math/big: replace division with multiplication by reciprocal word

Division is much slower than multiplication. And the method of using
multiplication by multiplying reciprocal and replacing division with it
can increase the speed of divWVW algorithm by three times,and at the
same time increase the speed of nats division.

The benchmark test on arm64 is as follows:
name                     old time/op    new time/op    delta
DivWVW/1-4                 13.1ns ± 4%    13.3ns ± 4%      ~     (p=0.444 n=5+5)
DivWVW/2-4                 48.6ns ± 1%    51.2ns ± 2%    +5.39%  (p=0.008 n=5+5)
DivWVW/3-4                 82.0ns ± 1%    69.7ns ± 1%   -15.03%  (p=0.008 n=5+5)
DivWVW/4-4                  116ns ± 1%      71ns ± 2%   -38.88%  (p=0.008 n=5+5)
DivWVW/5-4                  152ns ± 1%      84ns ± 4%   -44.70%  (p=0.008 n=5+5)
DivWVW/10-4                 319ns ± 1%     155ns ± 4%   -51.50%  (p=0.008 n=5+5)
DivWVW/100-4               3.44µs ± 3%    1.30µs ± 8%   -62.30%  (p=0.008 n=5+5)
DivWVW/1000-4              33.8µs ± 0%    10.9µs ± 1%   -67.74%  (p=0.008 n=5+5)
DivWVW/10000-4              343µs ± 4%     111µs ± 5%   -67.63%  (p=0.008 n=5+5)
DivWVW/100000-4            3.35ms ± 1%    1.25ms ± 3%   -62.79%  (p=0.008 n=5+5)
QuoRem-4                   3.08µs ± 2%    2.21µs ± 4%   -28.40%  (p=0.008 n=5+5)
ModSqrt225_Tonelli-4        444µs ± 2%     457µs ± 3%      ~     (p=0.095 n=5+5)
ModSqrt225_3Mod4-4          136µs ± 1%     138µs ± 3%      ~     (p=0.151 n=5+5)
ModSqrt231_Tonelli-4        473µs ± 3%     483µs ± 4%      ~     (p=0.548 n=5+5)
ModSqrt231_5Mod8-4          164µs ± 9%     169µs ±12%      ~     (p=0.421 n=5+5)
Sqrt-4                     36.8µs ± 1%    28.6µs ± 0%   -22.17%  (p=0.016 n=5+4)
Div/20/10-4                50.0ns ± 3%    51.3ns ± 6%      ~     (p=0.238 n=5+5)
Div/40/20-4                49.8ns ± 2%    51.3ns ± 6%      ~     (p=0.222 n=5+5)
Div/100/50-4               85.8ns ± 4%    86.5ns ± 5%	   ~     (p=0.246 n=5+5)
Div/200/100-4               335ns ± 3%     296ns ± 2%   -11.60%  (p=0.008 n=5+5)
Div/400/200-4               442ns ± 2%     359ns ± 5%   -18.81%  (p=0.008 n=5+5)
Div/1000/500-4              858ns ± 3%     643ns ± 6%   -25.06%  (p=0.008 n=5+5)
Div/2000/1000-4            1.70µs ± 3%    1.28µs ± 4%   -24.80%  (p=0.008 n=5+5)
Div/20000/10000-4          45.0µs ± 5%    41.8µs ± 4%    -7.17%  (p=0.016 n=5+5)
Div/200000/100000-4        1.51ms ± 7%    1.43ms ± 3%    -5.42%  (p=0.016 n=5+5)
Div/2000000/1000000-4      57.6ms ± 4%    57.5ms ± 3%      ~     (p=1.000 n=5+5)
Div/20000000/10000000-4     2.08s ± 3%     2.04s ± 1%      ~     (p=0.095 n=5+5)

name                     old speed      new speed      delta
DivWVW/1-4               4.87GB/s ± 4%  4.80GB/s ± 4%      ~     (p=0.310 n=5+5)
DivWVW/2-4               2.63GB/s ± 1%  2.50GB/s ± 2%    -5.07%  (p=0.008 n=5+5)
DivWVW/3-4               2.34GB/s ± 1%  2.76GB/s ± 1%   +17.70%  (p=0.008 n=5+5)
DivWVW/4-4               2.21GB/s ± 1%  3.61GB/s ± 2%   +63.42%  (p=0.008 n=5+5)
DivWVW/5-4               2.10GB/s ± 2%  3.81GB/s ± 4%   +80.89%  (p=0.008 n=5+5)
DivWVW/10-4              2.01GB/s ± 0%  4.13GB/s ± 4%  +105.91%  (p=0.008 n=5+5)
DivWVW/100-4             1.86GB/s ± 2%  4.95GB/s ± 7%  +165.63%  (p=0.008 n=5+5)
DivWVW/1000-4            1.89GB/s ± 0%  5.86GB/s ± 1%  +209.96%  (p=0.008 n=5+5)
DivWVW/10000-4           1.87GB/s ± 4%  5.76GB/s ± 5%  +208.96%  (p=0.008 n=5+5)
DivWVW/100000-4          1.91GB/s ± 1%  5.14GB/s ± 3%  +168.85%  (p=0.008 n=5+5)

Change-Id: I049f1196562b20800e6ef8a6493fd147f93ad830
Reviewed-on: https://go-review.googlesource.com/c/go/+/250417
Trust: Giovanni Bajo <rasky@develer.com>
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

15 files changed, 136 insertions, 186 deletions

diff --git a/src/math/big/arith.go b/src/math/big/arith.go
index b0885f261f..750ce8aa39 100644
--- a/src/math/big/arith.go
+++ b/src/math/big/arith.go
@@ -60,12 +60,6 @@ func nlz(x Word) uint {
 	return uint(bits.LeadingZeros(uint(x)))
 }
 
-// q = (u1<<_W + u0 - r)/v
-func divWW_g(u1, u0, v Word) (q, r Word) {
-	qq, rr := bits.Div(uint(u1), uint(u0), uint(v))
-	return Word(qq), Word(rr)
-}
-
 // The resulting carry c is either 0 or 1.
 func addVV_g(z, x, y []Word) (c Word) {
 	// The comment near the top of this file discusses this for loop condition.
@@ -207,10 +201,87 @@ func addMulVVW_g(z, x []Word, y Word) (c Word) {
 	return
 }
 
-func divWVW_g(z []Word, xn Word, x []Word, y Word) (r Word) {
+// q = ( x1 << _W + x0 - r)/y. m = floor(( _B^2 - 1 ) / d - _B). Requiring x1<y.
+// An approximate reciprocal with a reference to "Improved Division by Invariant Integers
+// (IEEE Transactions on Computers, 11 Jun. 2010)"
+func divWW(x1, x0, y, m Word) (q, r Word) {
+	s := nlz(y)
+	if s != 0 {
+		x1 = x1<<s | x0>>(_W-s)
+		x0 <<= s
+		y <<= s
+	}
+	d := uint(y)
+	// We know that
+	//   m = ⎣(B^2-1)/d⎦-B
+	//   ⎣(B^2-1)/d⎦ = m+B
+	//   (B^2-1)/d = m+B+delta1    0 <= delta1 <= (d-1)/d
+	//   B^2/d = m+B+delta2        0 <= delta2 <= 1
+	// The quotient we're trying to compute is
+	//   quotient = ⎣(x1*B+x0)/d⎦
+	//            = ⎣(x1*B*(B^2/d)+x0*(B^2/d))/B^2⎦
+	//            = ⎣(x1*B*(m+B+delta2)+x0*(m+B+delta2))/B^2⎦
+	//            = ⎣(x1*m+x1*B+x0)/B + x0*m/B^2 + delta2*(x1*B+x0)/B^2⎦
+	// The latter two terms of this three-term sum are between 0 and 1.
+	// So we can compute just the first term, and we will be low by at most 2.
+	t1, t0 := bits.Mul(uint(m), uint(x1))
+	_, c := bits.Add(t0, uint(x0), 0)
+	t1, _ = bits.Add(t1, uint(x1), c)
+	// The quotient is either t1, t1+1, or t1+2.
+	// We'll try t1 and adjust if needed.
+	qq := t1
+	// compute remainder r=x-d*q.
+	dq1, dq0 := bits.Mul(d, qq)
+	r0, b := bits.Sub(uint(x0), dq0, 0)
+	r1, _ := bits.Sub(uint(x1), dq1, b)
+	// The remainder we just computed is bounded above by B+d:
+	// r = x1*B + x0 - d*q.
+	//   = x1*B + x0 - d*⎣(x1*m+x1*B+x0)/B⎦
+	//   = x1*B + x0 - d*((x1*m+x1*B+x0)/B-alpha)                                   0 <= alpha < 1
+	//   = x1*B + x0 - x1*d/B*m                         - x1*d - x0*d/B + d*alpha
+	//   = x1*B + x0 - x1*d/B*⎣(B^2-1)/d-B⎦             - x1*d - x0*d/B + d*alpha
+	//   = x1*B + x0 - x1*d/B*⎣(B^2-1)/d-B⎦             - x1*d - x0*d/B + d*alpha
+	//   = x1*B + x0 - x1*d/B*((B^2-1)/d-B-beta)        - x1*d - x0*d/B + d*alpha   0 <= beta < 1
+	//   = x1*B + x0 - x1*B + x1/B + x1*d + x1*d/B*beta - x1*d - x0*d/B + d*alpha
+	//   =        x0        + x1/B        + x1*d/B*beta        - x0*d/B + d*alpha
+	//   = x0*(1-d/B) + x1*(1+d*beta)/B + d*alpha
+	//   <  B*(1-d/B) +  d*B/B          + d          because x0<B (and 1-d/B>0), x1<d, 1+d*beta<=B, alpha<1
+	//   =  B - d     +  d              + d
+	//   = B+d
+	// So r1 can only be 0 or 1. If r1 is 1, then we know q was too small.
+	// Add 1 to q and subtract d from r. That guarantees that r is <B, so
+	// we no longer need to keep track of r1.
+	if r1 != 0 {
+		qq++
+		r0 -= d
+	}
+	// If the remainder is still too large, increment q one more time.
+	if r0 >= d {
+		qq++
+		r0 -= d
+	}
+	return Word(qq), Word(r0 >> s)
+}
+
+func divWVW(z []Word, xn Word, x []Word, y Word) (r Word) {
 	r = xn
+	if len(x) == 1 {
+		qq, rr := bits.Div(uint(r), uint(x[0]), uint(y))
+		z[0] = Word(qq)
+		return Word(rr)
+	}
+	rec := reciprocalWord(y)
 	for i := len(z) - 1; i >= 0; i-- {
-		z[i], r = divWW_g(r, x[i], y)
+		z[i], r = divWW(r, x[i], y, rec)
 	}
-	return
+	return r
+}
+
+// reciprocalWord return the reciprocal of the divisor. rec = floor(( _B^2 - 1 ) / u - _B). u = d1 << nlz(d1).
+func reciprocalWord(d1 Word) Word {
+	u := uint(d1 << nlz(d1))
+	x1 := ^u
+	x0 := uint(_M)
+	rec, _ := bits.Div(x1, x0, u) // (_B^2-1)/U-_B = (_B*(_M-C)+_M)/U
+	return Word(rec)
 }
diff --git a/src/math/big/arith_386.s b/src/math/big/arith_386.s
index f61da2aba7..d0ea949fe6 100644
--- a/src/math/big/arith_386.s
+++ b/src/math/big/arith_386.s
@@ -18,16 +18,6 @@ TEXT ·mulWW(SB),NOSPLIT,$0
 	RET
 
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB),NOSPLIT,$0
-	MOVL x1+0(FP), DX
-	MOVL x0+4(FP), AX
-	DIVL y+8(FP)
-	MOVL AX, q+12(FP)
-	MOVL DX, r+16(FP)
-	RET
-
-
 // func addVV(z, x, y []Word) (c Word)
 TEXT ·addVV(SB),NOSPLIT,$0
 	MOVL z+0(FP), DI
@@ -251,21 +241,4 @@ E6:	CMPL BX, $0		// i < 0
 	RET
 
 
-// func divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
-TEXT ·divWVW(SB),NOSPLIT,$0
-	MOVL z+0(FP), DI
-	MOVL xn+12(FP), DX	// r = xn
-	MOVL x+16(FP), SI
-	MOVL y+28(FP), CX
-	MOVL z_len+4(FP), BX	// i = z
-	JMP E7
 
-L7:	MOVL (SI)(BX*4), AX
-	DIVL CX
-	MOVL AX, (DI)(BX*4)
-
-E7:	SUBL $1, BX		// i--
-	JGE L7			// i >= 0
-
-	MOVL DX, r+32(FP)
-	RET
diff --git a/src/math/big/arith_amd64.s b/src/math/big/arith_amd64.s
index b75639f540..61043ca2d9 100644
--- a/src/math/big/arith_amd64.s
+++ b/src/math/big/arith_amd64.s
@@ -18,14 +18,6 @@ TEXT ·mulWW(SB),NOSPLIT,$0
 	RET
 
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB),NOSPLIT,$0
-	MOVQ x1+0(FP), DX
-	MOVQ x0+8(FP), AX
-	DIVQ y+16(FP)
-	MOVQ AX, q+24(FP)
-	MOVQ DX, r+32(FP)
-	RET
 
 // The carry bit is saved with SBBQ Rx, Rx: if the carry was set, Rx is -1, otherwise it is 0.
 // It is restored with ADDQ Rx, Rx: if Rx was -1 the carry is set, otherwise it is cleared.
@@ -531,21 +523,3 @@ adx_short:
 
 
 
-// func divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
-TEXT ·divWVW(SB),NOSPLIT,$0
-	MOVQ z+0(FP), R10
-	MOVQ xn+24(FP), DX	// r = xn
-	MOVQ x+32(FP), R8
-	MOVQ y+56(FP), R9
-	MOVQ z_len+8(FP), BX	// i = z
-	JMP E7
-
-L7:	MOVQ (R8)(BX*8), AX
-	DIVQ R9
-	MOVQ AX, (R10)(BX*8)
-
-E7:	SUBQ $1, BX		// i--
-	JGE L7			// i >= 0
-
-	MOVQ DX, r+64(FP)
-	RET
diff --git a/src/math/big/arith_arm.s b/src/math/big/arith_arm.s
index 33aa36f709..cbf7445e7a 100644
--- a/src/math/big/arith_arm.s
+++ b/src/math/big/arith_arm.s
@@ -272,17 +272,6 @@ E9:
 	RET
 
 
-// func divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
-TEXT ·divWVW(SB),NOSPLIT,$0
-	// ARM has no multiword division, so use portable code.
-	B ·divWVW_g(SB)
-
-
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB),NOSPLIT,$0
-	// ARM has no multiword division, so use portable code.
-	B ·divWW_g(SB)
-
 
 // func mulWW(x, y Word) (z1, z0 Word)
 TEXT ·mulWW(SB),NOSPLIT,$0
diff --git a/src/math/big/arith_arm64.s b/src/math/big/arith_arm64.s
index da6e408e19..22357d088e 100644
--- a/src/math/big/arith_arm64.s
+++ b/src/math/big/arith_arm64.s
@@ -23,11 +23,6 @@ TEXT ·mulWW(SB),NOSPLIT,$0
 	RET
 
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB),NOSPLIT,$0
-	B	·divWW_g(SB) // ARM64 has no multiword division
-
-
 // func addVV(z, x, y []Word) (c Word)
 TEXT ·addVV(SB),NOSPLIT,$0
 	MOVD	z_len+8(FP), R0
@@ -585,6 +580,4 @@ done:
 	MOVD	R4, c+56(FP)
 	RET
 
-// func divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
-TEXT ·divWVW(SB),NOSPLIT,$0
-	B ·divWVW_g(SB)
+
diff --git a/src/math/big/arith_decl.go b/src/math/big/arith_decl.go
index 41e592334c..d519bdc87b 100644
--- a/src/math/big/arith_decl.go
+++ b/src/math/big/arith_decl.go
@@ -8,7 +8,6 @@ package big
 
 // implemented in arith_$GOARCH.s
 func mulWW(x, y Word) (z1, z0 Word)
-func divWW(x1, x0, y Word) (q, r Word)
 func addVV(z, x, y []Word) (c Word)
 func subVV(z, x, y []Word) (c Word)
 func addVW(z, x []Word, y Word) (c Word)
@@ -17,4 +16,3 @@ func shlVU(z, x []Word, s uint) (c Word)
 func shrVU(z, x []Word, s uint) (c Word)
 func mulAddVWW(z, x []Word, y, r Word) (c Word)
 func addMulVVW(z, x []Word, y Word) (c Word)
-func divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
diff --git a/src/math/big/arith_decl_pure.go b/src/math/big/arith_decl_pure.go
index 305f7ee03b..5faa3bd281 100644
--- a/src/math/big/arith_decl_pure.go
+++ b/src/math/big/arith_decl_pure.go
@@ -10,10 +10,6 @@ func mulWW(x, y Word) (z1, z0 Word) {
 	return mulWW_g(x, y)
 }
 
-func divWW(x1, x0, y Word) (q, r Word) {
-	return divWW_g(x1, x0, y)
-}
-
 func addVV(z, x, y []Word) (c Word) {
 	return addVV_g(z, x, y)
 }
@@ -55,7 +51,3 @@ func mulAddVWW(z, x []Word, y, r Word) (c Word) {
 func addMulVVW(z, x []Word, y Word) (c Word) {
 	return addMulVVW_g(z, x, y)
 }
-
-func divWVW(z []Word, xn Word, x []Word, y Word) (r Word) {
-	return divWVW_g(z, xn, x, y)
-}
diff --git a/src/math/big/arith_mips64x.s b/src/math/big/arith_mips64x.s
index 983510ee3d..804b9fe06e 100644
--- a/src/math/big/arith_mips64x.s
+++ b/src/math/big/arith_mips64x.s
@@ -12,9 +12,6 @@
 TEXT ·mulWW(SB),NOSPLIT,$0
 	JMP ·mulWW_g(SB)
 
-TEXT ·divWW(SB),NOSPLIT,$0
-	JMP ·divWW_g(SB)
-
 TEXT ·addVV(SB),NOSPLIT,$0
 	JMP ·addVV_g(SB)
 
@@ -39,5 +36,3 @@ TEXT ·mulAddVWW(SB),NOSPLIT,$0
 TEXT ·addMulVVW(SB),NOSPLIT,$0
 	JMP ·addMulVVW_g(SB)
 
-TEXT ·divWVW(SB),NOSPLIT,$0
-	JMP ·divWVW_g(SB)
diff --git a/src/math/big/arith_mipsx.s b/src/math/big/arith_mipsx.s
index 54cafbd9c0..efdecb80f3 100644
--- a/src/math/big/arith_mipsx.s
+++ b/src/math/big/arith_mipsx.s
@@ -12,9 +12,6 @@
 TEXT ·mulWW(SB),NOSPLIT,$0
 	JMP	·mulWW_g(SB)
 
-TEXT ·divWW(SB),NOSPLIT,$0
-	JMP	·divWW_g(SB)
-
 TEXT ·addVV(SB),NOSPLIT,$0
 	JMP	·addVV_g(SB)
 
@@ -39,5 +36,3 @@ TEXT ·mulAddVWW(SB),NOSPLIT,$0
 TEXT ·addMulVVW(SB),NOSPLIT,$0
 	JMP	·addMulVVW_g(SB)
 
-TEXT ·divWVW(SB),NOSPLIT,$0
-	JMP	·divWVW_g(SB)
diff --git a/src/math/big/arith_ppc64x.s b/src/math/big/arith_ppc64x.s
index 409e10ab48..b299ccc2fb 100644
--- a/src/math/big/arith_ppc64x.s
+++ b/src/math/big/arith_ppc64x.s
@@ -478,44 +478,4 @@ done:
 	MOVD R4, c+56(FP)
 	RET
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB), NOSPLIT, $0
-	MOVD x1+0(FP), R4
-	MOVD x0+8(FP), R5
-	MOVD y+16(FP), R6
-
-	CMPU R4, R6
-	BGE  divbigger
-
-	// from the programmer's note in ch. 3 of the ISA manual, p.74
-	DIVDEU R6, R4, R3
-	DIVDU  R6, R5, R7
-	MULLD  R6, R3, R8
-	MULLD  R6, R7, R20
-	SUB    R20, R5, R10
-	ADD    R7, R3, R3
-	SUB    R8, R10, R4
-	CMPU   R4, R10
-	BLT    adjust
-	CMPU   R4, R6
-	BLT    end
-
-adjust:
-	MOVD $1, R21
-	ADD  R21, R3, R3
-	SUB  R6, R4, R4
-
-end:
-	MOVD R3, q+24(FP)
-	MOVD R4, r+32(FP)
 
-	RET
-
-divbigger:
-	MOVD $-1, R7
-	MOVD R7, q+24(FP)
-	MOVD R7, r+32(FP)
-	RET
-
-TEXT ·divWVW(SB), NOSPLIT, $0
-	BR ·divWVW_g(SB)
diff --git a/src/math/big/arith_riscv64.s b/src/math/big/arith_riscv64.s
index 59065c3f7b..a2f7666c7b 100644
--- a/src/math/big/arith_riscv64.s
+++ b/src/math/big/arith_riscv64.s
@@ -19,9 +19,6 @@ TEXT ·mulWW(SB),NOSPLIT,$0
 	MOV	X8, z0+24(FP)
 	RET
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB),NOSPLIT,$0
-	JMP ·divWW_g(SB)		// riscv64 has no multiword division
 
 TEXT ·addVV(SB),NOSPLIT,$0
 	JMP ·addVV_g(SB)
@@ -47,5 +44,3 @@ TEXT ·mulAddVWW(SB),NOSPLIT,$0
 TEXT ·addMulVVW(SB),NOSPLIT,$0
 	JMP ·addMulVVW_g(SB)
 
-TEXT ·divWVW(SB),NOSPLIT,$0
-	JMP ·divWVW_g(SB)
diff --git a/src/math/big/arith_s390x.s b/src/math/big/arith_s390x.s
index 4891768111..242aca7434 100644
--- a/src/math/big/arith_s390x.s
+++ b/src/math/big/arith_s390x.s
@@ -17,15 +17,6 @@ TEXT ·mulWW(SB), NOSPLIT, $0
 	MOVD   R11, z0+24(FP)
 	RET
 
-// func divWW(x1, x0, y Word) (q, r Word)
-TEXT ·divWW(SB), NOSPLIT, $0
-	MOVD x1+0(FP), R10
-	MOVD x0+8(FP), R11
-	MOVD y+16(FP), R5
-	WORD $0xb98700a5   // dlgr r10,r5
-	MOVD R11, q+24(FP)
-	MOVD R10, r+32(FP)
-	RET
 
 // DI = R3, CX = R4, SI = r10, r8 = r8, r9=r9, r10 = r2 , r11 = r5, r12 = r6, r13 = r7, r14 = r1 (R0 set to 0) + use R11
 // func addVV(z, x, y []Word) (c Word)
@@ -990,27 +981,3 @@ E6:
 	MOVD R4, c+56(FP)
 	RET
 
-// func divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
-// CX = R4, r8 = r8, r9=r9, r10 = r2 , r11 = r5, AX = r11, DX = R6, r12=r12, BX = R1(*8) , (R0 set to 0) + use R11 + use R7 for i
-TEXT ·divWVW(SB), NOSPLIT, $0
-	MOVD z+0(FP), R2
-	MOVD xn+24(FP), R10  // r = xn
-	MOVD x+32(FP), R8
-	MOVD y+56(FP), R9
-	MOVD z_len+8(FP), R7 // i = z
-	SLD  $3, R7, R1      // i*8
-	MOVD $0, R0          // make sure it's zero
-	BR   E7
-
-L7:
-	MOVD (R8)(R1*1), R11
-	WORD $0xB98700A9     // DLGR R10,R9
-	MOVD R11, (R2)(R1*1)
-
-E7:
-	SUB $1, R7 // i--
-	SUB $8, R1
-	BGE L7     // i >= 0
-
-	MOVD R10, r+64(FP)
-	RET
diff --git a/src/math/big/arith_test.go b/src/math/big/arith_test.go
index fc205934c5..808d178459 100644
--- a/src/math/big/arith_test.go
+++ b/src/math/big/arith_test.go
@@ -7,6 +7,7 @@ package big
 import (
 	"fmt"
 	"internal/testenv"
+	"math/bits"
 	"math/rand"
 	"strings"
 	"testing"
@@ -493,7 +494,6 @@ func TestFunVWW(t *testing.T) {
 
 		if a.y != 0 && a.r < a.y {
 			arg := argWVW{a.x, a.c, a.z, a.y, a.r}
-			testFunWVW(t, "divWVW_g", divWVW_g, arg)
 			testFunWVW(t, "divWVW", divWVW, arg)
 		}
 	}
@@ -536,6 +536,42 @@ func TestMulAddWWW(t *testing.T) {
 	}
 }
 
+var divWWTests = []struct {
+	x1, x0, y Word
+	q, r      Word
+}{
+	{_M >> 1, 0, _M, _M >> 1, _M >> 1},
+	{_M - (1 << (_W - 2)), _M, 3 << (_W - 2), _M, _M - (1 << (_W - 2))},
+}
+
+const testsNumber = 1 << 16
+
+func TestDivWW(t *testing.T) {
+	i := 0
+	for i, test := range divWWTests {
+		rec := reciprocalWord(test.y)
+		q, r := divWW(test.x1, test.x0, test.y, rec)
+		if q != test.q || r != test.r {
+			t.Errorf("#%d got (%x, %x) want (%x, %x)", i, q, r, test.q, test.r)
+		}
+	}
+	//random tests
+	for ; i < testsNumber; i++ {
+		x1 := rndW()
+		x0 := rndW()
+		y := rndW()
+		if x1 >= y {
+			continue
+		}
+		rec := reciprocalWord(y)
+		qGot, rGot := divWW(x1, x0, y, rec)
+		qWant, rWant := bits.Div(uint(x1), uint(x0), uint(y))
+		if uint(qGot) != qWant || uint(rGot) != rWant {
+			t.Errorf("#%d got (%x, %x) want (%x, %x)", i, qGot, rGot, qWant, rWant)
+		}
+	}
+}
+
 func BenchmarkMulAddVWW(b *testing.B) {
 	for _, n := range benchSizes {
 		if isRaceBuilder && n > 1e3 {
@@ -570,3 +606,19 @@ func BenchmarkAddMulVVW(b *testing.B) {
 		})
 	}
 }
+func BenchmarkDivWVW(b *testing.B) {
+	for _, n := range benchSizes {
+		if isRaceBuilder && n > 1e3 {
+			continue
+		}
+		x := rndV(n)
+		y := rndW()
+		z := make([]Word, n)
+		b.Run(fmt.Sprint(n), func(b *testing.B) {
+			b.SetBytes(int64(n * _W))
+			for i := 0; i < b.N; i++ {
+				divWVW(z, 0, x, y)
+			}
+		})
+	}
+}
diff --git a/src/math/big/arith_wasm.s b/src/math/big/arith_wasm.s
index 382597c694..add1064469 100644
--- a/src/math/big/arith_wasm.s
+++ b/src/math/big/arith_wasm.s
@@ -9,9 +9,6 @@
 TEXT ·mulWW(SB),NOSPLIT,$0
 	JMP ·mulWW_g(SB)
 
-TEXT ·divWW(SB),NOSPLIT,$0
-	JMP ·divWW_g(SB)
-
 TEXT ·addVV(SB),NOSPLIT,$0
 	JMP ·addVV_g(SB)
 
@@ -36,5 +33,3 @@ TEXT ·mulAddVWW(SB),NOSPLIT,$0
 TEXT ·addMulVVW(SB),NOSPLIT,$0
 	JMP ·addMulVVW_g(SB)
 
-TEXT ·divWVW(SB),NOSPLIT,$0
-	JMP ·divWVW_g(SB)
diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index 6a3989bf9d..c2f3787848 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -751,6 +751,7 @@ func (q nat) divBasic(u, v nat) {
 
 	// D2.
 	vn1 := v[n-1]
+	rec := reciprocalWord(vn1)
 	for j := m; j >= 0; j-- {
 		// D3.
 		qhat := Word(_M)
@@ -760,7 +761,7 @@ func (q nat) divBasic(u, v nat) {
 		}
 		if ujn != vn1 {
 			var rhat Word
-			qhat, rhat = divWW(ujn, u[j+n-1], vn1)
+			qhat, rhat = divWW(ujn, u[j+n-1], vn1, rec)
 
 			// x1 | x2 = q̂v_{n-2}
 			vn2 := v[n-2]