Do these floating-point instructions use vxrm?
No. Ordinary RVV floating-point operations and FP conversions use floating-point frm or an instruction-specified fixed rounding mode; vxrm is for fixed-point rounding instructions.
VFNCVT.X.F.W
RISC-V VFNCVT.X.F.W Instruction Details
Instruction ManualR-typeNarrow wide float to signed int, rounding from frm.
Instruction Syntax
vfncvt.x.f.w 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.
VVector Type ConvertFloating-Point
Instruction Behavior
VFNCVT.X.F.W performs RVV narrowing floating-point/integer conversion. Conversions that require rounding use frm. The source is 2*SEW wide and the result is SEW wide. Vector FP32/FP64 operation requires the corresponding scalar F/D support; FP16 is controlled by the relevant vector half-precision extensions; do not assume the base V extension includes half-precision arithmetic.
Quick Understanding & Search Notes
VFNCVT.X.F.W converts element types; first check whether the form is single-width, widening, or narrowing, then check where rounding comes from.
FP conversions without a fixed-rounding suffix round according to frm.
Narrowing conversion reduces element width, often after wider computation.
The operation applies only to active elements within vl; inactive and tail elements follow the current vma/vta policy.
Except for dedicated mask forms, vm=0 uses v0 as the execution mask and vm=1 is unmasked.
Floating-point operations follow the vector FP rules: normal FP operations use frm rounding and set FP exception flags; fixed-point vxrm does not control them.
Common Usage Scenarios
Precision Reduction
Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vfncvt.x.f.w v1, v2».
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
Dynamic frm rounding vs rtz fixed truncation — key difference.
Src EEW=2×SEW; mind register group sizes.