x86 doesn’t really have a dense instruction encoding. The old un-prefixed scalar instructions are dense, but they do much less work per instruction than SSE or

x86 doesn’t really have a dense instruction encoding. The old un-prefixed scalar instructions are dense, but they do much less work per instruction than SSE or AVX instructions, which have two or three byte prefixes (four with AVX-512). arm64 instructions are all four bytes, for comparison. The upshot is that simple scalar code tends to be somewhat more compact on x86, and tuned vector code (likely to be found in perf-critical routines that are CPU-bound) tends to be somewhat more compact on arm. More to the point, loop buffers and µop caches on the last couple generations of processors make encoding density mostly irrelevant to performance (though occasional pathological examples do still exist).