Is vd only a write destination for VNMSUB.VX?
No. This class is destructive: the old vd value participates as an accumulator or multiplicand.
VNMSUB.VX
RISC-V VNMSUB.VX Instruction Details
Instruction ManualR-typeVector integer negative multiply-subtract using vd as a multiplicand.
Instruction Syntax
vnmsub.vx vd, rs1, 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 IntegerMultiply-Subtract
Instruction Behavior
VNMSUB.VX computes vs2[i] - (x[rs1] * vd[i]) for active elements and writes the low SEW bits back to vd.
Quick Understanding & Search Notes
VNMSUB.VX is a destructive integer multiply-add class instruction; vd participates in the operation and receives the result.
VNMSUB uses old vd as one multiplicand and vs2 as the minuend.
The result is truncated to SEW width and does not raise integer overflow exceptions.
vm=0 uses v0 as the execution mask; vm=1 is unmasked.
Common Usage Scenarios
Polynomial Evaluation
Understand this scenario with real code like «vnmsub.vx v8, x5, v12, v0.t».
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
vnmsub uses old vd as one multiplicand.
vs2 is the third operand added to the negated product.
The low SEW bits are written and no integer overflow exception is raised.