Optimization: Use an identity table to synthesize direct A,X/A,Y operand modes

[asiekierka]

Memblers of the NESdev forums pointed out the existence of an ASM trick called the "identity table": https://www.nesdev.org/wiki/Identity_table - this uses an identity table (where LDA table,x == x for any given x) to allow using any opcode with an abs,x or abs,y addressing mode as a direct x or y addressing mode.

This is particularly interesting as some of the NES targets already feature what is essentially a (limited) identity table, for handling bus conflicts.

The way I would see this done is:

• Implement a compiler switch for enabling use of the identity table.
  • We certainly don't want this on by default, due to its compiler runtime requirements and 256-byte fixed cost.
  • However, given some NES mappers require handling bus conflicts, this optimization may be effectively free on those platforms.
• Implement the necessary lowering, using an __identity_table table to synthesize, for example, ORA x to ORA __identity_table,x.
• Measure if this actually provides meaningful code generation gains.
• In llvm-mos-sdk, patch the rompoke logic for handling bus conflicts:
  • Use the __identity_table symbol name and set it to maximum size (256 bytes), if the identity table is used by the compiler. This may be a little tricky to model.
  • Make an effort to place __identity_table at the beginning of the fixed bank - this will ensure it's page-aligned, and thus avoid hurting performance.

This is a rough sketch - there's obviously details to be figured out.