Looking through Zydis ( https://github.com/zyantific/zydis/blob/57be5b1d1b9dd99830b89caac928add64ad5d072/include/Zydis/Generated/EnumMnemonic.h ) mnemonics I found these:
ZYDIS_MNEMONIC_JKNZD,
ZYDIS_MNEMONIC_JKZD,
I couldn't find these mnemonics anywhere else; what instructions do they represent?
What operations do these instructions perform?
Z and NZ would likely indicate zero and not zero respectively, and J likely stands for jump, but K and D?
Edit: I found this old intel document, but it makes no sense:
It states (page 75) that JKZD is encoded as VEX.NDS.128.0F.W0 84 id.
You already answered your own question with that Intel Knight's Corner link. KNC had its own version of what later became AVX-512, with incompatible machine-code encodings.
As documented in that Intel manual, KNC had instructions like jrcxz but for mask registers (which KNC and AVX-512 call k0..k7) to let you compare-and-branch in one instruction on a vector compare-into-mask result. JKZD - Jump near if mask is zero. It has 2 forms, jkzd k1, rel32 and jkzd k1, rel8
These are not standard instructions and don't appear in Intel's mainstream manuals (which include AVX-512). In AVX-512, you branch on a mask register value with ktest k1,k1 or kortest k1,k1 to set regular FLAGS, then use a regular branch condition. (With unrolling to get 2 vector compare results, you can branch on either of them being non-zero with kortest k1,k2 with 2 separate registers.)