Bithacks

Bithacks are optimization tricks that utilize information in bits and bit manipulation to accomplish their tasks. Usually they work in a slightly non-obvious way, (the most famous being the fast inverse sqrt), and bit manipulation in general is harder on the 65c816. To that end here is a collection of some useful tricks.
Note: cycle counts are intended to be a worst case measure.

Math Bithacks

Signed Division By 2 (5 bytes / 10 cycles)
inputs: A
outputs: A

STA $00
ASL $00
ROR

Signed Division By 2ⁿ (6+2n bytes / 5+2n cycles)
inputs: A, n
outputs: A

; signed division by two, n times
macro SignedDiv_2N(n)
	BMI ?negative
	LSR #<n>
	BRA ?end
?negative:
	LSR #<n>
	ORA #($FF<<(8-<n>))	; sign extension
?end:
endmacro

Misc. Tricks

As this list grows tricks here will be consolidated into their own sections. Clever optimization tricks that aren't necessarily what someone might personally call a "bithack" are okay here as well!

Direction As Index (4 bytes / 6 cycles)
inputs: A
outputs: A

; Ever wonder why facing flags are 0=right and 1=left? This is why. It's incredibly cheap.
ASL : ROL : AND #$01	; get facing as index