C function: multiply 2 numbers using bit twiddling?

Question

I need some help answering the above question Any twiddling idea?

Are both numbers just any numbers unknown in advance? The question *could* make sense for the opposite case, e.g. if one of the numbers is constant. — Anton Kovalenko, Feb 12 '13 at 12:33
for example multiply 2 unsiged integers using only shift right and shift left. — KBE, Feb 12 '13 at 13:52

score 4 · Answer 1 · answered Feb 12 '13 at 12:32

4

You'll get a lot more here: Bit Twiddling Hacks

answered Feb 12 '13 at 12:32

Azodious

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score 3 · Answer 2 · answered Feb 12 '13 at 12:33

Treat one of the inputs as a bitmask. For each bit in it that is set, you want to add the other input, shifted that many spaces left, to your result. This assumes unsigned inputs: non-2's-complement signed inputs require special treatment of the sign bit.

I think I can safely predict that this will be less efficient than the CPU's built-in multiply operation.

C function: multiply 2 numbers using bit twiddling?

2 Answers2