Support identity table usage

[joshop]

There's an interesting 6502 assembly optimization where you put a 256-byte page aligned table in memory with each byte having the value of its index. Then you can make some pseudo-operations:

• ADC X with ADC table,X
• TXY with LDY table,X
• etc.
  These are faster and smaller than doing it with a temp byte, so if they offset the cost of the table it can be worth it. Could such a feature (probably in the form of a flag) be added to the compiler?

[KarolS]

This sounds like something worth pursuing. For example, currently there are a lot of places that have the following code pattern: PHA ... TSX ORA $101,X INX TXS which usually could be simplified to TAX ... ORA identity,X. Variable-to-register optimization could optimize more opcodes (and since these already use cost-to-benefit calculations, they won't have speed regressions). There are also several places that could use TXY/TYX right now (even if they don't use it even on 65816)

I think it would almost always lead to larger code though, so I agree that it should be a flag.

[KarolS]

So I checked and it turned out that the identity table is actually already used in few places at the -Ob level of optimization. I'll add more uses for it, I'll add some optimizations that target it, so that things like LDA identity,X or ORA identity+n are optimized, and I'll move it to a separate flag, so that it can be used without -Ob (which is an optimization level that is a bit ridiculous, with things like loop unrolling etc., and blows up the code size too much).