What are JKZD and JKNZD?

Question

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:

http://www.cism.ucl.ac.be/Services/Formations/ICS/ics_2013.0.028/vtune_amplifier_xe/documentation/en/instructions/327364001EN.pdf

It states (page 75) that JKZD is encoded as VEX.NDS.128.0F.W0 84 id.

What's Zydis? Those aren't x86 instructions, and you don't have any other tags that would shed any light on where they come from. — Peter Cordes, Aug 23 '20 at 08:30
Given that it's not referenced anywhere else in the source tree (just the auto-generated and the `ZYDIS_MAKE_SHORTSTRING("jkzd")` https://github.com/zyantific/zydis/search?q=JKZD&unscoped_q=JKZD), there's no way to know why it's there. Except maybe if there's a mention in a git commit message. Unless something indexes that array element with a numeric constant or reaches it in a loop? Anyway, it's not an x86 instruction. You might want to open an issue on their github to point out that these aren't instructions. Either it was a bug or they'll explain why it's there. — Peter Cordes, Aug 23 '20 at 08:35
Ah, interesting, yeah Knight's Corner has its own version of what later became AVX-512. So there you go, 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. — Peter Cordes, Aug 23 '20 at 08:41

score 7 · Accepted Answer · answered Aug 23 '20 at 08:45

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.)

What are JKZD and JKNZD?

1 Answers1