Count Leading Zeros

Count the leading zeros of a 32-bit register without using the CLZ instruction, using a binary search so the cost is fixed at five tests.

Constraints

• Input in r0, result in r0.
• Must return 32 for an input of zero.
• Do not use the CLZ instruction.
• Must be branch-light: at most 5 conditional steps.
• Preserve all registers except r0 to r3.
• Thumb-2 or ARM syntax both acceptable.