RISC-VのRVVにはFixed-Point命令セットが含まれており、完全にRVV1.0を名乗るためにはこれらもサポートしなければならない。
12. Vector Fixed-Point Arithmetic Instructions 12.1. Vector Single-Width Saturating Add and Subtract 12.2. Vector Single-Width Averaging Add and Subtract 12.3. Vector Single-Width Fractional Multiply with Rounding and Saturation 12.4. Vector Single-Width Scaling Shift Instructions 12.5. Vector Narrowing Fixed-Point Clip Instructions
Saturating Add and Subtractは、その名の通り加算・減算をした後オーバフローしたときに、最大値または最小値に張り付ける必要がある。
# Saturating adds of unsigned integers. vsaddu.vv vd, vs2, vs1, vm # Vector-vector vsaddu.vx vd, vs2, rs1, vm # vector-scalar vsaddu.vi vd, vs2, imm, vm # vector-immediate # Saturating adds of signed integers. vsadd.vv vd, vs2, vs1, vm # Vector-vector vsadd.vx vd, vs2, rs1, vm # vector-scalar vsadd.vi vd, vs2, imm, vm # vector-immediate # Saturating subtract of unsigned integers. vssubu.vv vd, vs2, vs1, vm # Vector-vector vssubu.vx vd, vs2, rs1, vm # vector-scalar # Saturating subtract of signed integers. vssub.vv vd, vs2, vs1, vm # Vector-vector vssub.vx vd, vs2, rs1, vm # vector-scalar
Averaging Add and Subtractは、演算結果を1ビットシフトしたうえで、vxrmに基づいて値を丸める。
# Averaging add # Averaging adds of unsigned integers. vaaddu.vv vd, vs2, vs1, vm # roundoff_unsigned(vs2[i] + vs1[i], 1) vaaddu.vx vd, vs2, rs1, vm # roundoff_unsigned(vs2[i] + x[rs1], 1) # Averaging adds of signed integers. vaadd.vv vd, vs2, vs1, vm # roundoff_signed(vs2[i] + vs1[i], 1) vaadd.vx vd, vs2, rs1, vm # roundoff_signed(vs2[i] + x[rs1], 1) # Averaging subtract # Averaging subtract of unsigned integers. vasubu.vv vd, vs2, vs1, vm # roundoff_unsigned(vs2[i] - vs1[i], 1) vasubu.vx vd, vs2, rs1, vm # roundoff_unsigned(vs2[i] - x[rs1], 1) # Averaging subtract of signed integers. vasub.vv vd, vs2, vs1, vm # roundoff_signed(vs2[i] - vs1[i], 1) vasub.vx vd, vs2, rs1, vm # roundoff_signed(vs2[i] - x[rs1], 1)
Fractional Multiply with Rounding and Saturationは、SEW同士の値を乗算して2SEWの積を作り、SEW-1ビットをシフトしたうえでvxrmに基づいて丸める。
# Signed saturating and rounding fractional multiply # See vxrm description for rounding calculation vsmul.vv vd, vs2, vs1, vm # vd[i] = clip(roundoff_signed(vs2[i]*vs1[i], SEW-1)) vsmul.vx vd, vs2, rs1, vm # vd[i] = clip(roundoff_signed(vs2[i]*x[rs1], SEW-1))
Scaling Shift Instructions命令は、入力オペランドを右にシフトしたうえで丸めを行う。
# Scaling shift right logical vssrl.vv vd, vs2, vs1, vm # vd[i] = roundoff_unsigned(vs2[i], vs1[i]) vssrl.vx vd, vs2, rs1, vm # vd[i] = roundoff_unsigned(vs2[i], x[rs1]) vssrl.vi vd, vs2, uimm, vm # vd[i] = roundoff_unsigned(vs2[i], uimm) # Scaling shift right arithmetic vssra.vv vd, vs2, vs1, vm # vd[i] = roundoff_signed(vs2[i],vs1[i]) vssra.vx vd, vs2, rs1, vm # vd[i] = roundoff_signed(vs2[i], x[rs1]) vssra.vi vd, vs2, uimm, vm # vd[i] = roundoff_signed(vs2[i], uimm)
Narrowing Fixed-Point Clip Instructions は、SEW2の入力データに対してClipingを行い、SEWのサイズに縮退させたうえでレジスタに書き込む。これだけSEW2の対処が必要なため、ちょっと面倒だ。
# Narrowing unsigned clip # SEW 2*SEW SEW vnclipu.wv vd, vs2, vs1, vm # vd[i] = clip(roundoff_unsigned(vs2[i], vs1[i])) vnclipu.wx vd, vs2, rs1, vm # vd[i] = clip(roundoff_unsigned(vs2[i], x[rs1])) vnclipu.wi vd, vs2, uimm, vm # vd[i] = clip(roundoff_unsigned(vs2[i], uimm)) # Narrowing signed clip vnclip.wv vd, vs2, vs1, vm # vd[i] = clip(roundoff_signed(vs2[i], vs1[i])) vnclip.wx vd, vs2, rs1, vm # vd[i] = clip(roundoff_signed(vs2[i], x[rs1])) vnclip.wi vd, vs2, uimm, vm # vd[i] = clip(roundoff_signed(vs2[i], uimm))
