Question

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performance of unsigned vs signed integers

I have read somewhere that it’s a tiny bit faster on x86_64 to compare signed ints in C/C++ compared to unsigned ints, e.g. for (int i...) is “faster” than for (uint i...).

Is that true? Why is that true? I know the difference is super small, but anyway.

Answer 1

You’d better cite a source for such a claim, which is ridiculous on the face of it.

We’re talking about x86_64, which means modern processors. These ALUs will complete integer addition, subtraction, and/or comparison in a single clock cycle (cache misses will of course take longer, but depend only on the size and memory layout of the data, not signed-ness). Or even less, with the SIMD coprocessor.

I’d more likely to believe that there’s a slight power difference between the two types of comparison, but not a speed difference.

Now, it is possible that for a particular compiler, code generation is worse for one data type vs the other when targeting the x86_64 platform. But that would be a very specialized case, and not likely to apply to all x86_64 compilers. And still, I’d suspect cache effects or background processes were affecting the performance measurement (even performance counters that measure time spent per-process will be affected by a context switch invalidating caches).