math/big: implement addMulVVW in riscv64 assembly

This provides an assembly implementation of addMulVVW for riscv64,
processing up to four words per loop, resulting in a significant
performance gain.

On a StarFive VisionFive 2:

                   │ addmulvvw.1  │             addmulvvw.2             │
                   │    sec/op    │   sec/op     vs base                │
AddMulVVW/1-4         65.49n ± 0%   50.79n ± 0%  -22.44% (p=0.000 n=10)
AddMulVVW/2-4         82.81n ± 0%   66.83n ± 0%  -19.29% (p=0.000 n=10)
AddMulVVW/3-4        100.20n ± 0%   82.87n ± 0%  -17.30% (p=0.000 n=10)
AddMulVVW/4-4        117.50n ± 0%   84.20n ± 0%  -28.34% (p=0.000 n=10)
AddMulVVW/5-4         134.9n ± 0%   100.3n ± 0%  -25.69% (p=0.000 n=10)
AddMulVVW/10-4        221.7n ± 0%   164.4n ± 0%  -25.85% (p=0.000 n=10)
AddMulVVW/100-4       1.794µ ± 0%   1.250µ ± 0%  -30.32% (p=0.000 n=10)
AddMulVVW/1000-4      17.42µ ± 0%   12.08µ ± 0%  -30.68% (p=0.000 n=10)
AddMulVVW/10000-4     254.9µ ± 0%   214.8µ ± 0%  -15.75% (p=0.000 n=10)
AddMulVVW/100000-4    2.569m ± 0%   2.178m ± 0%  -15.20% (p=0.000 n=10)
geomean               1.443µ        1.107µ       -23.29%

                   │ addmulvvw.1  │              addmulvvw.2              │
                   │     B/s      │      B/s       vs base                │
AddMulVVW/1-4        932.0Mi ± 0%   1201.6Mi ± 0%  +28.93% (p=0.000 n=10)
AddMulVVW/2-4        1.440Gi ± 0%    1.784Gi ± 0%  +23.90% (p=0.000 n=10)
AddMulVVW/3-4        1.785Gi ± 0%    2.158Gi ± 0%  +20.87% (p=0.000 n=10)
AddMulVVW/4-4        2.029Gi ± 0%    2.832Gi ± 0%  +39.59% (p=0.000 n=10)
AddMulVVW/5-4        2.209Gi ± 0%    2.973Gi ± 0%  +34.55% (p=0.000 n=10)
AddMulVVW/10-4       2.689Gi ± 0%    3.626Gi ± 0%  +34.86% (p=0.000 n=10)
AddMulVVW/100-4      3.323Gi ± 0%    4.770Gi ± 0%  +43.54% (p=0.000 n=10)
AddMulVVW/1000-4     3.421Gi ± 0%    4.936Gi ± 0%  +44.27% (p=0.000 n=10)
AddMulVVW/10000-4    2.338Gi ± 0%    2.776Gi ± 0%  +18.69% (p=0.000 n=10)
AddMulVVW/100000-4   2.320Gi ± 0%    2.736Gi ± 0%  +17.93% (p=0.000 n=10)
geomean              2.109Gi         2.749Gi       +30.36%

Change-Id: I6c7ee48233c53ff9b6a5a9002675886cd9bff5af
Reviewed-on: https://go-review.googlesource.com/c/go/+/595400


Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/math/big/arith_riscv64.s

@@ -375,5 +375,96 @@ done:
 	RET
 
 TEXT ·addMulVVW(SB),NOSPLIT,$0
-	JMP ·addMulVVW_g(SB)
+	MOV	x+24(FP), X5
+	MOV	y+48(FP), X6
+	MOV	z+0(FP), X7
+	MOV	z_len+8(FP), X30
+
+	MOV	$4, X28
+	MOV	$0, X29		// c = 0
+
+	BEQZ	X30, done
+	BLTU	X30, X28, loop1
+
+loop4:
+	MOV	0(X5), X8	// x[0]
+	MOV	0(X7), X10	// z[0]
+	MOV	8(X5), X11	// x[1]
+	MOV	8(X7), X13	// z[1]
+	MOV	16(X5), X14	// x[2]
+	MOV	16(X7), X16	// z[2]
+	MOV	24(X5), X17	// x[3]
+	MOV	24(X7), X19	// z[3]
+
+	MULHU	X8, X6, X9	// z_hi[0] = x[0] * y
+	MUL	X8, X6, X8	// z_lo[0] = x[0] * y
+	ADD	X8, X10, X21	// z_lo[0] = x[0] * y + z[0]
+	SLTU	X8, X21, X22
+	ADD	X9, X22, X9	// z_hi[0] = x[0] * y + z[0]
+	ADD	X21, X29, X10	// z[0] = x[0] * y + z[0] + c
+	SLTU	X21, X10, X22
+	ADD	X9, X22, X29	// next c
+
+	MULHU	X11, X6, X12	// z_hi[1] = x[1] * y
+	MUL	X11, X6, X11	// z_lo[1] = x[1] * y
+	ADD	X11, X13, X21	// z_lo[1] = x[1] * y + z[1]
+	SLTU	X11, X21, X22
+	ADD	X12, X22, X12	// z_hi[1] = x[1] * y + z[1]
+	ADD	X21, X29, X13	// z[1] = x[1] * y + z[1] + c
+	SLTU	X21, X13, X22
+	ADD	X12, X22, X29	// next c
+
+	MULHU	X14, X6, X15	// z_hi[2] = x[2] * y
+	MUL	X14, X6, X14	// z_lo[2] = x[2] * y
+	ADD	X14, X16, X21	// z_lo[2] = x[2] * y + z[2]
+	SLTU	X14, X21, X22
+	ADD	X15, X22, X15	// z_hi[2] = x[2] * y + z[2]
+	ADD	X21, X29, X16	// z[2] = x[2] * y + z[2] + c
+	SLTU	X21, X16, X22
+	ADD	X15, X22, X29	// next c
+
+	MULHU	X17, X6, X18	// z_hi[3] = x[3] * y
+	MUL	X17, X6, X17	// z_lo[3] = x[3] * y
+	ADD	X17, X19, X21	// z_lo[3] = x[3] * y + z[3]
+	SLTU	X17, X21, X22
+	ADD	X18, X22, X18	// z_hi[3] = x[3] * y + z[3]
+	ADD	X21, X29, X19	// z[3] = x[3] * y + z[3] + c
+	SLTU	X21, X19, X22
+	ADD	X18, X22, X29	// next c
 
+	MOV	X10, 0(X7)	// z[0]
+	MOV	X13, 8(X7)	// z[1]
+	MOV	X16, 16(X7)	// z[2]
+	MOV	X19, 24(X7)	// z[3]
+
+	ADD	$32, X5
+	ADD	$32, X7
+	SUB	$4, X30
+
+	BGEU	X30, X28, loop4
+	BEQZ	X30, done
+
+loop1:
+	MOV	0(X5), X10	// x
+	MOV	0(X7), X11	// z
+
+	MULHU	X10, X6, X12	// z_hi = x * y
+	MUL	X10, X6, X10	// z_lo = x * y
+	ADD	X10, X11, X13	// z_lo = x * y + z
+	SLTU	X10, X13, X15
+	ADD	X12, X15, X12	// z_hi = x * y + z
+	ADD	X13, X29, X10	// z = x * y + z + c
+	SLTU	X13, X10, X15
+	ADD	X12, X15, X29	// next c
+
+	MOV	X10, 0(X7)	// z
+
+	ADD	$8, X5
+	ADD	$8, X7
+	SUB	$1, X30
+
+	BNEZ	X30, loop1
+
+done:
+	MOV	X29, c+56(FP)	// return c
+	RET