What determines the element count for vwmulu.vx?
The current vl and vtype determine it, with vstart, LMUL, SEW, mask state, and tail policy also affecting execution.
VWMULU.VX
RISC-V VWMULU.VX Instruction Details
Instruction ManualR-typeVWMULU.VX produces a 2*SEW widening product without accumulation.
Instruction Syntax
vwmulu.vx 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.
VVector Operations
Instruction Behavior
vwmulu.vx is a V extension widening multiply instruction. It interprets operands as unsigned-by-unsigned and writes the 2*SEW-wide product to vd.
Quick Understanding & Search Notes
vwmulu.vx is a V extension vector instruction. Active elements are controlled by vl, vtype, vstart, and the optional v0.t mask; it produces a 2*SEW product using unsigned-by-unsigned interpretation.
Official syntax is `vwmulu.vx vd, vs2, rs1, vm`; without a mask operand it is unmasked, while `, v0.t` updates only selected active elements.
Tail elements and masked-off elements follow the current vtype tail/mask policy; the mnemonic alone does not imply zeroing.
The destination group is wider than the SEW source group, so LMUL/EMUL legality matters.
Common Usage Scenarios
Bit Operations & Masks
Understand this scenario with real code like «vwmulu.vx v8, v12, x5, v0.t».
Data Storing
Understand this scenario with real code like «vwmulu.vx v8, v12, x5, v0.t».
Vector Operations
Understand this scenario with real code like «vwmulu.vx v8, v12, x5, 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
The result width is 2*SEW, so the destination group EMUL is twice the source group EMUL.
This is multiply, not multiply-accumulate; old vd is not used as an accumulator.
Masking, vl, vtype, and tail policy control active and inactive elements.
FAQ
Does vwmulu.vx always process the whole vector register?
No. V instructions operate on active elements; register grouping and inactive-element behavior are controlled by vtype and policy bits.