That __restrict isn't useful to the compiler in this function, since it can already infer that there is no mutation from the access patterns: it's a leaf function with only non-atomic reads and no writes. (It's mostly only useful when you interleave reading from one pointer with writing to another with the same type, or when you call an opaque function before reading from a pointer.) Indeed, if you look at the generated assembly, the two are identical. The timing differences are likely random variation.