[bonzini]

Yes, two in fact. One is the same prefix (with subtle differences of course, it's x86) that was introduced for AVX512's 32 registers. The other is new and it's a two byte extension (0xd6 0x??) of the REX prefix (0x40-0x4f).

The longer prefix has extra functionality such as adding a third operand (i.e. add r8, r15, r16), blocking flags update, and accessing a few new instructions (push2, pop2, ccmp, ctest, cfcmov).