[LLVMdev] XOR Optimization

[Alistair Lynn]

Hi-

> I haven't seen a machine in which OR is faster than ADD nor more energy-efficient. They're all done by the same ALU circuitry which delays the pipeline by its worstcase path timing. So, for modern processor hardware purposes, OR is exactly equal ADD. Transforming ADD to OR isn't strenght reduction at all. Maybe this is benefical only if you have a backend generating circuitry (programming FPGAs).

I believe that in cases where ADD and OR are equivalent, LLVM prefers the latter because it's easier to reason about the bits in the result of an OR in complex cases. The x86 backend, for instance, transforms ORs in such cases back into adds, presumably in case it may be matched to an lea where that's beneficial.

Alistair