[pcwalton]

Not sure if x86-64 REX prefixes count as "core" by your definition, but REX prefixes are incredibly wasteful and basically throw away all the code size gains that x86 would otherwise get over competing instruction sets. For most instructions, 4 bits are wasted to signal REX, and usually at least 1 more bit is wasted on register extensions not used by the instruction, particularly the SIB index extension. If you limit yourself to 32-bit x86, then yeah, x86 is pretty compact.

[woodruffw]

Yeah, I was talking about 32-bit x86. REX is indeed very wasteful :-)

(Even then, there's a lot of waste in the "legacy" prefix bytes, and I've always wondered who hacked those into the ISA instead of designing something more compact.)

[userbinator]

I've always thought x86-64 was a very weird "not quite 64-bit" extension of x86 that was done awkwardly, unlike the 16- to 32-bit transition. The fact that AMD designed it and not Intel may have been one of the reasons. Then again, "full 64-bit" wasn't really necessary (and even now, a lot of code is fine with 32-bit ints).