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[Syntax] Start to move trivial Node class definitions to TableGen. NFC

Authored by sammccall on Oct 31 2020, 1:09 PM.

Description

[Syntax] Start to move trivial Node class definitions to TableGen. NFC

This defines two node archetypes with trivial class definitions:

  • Alternatives: the generated abstract classes are trivial as all functionality is via LLVM RTTI
  • Unconstrained: this is a placeholder, I think all of these are going to be Lists but today they have no special accessors etc, so we just say "could contain anything", and migrate them one-by-one to Sequence later.

Compared to Dmitri's prototype, Nodes.td looks more like a class hierarchy and
less like a grammar. (E.g. variants list the Alternatives parent rather than
vice versa).
The main reasons for this:

  • the hierarchy is an important part of the API we want direct control over.
    • e.g. we may introduce abstract bases like "loop" that the grammar doesn't care about in order to model is-a concepts that might make refactorings more expressive. This is less natural in a grammar-like idiom.
    • e.g. we're likely to have to model some alternatives as variants and others as class hierarchies, the choice will probably be based on natural is-a relationships.
  • it reduces the cognitive load of switching from editing *.td to working with code that uses the generated classes

Differential Revision: https://reviews.llvm.org/D90543

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