TableGen: more powerful generic tables as successor of SearchableTable

Hello all,

I'd like to briefly let you know that I want to extend what the "searchable tables" backend of TableGen can do in terms of emitting generic enums and tables.

The patch is at

This is motivated by the need to express the structure of image load/store/sample instructions and their corresponding intrinsics in the AMDGPU backend, but I'm writing this mail to raise awareness in case others find this functionality useful.

For an example usage, there are machine instructions for loading data from images that are equivalent except for the size of the vectors they return. At certain points, we need to map from an instruction with vector size N to an instruction with vector size M.

The old approach used InstrMapping, which has the disadvantage of only allowing 1:M mappings -- so in fact we had multiple InstrMappings to support the different values of N.

The extended generic tables allow us to instead have a generic table with the fields [Opcode, BaseOpcode, NumDwords] and two lookup functions that allow:

- looking up a row of the table by Opcode
- looking up a row of the table by (BaseOpcode, NumDwords)

This then allows mapping the old Opcode to the corresponding BaseOpcode, and from there to the new Opcode (matched with the required vector size).

[The actual implementation of this is more involved, because in reality the matrix of Opcodes derived from a single BaseOpcode depends on more factors than just a single vector size. The gory details are at ⚙ D48016 AMDGPU: Refactor MIMG instruction TableGen using generic tables and the stack of patches it belongs to.]

I also have patches that transition the existing uses of SearchableTable to the new primitives. I believe it makes sense to remove the old SearchableTable once the dust has settled.

Feedback is welcome!