Is the estimate result implementation-defined?
No. The V extension defines the 7-bit estimate and special-input handling rules.
VFREC7.V
RISC-V VFREC7.V Instruction Details
Instruction ManualR-typeCompute approximate reciprocal (7-bit accuracy) of each float element in vs2, writing to vd.
Instruction Syntax
vfrec7.v vd, vs2, vm
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
VZvfhVector OperationsFloating-Point
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.
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.