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VFREC7.V

RISC-V VFREC7.V Instruction Details

Instruction ManualR-type

Compute approximate reciprocal (7-bit accuracy) of each float element in vs2, writing to vd.

Instruction Syntax

vfrec7.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.
VZvfhVector OperationsFloating-Point
Related Search Terms
VFREC7.V instructionRISC-V VFREC7.Vvfrec7.v syntaxvfrec7.v RVVreciprocal estimate7-bit estimate

Instruction Behavior

VFREC7.V produces the 7-bit floating-point estimate defined by the V extension. Special inputs such as zeros, infinities, NaNs, and negatives follow the spec tables; do not describe it as ordinary division/square-root rounded by frm. Vector FP32/FP64 operation requires the corresponding scalar F/D support; FP16 is controlled by the relevant vector half-precision extensions, and the base V extension does not automatically include half-precision arithmetic.

Quick Understanding & Search Notes

VFREC7.V is a table-defined 7-bit estimate instruction, not ordinary FP divide or square-root.

The operation applies only to active elements within vl; inactive and tail elements follow the current vma/vta policy.
vm=0 uses v0 as the execution mask and vm=1 is unmasked.
Special-case results and exception flags should be handled using the official estimate-instruction tables.
Vector FP32/FP64 operation requires the corresponding scalar F/D support; FP16 is controlled by the relevant vector half-precision extensions, and the base V extension does not automatically include half-precision arithmetic.
RISC-V Unprivileged ISA Manual: V Standard Extension for Vector Operations

Vector Execution Context

When reading VFREC7.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

Division Optimization

Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vfrec7.v v1, v2 # v1[i] ≈ 1.0/v2[i] (7-bit)».

Newton-Raphson

Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vfrec7.v v1, v2 # v1[i] ≈ 1.0/v2[i] (7-bit)».

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

Check the official special-case tables for zeros, infinities, NaNs, and negative inputs; do not label the zero-input result implementation-defined.
The estimate has limited precision and usually needs later refinement for high precision.
Vector FP32/FP64 operation requires the corresponding scalar F/D support; FP16 is controlled by the relevant vector half-precision extensions, and the base V extension does not automatically include half-precision arithmetic.

FAQ

Is the estimate result implementation-defined?

No. The V extension defines the 7-bit estimate and special-input handling rules.