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VCLZ.V

RISC-V VCLZ.V Instruction Details

Instruction ManualR-type

Vector count leading zeros: count leading zero bits in each element

Instruction Syntax

vclz.v vd, vs2, 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.

ZvbbVector Bit Manipulation

Related Search Terms

vclz.v instruction RISC-V vclz.v vclz.v syntax vclz.v encoding Zvbb vector count leading zeros

Instruction Behavior

vclz.v is the Zvbb vector count-leading-zeros instruction. Counts leading zero bits in each SEW element, writing the count to the corresponding destination element. Supports all SEW.

Quick Understanding & Search Notes

VCLZ.V is a Zvbb vector instruction for vector count leading zeros. This page is checked against the official vector crypto extension and V-extension execution model.

Each element returns the number of leading zero bits from the most-significant bit; an all-zero element returns SEW.

It follows V-extension vl, vstart, vtype, and optional vm mask rules.

RISC-V Unprivileged ISA Manual: Vector cryptography extensions 1.0 RISC-V Unprivileged ISA Manual: V standard vector extension

Vector Execution Context

When reading VCLZ.V, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. The suffix and operand form determine whether sources are vector, scalar, or immediate values.

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 «vclz.v vd, vs2».

Vector Acceleration

Understand this scenario with real code like «vclz.v vd, vs2».

Pre-Use Checklist

Syntax Check

Confirm the current instruction format is R-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

Result is count (0..SEW), not a bit position index. All-zero element returns SEW.

FAQ

Can VCLZ.V 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 VCLZ.V?

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