70d7c847fd
This is part of https://discourse.llvm.org/t/rfc-introduce-opasm-type-attr-interface-for-pretty-print-in-asmprinter/83792. OpAsmOpInterface controls the SSA Name/Block Name and Default Dialect Prefix. This PR adds the usage of them by existing examples in MLIR.
127 lines
4.2 KiB
Markdown
127 lines
4.2 KiB
Markdown
# Customizing Assembly Behavior
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[TOC]
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## Generating Aliases
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To reduce verbosity in the resulting assembly, `AsmPrinter` can generate aliases for frequently used types and attributes.
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For example, `!my_dialect.type<a=3,b=4,c=5,d=tuple,e=another_type>` and `#my_dialect.attr<a=3>` can be aliased to `!my_dialect_type` and `#my_dialect_attr`, simplifying further references.
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To enable this, the owning dialect of these types/attributes can define an interface to hook into the `AsmPrinter`. This is effective only when the assembly is not printed in generic form.
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```cpp
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// OpAsmDialectInterface is defined in
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// https://github.com/llvm/llvm-project/blob/91ab10e8d6c256d841da1a1a1b47c334e08d95b9/mlir/include/mlir/IR/OpImplementation.h#L1738
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struct MyDialectOpAsmDialectInterface : public OpAsmDialectInterface {
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public:
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using OpAsmDialectInterface::OpAsmDialectInterface;
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AliasResult getAlias(Type type, raw_ostream& os) const override {
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if (mlir::isa<MyType>(type)) {
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os << "my_dialect_type";
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// Could return OverridableAlias when
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// allowing other dialect to override the alias.
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//
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// Other dialects are allowed to provide alias for
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// type/attribute not owned by them
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// but the final result would depend on the registration order
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// of these dialects in the MLIRContext
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return AliasResult::FinalAlias;
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}
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return AliasResult::NoAlias;
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}
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AliasResult getAlias(Attribute attr, raw_ostream& os) const override {
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if (mlir::isa<MyAttribute>(attr)) {
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os << "my_dialect_attr";
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return AliasResult::FinalAlias;
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}
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return AliasResult::NoAlias;
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}
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};
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void MyDialect::initialize() {
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// register the interface to the dialect
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addInterface<MyDialectOpAsmDialectInterface>();
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}
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```
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* If `getAlias` provides an alias with a trailing digit, `AsmPrinter` appends an underscore to avoid conflicts with autogenerated IDs.
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* If multiple types/attributes have the same alias from `getAlias`, a number is appended to the alias to avoid conflicts.
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## Suggesting SSA/Block Names
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An `Operation` can suggest the SSA name prefix using `OpAsmOpInterface`.
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For example, `arith.constant` will suggest a name like `%c42_i32` for its result:
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```tablegen
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include "mlir/IR/OpAsmInterface.td"
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def Arith_ConstantOp : Op<Arith_Dialect, "constant",
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[ConstantLike, Pure,
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DeclareOpInterfaceMethods<OpAsmOpInterface, ["getAsmResultNames"]>]> {
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...
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}
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```
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And the corresponding method:
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```cpp
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// from https://github.com/llvm/llvm-project/blob/5ce271ef74dd3325993c827f496e460ced41af11/mlir/lib/Dialect/Arith/IR/ArithOps.cpp#L184
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void arith::ConstantOp::getAsmResultNames(
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function_ref<void(Value, StringRef)> setNameFn) {
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auto type = getType();
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if (auto intCst = llvm::dyn_cast<IntegerAttr>(getValue())) {
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auto intType = llvm::dyn_cast<IntegerType>(type);
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// Sugar i1 constants with 'true' and 'false'.
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if (intType && intType.getWidth() == 1)
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return setNameFn(getResult(), (intCst.getInt() ? "true" : "false"));
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// Otherwise, build a complex name with the value and type.
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SmallString<32> specialNameBuffer;
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llvm::raw_svector_ostream specialName(specialNameBuffer);
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specialName << 'c' << intCst.getValue();
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if (intType)
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specialName << '_' << type;
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setNameFn(getResult(), specialName.str());
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} else {
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setNameFn(getResult(), "cst");
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}
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}
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```
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Similarly, an `Operation` can suggest the name for its block arguments using `getAsmBlockArgumentNames` method in `OpAsmOpInterface`.
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For custom block names, `OpAsmOpInterface` has a method `getAsmBlockNames` so that
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the operation can suggest a custom prefix instead of a generic `^bb0`.
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## Defining Default Dialect
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An `Operation` can indicate that the nested region in it has a default dialect prefix, and the operations in the region could elide the dialect prefix.
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For example, in a `func.func` op all `func` prefix could be omitted:
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```tablegen
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include "mlir/IR/OpAsmInterface.td"
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def FuncOp : Func_Op<"func", [
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OpAsmOpInterface
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...
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]> {
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let extraClassDeclaration = [{
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/// Allow the dialect prefix to be omitted.
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static StringRef getDefaultDialect() { return "func"; }
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}];
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}
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```
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```mlir
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func.func @main() {
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// actually func.call
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call @another()
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}
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```
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