How is FMSUB.D different from FMUL plus add/subtract?
The fused instruction performs one final rounding; two separate instructions round the product first and then round the add/subtract result.
FMSUB.D
RISC-V FMSUB.D Instruction Details
Instruction ManualR4-typeDouble-precision fused multiply-subtract: rd = (rs1 * rs2) - rs3, rounded once.
Instruction Syntax
fmsub.d rd, rs1, rs2, rs3, rm
Operand Breakdown
rd: destination floating-point register.
rs1, rs2, rs3: three source floating-point registers; R4 is not an ordinary three-register R-type form.
rm: rounding-mode field for the final fused multiply-add/subtract result.
DFloating-Point Arithmetic
Instruction Behavior
FMSUB.D uses the R4 four-register format for double-precision fused multiply-subtract. The multiply and add/subtract are fused, so the exact intermediate result is rounded only once at the end according to rm before being written to rd.
Quick Understanding & Search Notes
FMSUB.D's key property is fusion: the product and add/subtract term are not written as an intermediate result and are rounded once at the final step.
The R4 format includes rs3; do not read it as an ordinary three-register R-type instruction.
The sign rules follow the official definitions: FMADD adds rs3, FMSUB subtracts rs3, and FNMADD/FNMSUB use the negated fused-result variants.
Common Usage Scenarios
Fused Floating Point
Understand this scenario with real code like «fmsub.d f0, f1, f2, f3 # f0 = f1*f2-f3».
Numerical Computing
Understand this scenario with real code like «fmsub.d f0, f1, f2, f3 # f0 = f1*f2-f3».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is R4-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
FMA rounds once and is not equivalent to a separate FMUL followed by FADD/FSUB with two roundings.
The negated FNMSUB/FNMADD sign rules are easy to confuse with ordinary FMSUB/FMADD.
rm controls the single final rounding; exception flags accrue for the fused operation.
FAQ
Why is FMSUB.D R4-type?
It needs rd, rs1, rs2, and rs3 register fields for the two multiply sources and the add/subtract term.