Bit Reversal for FFT Addressing

Reverse the low N bits of an index in assembly, which is the address permutation a radix-2 FFT needs before its butterflies start.

Constraints

• Index in r0, bit width in r1, result in r0.
• Only the low r1 bits participate.
• Must handle widths from 1 to 16.
• Use the RBIT instruction plus a shift, not a loop.
• Preserve r2 and above.
• Result must have zeros above bit r1-1.