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VCLMUL.VX

RISC-V VCLMUL.VX Instruction Details

Instruction ManualV-type

Vector-scalar carry-less multiply (low): GF(2) multiply with broadcast rs1, low SEW bits

Instruction Syntax

vclmul.vx vd, vs2, rs1, vm
Operand Breakdown
vd: destination vector register group.
vs2/vs1 or scalar source: selected by suffixes such as .vv, .vx, .vi, or .vf.
vm: when present, vm=0 uses v0 as the execution mask and vm=1 is unmasked.
ZvbcVector CryptoBit Manipulation
Related Search Terms
vclmul.vx instructionRISC-V vclmul.vxvclmul.vx syntaxvclmul.vx encodingZvbcvector-scalar carry-less multiply low

Instruction Behavior

vclmul.vx zero-extends or truncates x[rs1] to SEW, carry-less multiplies it with vs2[i], and writes the low 64 bits. It is defined only for SEW=64.

VCLMUL.VX Decode & Execute Animation

This bounded model shows only lane 0 ISA-visible state; it does not model a pipeline, a cryptographic protocol, or unspecified inactive/tail values.

Bounded execution context
32-bit OP-V encoding fields
31..26
funct6
001100
25
vm
1
24..20
vs2
01000
19..15
rs1
01011
14..12
funct3
110
11..7
vd
00100
6..0
OP-V
1010111
Execution data path

funct6=001100, funct3=110, Zvbc

v4[0] <- pending

Quick Understanding & Search Notes

VCLMUL.VX is a Zvbc vector instruction for vector-scalar carry-less multiply low. This page is checked against the official vector crypto extension and V-extension execution model.

x[rs1] is zero-extended or truncated to 64 bits, then carry-less multiplied with each element in GF(2), returning the low 64 bits; encodings with SEW other than 64 are reserved.
It follows V-extension vl, vstart, vtype, and optional vm mask rules.
Zvbc vclmul/vclmulh are defined only for SEW=64.
RISC-V Unprivileged ISA Manual: Vector cryptography extensions 1.0RISC-V Unprivileged ISA Manual: V standard vector extension

Vector Execution Context

When reading VCLMUL.VX, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. .vx: one vector source and one integer scalar source participate.

Check vl first

The current vl determines the number of body elements. Typical code executes vsetvli, vsetivli, or vsetvl before this instruction.

Then check vtype

The current vtype supplies SEW, LMUL, tail policy, and mask policy; these affect element width, register-group size, and inactive/tail destination elements.

Then check vm/v0

For ordinary vector instructions with vm, vm=0 uses v0 as the execution mask and vm=1 is unmasked. A few forms such as VMERGE use v0 as data-selection input.

Official source: RISC-V V Standard Extension for Vector Operations

Common Usage Scenarios

Crypto & Security

Understand this scenario with real code like «vclmul.vx vd, vs2, a1».

Vector Acceleration

Understand this scenario with real code like «vclmul.vx vd, vs2, a1».

Pre-Use Checklist

Syntax Check
  • Confirm the current instruction format is V-type.
  • Confirm the operand order matches the example.
Semantic Check
  • Ensure the destination register usage is compatible with the calling convention.
  • Confirm this is not the lower-level form of a pseudo-instruction expansion.

Pitfalls / Common Confusions

SEW must be 64; other SEW encodings are reserved. x[rs1] is zero-extended or truncated to 64 bits for each element, and the result is the low 64 bits.

FAQ

Can VCLMUL.VX always use a v0.t mask?

It can use the vm mask operand shown in the syntax; omitting it gives the unmasked form.

What determines the element width for VCLMUL.VX?

The current vtype SEW determines it, subject to any instruction-specific SEW restrictions in the extension.