Target
registered compiler flow customisationIn order to enable flexibility in how individual targets are lowered and built within TVM, this RFC proposes additional hooks on the Target
and that the target becomes the central place for such hooks, for example:
using FTVMRelayToTIR = Pass; using FTVMTIRToRuntime = runtime::TypedPackedFunc<runtime::Module(IRModule, Target)>; TVM_REGISTER_TARGET_KIND("cmsisnn", kDLCPU) .set_attr<FTVMRelayToTIR>("RelayToTIR", CMSISNNLowering) .set_attr<FTVMTIRToRuntime>("TIRToRuntime", CMSISNNCodeGen);
This defines two new hooks as attributes on the target, referencing functions registered into the central TVM registry. In similar fashion, external code generators (registered under the relay.ext.
namespace currently) would be grouped with an appropriate Target
as well:
using FTVMRelayToRuntime = runtime::TypedPackedFunc<runtime::Module(const Function&)>; using FTVMConstantUpdater = runtime::TypedPackedFunc<Map<String, runtime::NDArray>(Expr, std::string)>; TVM_REGISTER_TARGET_KIND("ethos-n", kDLCPU) .set_attr<FTVMRelayToRuntime>("RelayToRuntime", EthosNCodeGen) .set_attr<FTVMConstantUpdater>("UpdateConstants", EthosNConstantUpdater);
Collecting all targets under the Target
functionality (as opposed to registering additional Target
s through the function registry using the namespace relay.ext.
) and makes it clearer which hooks apply to each target.
We want to make external code generation (otherwise known as BYOC) more modular; instead of going from a Relay IRModule
to runtime::Module
in one big step, you can break it into phases and make use of existing transformations between phases.
Currently to introduce an external code generator, the entire compilation pipeline must be recreated; this is necessary for some targets but in the case of simply re-using existing libraries or introducing a function call to use for an operator it can become more than is necessary; to implement an external code generator requires going directly from Relay to a runtime::Module
and re-implementing any compiler passes and code generation functionality rather than being able to extend upon the existing compiler infrastructure.
The generated runtime::Module
also exists outside of the main graph, meaning it can't be inspected in combination with other operators; this limits the effectiveness of techniques such as memory planning. By introducing the hook RelayToTIR
, which is similar to the default LowerTEPass
in that it returns TIR, it can be inspected by the memory planner and other analysis passes that only work at the TIR level. If all that is necessary is transforming into a flat call_extern
(such is the case for the CMSIS NN Softmax function) then the hook may simply return that TIR to be collected by the host code generation.
In the more complex case, we still want to take advantage of memory planning by using RelayToTIR
and inspecting the liveness within the TIR graph, but instead want to generate out more complex calls (such as using the CMSIS NN Structures); the TIRToRuntime
hook can be used to build our intermediary TIR into a Runtime module similarly to how the existing external code generation works. This allows writing of external code generators that also get the benefits of any intermediary analysis or transformation that TVM offers. Alongside being able to use the analysis passes, code generators can extend from existing host code generators, customising only the generation which is relevant to them and gaining maximum benefit from the existing optimisations made in TVM.
As a user, you can pick from additional hooks to bypass certain behaviours of the Target
:
RelayToTIR
- Customize the lowering flow to TIRTIRToRuntime
- Customize code generation into a runtime module from TIRRelayToRuntime
- Full compilation flow from Relay to a runtime moduleTo illustrate where the hooks are placed, please refer to the following diagram:
These can be registered on targets using set_attr
:
TVM_REGISTER_TARGET_KIND("cmsisnn", kDLCPU) .set_attr<FTVMRelayToTIR>("RelayToTIR", CMSISNNLowering) .set_attr<FTVMTIRToRuntime>("TIRToRuntime", CMSISNNCodeGen); TVM_REGISTER_TARGET_KIND("ethos-n", kDLCPU) .set_attr<FTVMRelayToRuntime>("RelayToRuntime", EthosNCodeGen) .set_attr<FTVMConstantUpdater>("UpdateConstants", EthosNConstantUpdater);
With this change, this path splits, depending on whether you wanted to generate a full Module
or introduce some specific TIR nodes into the code generation flow. The RelayToTIR
hook is a full IRModule
Pass
which expects that Function
s will either be annotated with kTarget
or kCompiler
as part of a previous Pass
, and the resultant IRModule
is also expected to have any created PrimFunc
s annotated. The addition of the RelayToTIR
hook allows you to write trivial external TIR generators such as calling out to a third party library:
void CallExternalLibraryInTIR(const GlobalVar& new_global_var, const Function& func) { tir::Buffer x_buffer = tir::decl_buffer({8}, DataType::Float(32), "x"); tir::Var x_var("x", DataType::Handle()); Map<String, ObjectRef> dict_attrs; dict_attrs.Set("global_symbol", new_global_var->name_hint); dict_attrs.Set("tir.noalias", Bool(true)); Map<tir::Var, tir::Buffer> buffer_map = {{x_var, x_buffer}}; tir::Stmt body = tir::Evaluate(tvm::tir::Call(DataType::Int(8), tir::builtin::call_extern(), {x->data})); tir::PrimFunc replacement_func = tir::PrimFunc({x_var}, body, VoidType(), buffer_map, DictAttrs(dict_attrs)); replacement_func = WithAttr(replacement_func, ::tvm::attr::kTarget, host_target_); ir_module_->Add(new_global_var, replacement_func); }
This is then registered on a target:
TVM_REGISTER_TARGET_KIND("woofles", kDLCPU) .set_attr<FTVMRelayToTIR>("RelayToTIR", relay::contrib::woofles::RelayToTIR());
The signature for this hook is as the same as any other Pass
, which takes an IRModule
with Function
s and returns an IRModule
with transformed PrimFunc
s. The registered RelayToTIR
Pass
is responsible for both establishing the PrimFunc
definitions (with any caching) and rewriting Relay calls to those functions. At this time we feel it's not worth worrying about code sharing between different custom passes.
Extending from the above, a second hook is introduced to do further transformations from TIR -> Runtime named TIRToRuntime
, this bypasses the default target.build.X
and instead uses the registered TIRToRuntime
build:
runtime::Module BuildWooflesHost(IRModule mod, Target target) { // ... Custom Code generation here } TVM_REGISTER_TARGET_KIND("woofles", kDLCPU) .set_attr<FTVMTIRToRuntime>("TIRToRuntime", BuildWooflesHost);
Notably the generation hook is passed the unified IRModule
and is responsible for plucking the Target
relevant functions into the eventual runtime::Module
.
This functionality is an extension of the existing use of attr::kCompiler
to provide a hint that we can use to lookup attached target attribute, the compiler and code generation flows can choose to store TIR and/or generate runtime modules based on the registered hooks.
This can be added before the LowerTEPass
, as a Pass
which iterates over Target
s and transforming the relevant functions which will then be skipped by the Function
-level passes until the PrimFunc
passes begin:
for (Target target : targets_) { auto target_kind = target->kind; auto map = tvm::TargetKind::GetAttrMap<FTVMRelayToTIR>("RelayToTIR"); if (map.count(target_kind)) { ir_mod = map[target_kind](ir_mod, pass_context); } }
By placing this above the LowerTEPass
, this means any functions which are not processed in this way can be processed by the default lowering without interfering with LowerTEPass
. To achieve this initially kCompiler
would be used to carry the relevant target information, but the goal is to ensure all Target
s are visible as kTarget
.
return tvm::transform::Sequential({tvm::relay::transform::RelayToTIRTargetHook(), // Additional Pass to call RelayToTIR tvm::transform::CreateModulePass(pass_func, 0, "LowerTE", {}), InferType()});
It is proposed that this hook is implemented as part of codegen.cc
as a direct override of the code generation:
runtime::Module Build(IRModule mod, Target target) { if (transform::PassContext::Current() ->GetConfig<Bool>("tir.disable_assert", Bool(false)) .value()) { mod = tir::transform::SkipAssert()(mod); } if (target->kind->HasHook("TIRToRuntime")) { // Hooked here for Codegen return target->kind->GetAttr<FTVMTIRToRuntime>("TIRToRuntime")(mod, target); } // the build function. std::string build_f_name = "target.build." + target->kind->name; const PackedFunc* bf = runtime::Registry::Get(build_f_name); ICHECK(bf != nullptr) << build_f_name << " is not enabled"; return (*bf)(mod, target); }
See Relay to TIR Hook for how the TargetKind
registry would be used.
This would replace the existing relay.ext.<target>
lookup in te_compiler.cc
with a Pass
which runs beforehand, essentially using the same logic as Relay to TIR Hook to cross reference with kCompiler
.
return tvm::transform::Sequential({tvm::relay::transform::RelayToTIRTargetHook(), tvm::relay::transform::RelayToRuntimeTargetHook(), // Additional Pass to call RelayToRuntime tvm::transform::CreateModulePass(pass_func, 0, "LowerTE", {}), InferType()});
This is all based upon the existing external code generation infrastructure within TVM by placing additional hooks in the same areas as existing external generation. Instead of replicating this with named functions in the relay.ext.
namespace of the function registry it instead begins to follow the pattern outlined as B1 in https://discuss.tvm.apache.org/t/target-and-attributes/6013/6 by @tqchen.
In future, this approach enables rapid integration of anything that can be represented in TIR into the main compilation graph; this simplifies the transformation process for a multitude of external libraries.
Alongside this, adding further hooks means external code generation can gain benefits from the normal lower
and build
flow in TVM. This then expands to exposing more granular methods in the driver api to leverage the compiler passes in TVM, similar to how they've been exposed in https://github.com/apache/tvm/pull/8110 with lower_primfunc
and lower_schedule
. This can is then regulated by the normal Target mechanism to route as appropriate.
Refactoring the target splitting logic into build_module.cc
alongside any external module generation makes this a first class series of hooks into a simplified compilation flow; this would enable the removal of external generators from executor code generators which currently proxy to te_compiler.cc
. Eventually this could also be used for CPU/GPU split as a specialisation of a Target
/Target
s split.