Quick Java Optimization Question

Question

Will the Eclipse compiler automatically convert multiplication by a power of two into a bit shift, or should I do that manually? Thanks for the help.

Answer 1

Short answer: No. The source code compiler won't replace a multiplication by two with a bit shift.

Long answer: It won't, because it can't know if a bit shift is faster than a multiplication on the platform the code eventually will be running. So, the question should rather be if a specific VM will replace the multiplication with a bit shift, and it probably will. I experimented a little bit with this to optimize a code block and it's interesting that Sun's Hotspot shows different behaviour here, depending on if the program runs on an AMD or on an Intel CPU (at least with the CPUs I tested). In either case, a multiplication with a power of two is replaced with a bit shift, but for multiplications with a power of two +/- 1 (3, 5, 7, 9, 15, 17, ...), Hotspot will generate a bit shift and an addition or a subtraction for Intel CPUs, while generating a multiplication for AMD CPUs, since the AMD CPU executes a multiplication much faster than the Intel CPU. It's of course possible, that this behaviour differs between different CPU models from each vendor.

If you are interested in knowing what the VM actually is doing, it is quite benefical to get the debug build of jdk7 and enable dumps of the assembler code generated by the Hotspot compiler.