https://github.com/apache/tvm/blob/d2db9cb0d839e32778f461b77e59f6418282a511/src/target/target.cc#L810-L814
+
+The new `Target` parser will live in addition to the `preprocessor` until such a time as the `preprocessor` can be fully removed. This extends `TargetKind` to support both `preprocessor` and `target_parser`:
+
+```c++
+using TargetJSON = Map<String, ObjectRef>;
+using FTVMTargetParser = TypedPackedFunc<TargetJSON(TargetJSON)>;
+
+class TargetKind {
+    ...
+    PackedFunc preprocessor;
+    FTVMTargetParser target_parser;
+
+    ...
+}
+```
+
+Implementations for `Target` parsers will be stored under `src/target/parsers/<parser_identifier>.{cc.h}`, allowing them to be composed together (as shown above), such as:
+
+* src/target/parsers/cuda.cc
+* src/target/parsers/aarch64.cc
+* src/target/parsers/cpu.cc
+
+Where the `cpu` pre-processor can utilise the `aarch64` pre-processor if detected and `cuda` is an independent parser specific to that `Target`.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+By adding these new pre-processing options to `Target` we increase the amount of work incurred when instantiating a `Target`, it was ultimately considered that this one-time cost would be similar or less than repeatedly querying the `Target` attributes. 
+
+Providing the ability to completely change a `Target` on parsing could allow an extensive mutation of the input `Target`. 
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Instead of providing a single parser entrypoint, we can instead use several parsers for each attribute - this clearly separates the responsibility of each parser but also means maintaining many entrypoints to `Target` parsing.
+
+# Prior art
+[prior-art]: #prior-art
+
+## Other Compilers
+Taking the example of LLVM, it follows a similar methodology, resulting in a `Features` vector:
+* `clang` uses the LLVM parsers to determine available features for a given set of `Target` parameters such as `mcpu` and `mtune`: https://github.com/llvm/llvm-project/blob/43d758b142bbdf94a1c55dc0950637ae74f825b9/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
+* LLVM implements the `Features` parsers: https://github.com/llvm/llvm-project/blob/09c2b7c35af8c4bad39f03e9f60df8bd07323028/llvm/lib/Support/AArch64TargetParser.cpp
+* The parser is tested in insolation: https://github.com/llvm/llvm-project/blob/09c2b7c35af8c4bad39f03e9f60df8bd07323028/llvm/unittests/Support/TargetParserTest.cpp
+
+## Existing TVM RFCs
+This RFC builds upon the following existing TVM RFCs:
+* This follows the original Target Specification RFC: https://discuss.tvm.apache.org/t/rfc-tvm-target-specification/6844
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+# Future possibilities
+[future-possibilities]: #future-possibilities