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Comment by riledhel | Hacker News Reader

Please don't read just the selected answer, because he got the optimizations wrong. Bit shifting operations are always faster than multiplications (as the guy with the second most voted answer explains).

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In fact, bit shifting is always faster than anything. At the hardware level, it's just wiring; there is no logic involved in shifting the bits, so there is no propagation delay involved, unlike even the single logic gate operations like &, |, ~, etc.

It's so much faster, that many compilers, for integer multiplication, will optimize these multiplications by converting them to shifts and adds. Integer division, however, usually just involves a lookup table.

> In fact, bit shifting is always faster than anything.

This is overly simplistic. A fixed bitwise mapping is simple wiring and about as fast as anything can be, yes. Extending that, the "fastest" 32-bit shifter might be to put down 31 circuits, one for each possible shift amount. But that still requires a mux tree to select between the different circuits, which already goes beyond simple wiring.

While that might be the minimum-delay implementation, it's also very area inefficient. A more area-efficient design would be to cascade lg(32) = 5 muxed shifters, one for each bit in the shift amount. For example, x << 19 is the same as ((x << 16) << 2) << 1. If you read a VLSI design textbook, you'll see that there are all kinds of low-level tricks for designing barrel shifters, especially when it comes to layout.

Finally, the practical reality for programmers is that variable-amount shifts and rotates are relatively expensive on quite a few processors.

As well as the practical details of barrel shifter implementation design that psykotic points out, older CPUs couldn't afford the space for a barrel shifter and implemented shifts as a sequence of one-bit shifts. So for instance the 8086 took 8 clock cycles plus another 4 cycles per bit shift, and on that kind of CPU it was definitely not as fast as an addition.

Incidentally there is a standard trick for integer division by constant which allows you to convert it into a series of 3 to 5 shift and add/subtract operations; any decent C compiler will implement this. _Hacker's Delight_ has the explanation of the algorithm, I think.

Here's an article explaining the trick:

http://ridiculousfish.com/blog/posts/labor-of-division-episo...

And a library from the same guy for generating code at runtime to do fast division by constants:

http://libdivide.com/

You're reading it backwards.

> A good optimizing compiler will substitute shifts for multiplications when possible.

"substitute shifts for multiplications" means that multiplications will be replaced with shifts.

The wordier phrase would be "substitute with shifts for multiplications" or "substitute for multiplications with shifts".

Also, although the question is tagged for "c", the top answer has some Java specific examples.

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