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VFWSUB.VF

RISC-V VFWSUB.VF Instruction Details

Instruction ManualR-type

Subtract floating-point scalar f[rs1] from narrow vector vs2 and write the 2*SEW result to vd.

Instruction Syntax

vfwsub.vf vd, vs2, rs1, 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
Related Search Terms
VFWSUB.VF instructionRISC-V VFWSUB.VFvfwsub.vf syntaxvfwsub.vf RVVwidening FP subtractRVV floating point

Instruction Behavior

VFWSUB.VF is a RISC-V V widening floating-point subtract instruction. Both inputs are widened to 2*SEW before subtraction. The result is written to 2*SEW vd elements, with floating-point rounding, exception flags, and NaN handling following RVV FP rules.

Quick Understanding & Search Notes

VFWSUB.VF subtracts f[rs1] from vs2; it is not a reversed subtract.

For .VV/.VF forms, narrow inputs are widened first, and vd elements are 2*SEW wide.
Floating-point operations use frm or instruction-defined FP rounding rules; vxrm is only for fixed-point rounding instructions.
Vector FP32/FP64 operations require the corresponding scalar F/D support; half precision is controlled by the relevant vector half-precision extension.
vm=0 uses v0 as the execution mask and vm=1 is unmasked; inactive and tail elements follow the current vma/vta policy.
RISC-V Unprivileged ISA Manual: V Standard Extension for Vector Operations

Common Usage Scenarios

High-Precision Difference

e.g., sub t0, a0, a1 — compute the difference a0 - a1.

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

Dest EMUL=2*LMUL needs double registers. Masking via v0. Mind operand order vs2-(scalar/vector).
Vector FP32/FP64 operations require matching scalar F/D support; FP16 operation is not implied by V alone.

FAQ

What is the destination width of VFWSUB.VF?

It writes 2*SEW floating-point results, so destination register-group use is based on the widened result.

How does VFWSUB.VF handle masking?

With vm=0, v0 selects active elements; with vm=1, all body elements participate. Inactive and tail elements follow the current policies.