big: implemented overlap-tolerant shifts in assembly

- no need to make copies in cases of aliases
- removed deprecated internal shift functions
- minor unrelated simplifications

This change improves pidigits -s -n10000 by almost 20%:

user 0m6.156s (old)
user 0m4.999s (new)

(pidigits -s -n20000 goes from ~25s to ~19s)

R=rsc
CC=golang-dev
http://codereview.appspot.com/1149041

5 files changed, 188 insertions, 164 deletions

diff --git a/src/pkg/big/arith.go b/src/pkg/big/arith.go
index d5060bb88..a0c7aa31a 100644
--- a/src/pkg/big/arith.go
+++ b/src/pkg/big/arith.go
@@ -325,10 +325,16 @@ func subVW_g(z, x *Word, y Word, n int) (c Word) {
 
 func shlVW(z, x *Word, s Word, n int) (c Word)
 func shlVW_g(z, x *Word, s Word, n int) (c Word) {
-	ŝ := _W - s
-	for i := 0; i < n; i++ {
-		w := *x.at(i)
-		c, *z.at(i) = w>>ŝ, w<<s|c
+	if n > 0 {
+		ŝ := _W - s
+		w1 := *x.at(n - 1)
+		c = w1 >> ŝ
+		for i := n - 1; i > 0; i-- {
+			w := w1
+			w1 = *x.at(i - 1)
+			*z.at(i) = w<<s | w1>>ŝ
+		}
+		*z.at(0) = w1 << s
 	}
 	return
 }
@@ -336,10 +342,16 @@ func shlVW_g(z, x *Word, s Word, n int) (c Word) {
 
 func shrVW(z, x *Word, s Word, n int) (c Word)
 func shrVW_g(z, x *Word, s Word, n int) (c Word) {
-	ŝ := _W - s
-	for i := n - 1; i >= 0; i-- {
-		w := *x.at(i)
-		c, *z.at(i) = w<<ŝ, w>>s|c
+	if n > 0 {
+		ŝ := _W - s
+		w1 := *x.at(0)
+		c = w1 << ŝ
+		for i := 0; i < n-1; i++ {
+			w := w1
+			w1 = *x.at(i + 1)
+			*z.at(i) = w>>s | w1<<ŝ
+		}
+		*z.at(n - 1) = w1 >> s
 	}
 	return
 }
diff --git a/src/pkg/big/arith_386.s b/src/pkg/big/arith_386.s
index 22fde9ccb..09904594c 100644
--- a/src/pkg/big/arith_386.s
+++ b/src/pkg/big/arith_386.s
@@ -101,53 +101,73 @@ E4:	CMPL BX, BP		// i < n
 
 // func shlVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shlVW(SB),7,$0
+	MOVL n+12(FP), BX	// i = n
+	SUBL $1, BX		// i--
+	JL X8b			// i < 0	(n <= 0)
+
+	// n > 0
 	MOVL z+0(FP), DI
 	MOVL x+4(FP), SI
 	MOVL s+8(FP), CX
-	MOVL n+12(FP), BX
-	LEAL (DI)(BX*4), DI
-	LEAL (SI)(BX*4), SI
-	NEGL BX			// i = -n
-	MOVL $0, AX		// c = 0
-	JMP E8
+	MOVL (SI)(BX*4), AX	// w1 = x[n-1]
+	MOVL $0, DX
+	SHLL CX, DX:AX		// w1>>ŝ
+	MOVL DX, c+16(FP)
 
-L8:	MOVL (SI)(BX*4), DX
-	MOVL DX, BP
-	SHLL CX, DX:AX
-	MOVL DX, (DI)(BX*4)
-	MOVL BP, AX
-	ADDL $1, BX		// i++
+	CMPL BX, $0
+	JLE X8a			// i <= 0
 
-E8:	CMPL BX, $0		// i < 0
-	JL L8
+	// i > 0
+L8:	MOVL AX, DX		// w = w1
+	MOVL -4(SI)(BX*4), AX	// w1 = x[i-1]
+	SHLL CX, DX:AX		// w<<s | w1>>ŝ
+	MOVL DX, (DI)(BX*4)	// z[i] = w<<s | w1>>ŝ
+	SUBL $1, BX		// i--
+	JG L8			// i > 0
 
-	MOVL $0, DX
-	SHLL CX, DX:AX
-	MOVL DX, c+16(FP)
+	// i <= 0
+X8a:	SHLL CX, AX		// w1<<s
+	MOVL AX, (DI)		// z[0] = w1<<s
+	RET
+
+X8b:	MOVL $0, c+16(FP)
 	RET
 
 
 // func shrVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shrVW(SB),7,$0
+	MOVL n+24(FP), BP
+	SUBL $1, BP		// n--
+	JL X9b			// n < 0	(n <= 0)
+
+	// n > 0
 	MOVL z+0(FP), DI
 	MOVL x+4(FP), SI
 	MOVL s+8(FP), CX
-	MOVL n+12(FP), BX	// i = n
-	MOVL $0, AX		// c = 0
+	MOVL (SI), AX		// w1 = x[0]
+	MOVL $0, DX
+	SHRL CX, DX:AX		// w1<<ŝ
+	MOVL DX, c+16(FP)
+
+	MOVL $0, BX		// i = 0
 	JMP E9
 
-L9:	MOVL (SI)(BX*4), DX
-	MOVL DX, BP
-	SHRL CX, DX:AX
-	MOVL DX, (DI)(BX*4)
-	MOVL BP, AX
+	// i < n-1
+L9:	MOVL AX, DX		// w = w1
+	MOVL 4(SI)(BX*4), AX	// w1 = x[i+1]
+	SHRL CX, DX:AX		// w>>s | w1<<ŝ
+	MOVL DX, (DI)(BX*4)	// z[i] = w>>s | w1<<ŝ
+	ADDL $1, BX		// i++
+	
+E9:	CMPL BX, BP
+	JL L9			// i < n-1
 
-E9:	SUBL $1, BX		// i--
-	JGE L9
+	// i >= n-1
+X9a:	SHRL CX, AX		// w1>>s
+	MOVL AX, (DI)(BP*4)	// z[n-1] = w1>>s
+	RET
 
-	MOVL $0, DX
-	SHRL CX, DX:AX
-	MOVL DX, c+16(FP)
+X9b:	MOVL $0, c+16(FP)
 	RET
 
 
diff --git a/src/pkg/big/arith_amd64.s b/src/pkg/big/arith_amd64.s
index 5f9b4782d..e21651029 100644
--- a/src/pkg/big/arith_amd64.s
+++ b/src/pkg/big/arith_amd64.s
@@ -13,8 +13,8 @@ TEXT ·addVV(SB),7,$0
 	MOVQ x+8(FP), R8
 	MOVQ y+16(FP), R9
 	MOVL n+24(FP), R11
-	MOVQ $0, BX         // i = 0
-	MOVQ $0, DX         // c = 0
+	MOVQ $0, BX		// i = 0
+	MOVQ $0, DX		// c = 0
 	JMP E1
 
 L1:	MOVQ (R8)(BX*8), AX
@@ -22,7 +22,7 @@ L1:	MOVQ (R8)(BX*8), AX
 	ADCQ (R9)(BX*8), AX
 	RCLQ $1, DX
 	MOVQ AX, (R10)(BX*8)
-	ADDL $1, BX			// i++
+	ADDL $1, BX		// i++
 
 E1:	CMPQ BX, R11		// i < n
 	JL L1
@@ -38,8 +38,8 @@ TEXT ·subVV(SB),7,$0
 	MOVQ x+8(FP), R8
 	MOVQ y+16(FP), R9
 	MOVL n+24(FP), R11
-	MOVQ $0, BX         // i = 0
-	MOVQ $0, DX         // c = 0
+	MOVQ $0, BX		// i = 0
+	MOVQ $0, DX		// c = 0
 	JMP E2
 
 L2:	MOVQ (R8)(BX*8), AX
@@ -47,9 +47,9 @@ L2:	MOVQ (R8)(BX*8), AX
 	SBBQ (R9)(BX*8), AX
 	RCLQ $1, DX
 	MOVQ AX, (R10)(BX*8)
-	ADDL $1, BX         // i++
+	ADDL $1, BX		// i++
 
-E2:	CMPQ BX, R11        // i < n
+E2:	CMPQ BX, R11		// i < n
 	JL L2
 
 	MOVQ DX, c+32(FP)
@@ -60,18 +60,18 @@ E2:	CMPQ BX, R11        // i < n
 TEXT ·addVW(SB),7,$0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
-	MOVQ y+16(FP), AX   // c = y
+	MOVQ y+16(FP), AX	// c = y
 	MOVL n+24(FP), R11
-	MOVQ $0, BX         // i = 0
+	MOVQ $0, BX		// i = 0
 	JMP E3
 
 L3:	ADDQ (R8)(BX*8), AX
 	MOVQ AX, (R10)(BX*8)
 	RCLQ $1, AX
 	ANDQ $1, AX
-	ADDL $1, BX         // i++
+	ADDL $1, BX		// i++
 
-E3:	CMPQ BX, R11        // i < n
+E3:	CMPQ BX, R11		// i < n
 	JL L3
 
 	MOVQ AX, c+32(FP)
@@ -82,9 +82,9 @@ E3:	CMPQ BX, R11        // i < n
 TEXT ·subVW(SB),7,$0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
-	MOVQ y+16(FP), AX   // c = y
+	MOVQ y+16(FP), AX	// c = y
 	MOVL n+24(FP), R11
-	MOVQ $0, BX         // i = 0
+	MOVQ $0, BX		// i = 0
 	JMP E4
 
 L4:	MOVQ (R8)(BX*8), DX	// TODO(gri) is there a reverse SUBQ?
@@ -92,9 +92,9 @@ L4:	MOVQ (R8)(BX*8), DX	// TODO(gri) is there a reverse SUBQ?
 	MOVQ DX, (R10)(BX*8)
 	RCLQ $1, AX
 	ANDQ $1, AX
-	ADDL $1, BX          // i++
+	ADDL $1, BX		// i++
 
-E4:	CMPQ BX, R11         // i < n
+E4:	CMPQ BX, R11		// i < n
 	JL L4
 
 	MOVQ AX, c+32(FP)
@@ -103,47 +103,93 @@ E4:	CMPQ BX, R11         // i < n
 
 // func shlVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shlVW(SB),7,$0
+	MOVL n+24(FP), BX	// i = n
+	SUBL $1, BX		// i--
+	JL X8b			// i < 0	(n <= 0)
+
+	// n > 0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
 	MOVQ s+16(FP), CX
-	MOVL n+24(FP), R11
-	MOVQ $0, AX         // c = 0
-	MOVQ $0, BX         // i = 0
-	JMP E8
+	MOVQ (R8)(BX*8), AX	// w1 = x[n-1]
+	MOVQ $0, DX
+	SHLQ CX, DX:AX		// w1>>ŝ
+	MOVQ DX, c+32(FP)
 
-L8:	MOVQ (R8)(BX*8), DX
-	MOVQ DX, R12
-	SHLQ CX, DX:AX
-	MOVQ DX, (R10)(BX*8)
-	MOVQ R12, AX
-	ADDL $1, BX          // i++
+	CMPL BX, $0
+	JLE X8a			// i <= 0
 
-E8:	CMPQ BX, R11         // i < n
-	JL L8
+	// i > 0
+L8:	MOVQ AX, DX		// w = w1
+	MOVQ -8(R8)(BX*8), AX	// w1 = x[i-1]
+	SHLQ CX, DX:AX		// w<<s | w1>>ŝ
+	MOVQ DX, (R10)(BX*8)	// z[i] = w<<s | w1>>ŝ
+	SUBL $1, BX		// i--
+	JG L8			// i > 0
 
-	MOVQ $0, DX
-	SHLQ CX, DX:AX
-	MOVQ DX, c+32(FP)
+	// i <= 0
+X8a:	SHLQ CX, AX		// w1<<s
+	MOVQ AX, (R10)		// z[0] = w1<<s
+	RET
+
+X8b:	MOVQ $0, c+32(FP)
 	RET
 
 
 // func shrVW(z, x *Word, s Word, n int) (c Word)
 TEXT ·shrVW(SB),7,$0
+	MOVL n+24(FP), R11
+	SUBL $1, R11		// n--
+	JL X9b			// n < 0	(n <= 0)
+
+	// n > 0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
 	MOVQ s+16(FP), CX
-	MOVL n+24(FP), BX   // i = n
-	MOVQ $0, AX         // c = 0
+	MOVQ (R8), AX		// w1 = x[0]
+	MOVQ $0, DX
+	SHRQ CX, DX:AX		// w1<<ŝ
+	MOVQ DX, c+32(FP)
+
+	MOVQ $0, BX		// i = 0
 	JMP E9
 
-L9:	MOVQ (R8)(BX*8), DX
+	// i < n-1
+L9:	MOVQ AX, DX		// w = w1
+	MOVQ 8(R8)(BX*8), AX	// w1 = x[i+1]
+	SHRQ CX, DX:AX		// w>>s | w1<<ŝ
+	MOVQ DX, (R10)(BX*8)	// z[i] = w>>s | w1<<ŝ
+	ADDL $1, BX		// i++
+	
+E9:	CMPQ BX, R11
+	JL L9			// i < n-1
+
+	// i >= n-1
+X9a:	SHRQ CX, AX		// w1>>s
+	MOVQ AX, (R10)(R11*8)	// z[n-1] = w1>>s
+	RET
+
+X9b:	MOVQ $0, c+32(FP)
+	RET
+
+
+// func shrVW(z, x *Word, s Word, n int) (c Word)
+TEXT ·shrVW_(SB),7,$0
+	MOVQ z+0(FP), R10
+	MOVQ x+8(FP), R8
+	MOVQ s+16(FP), CX
+	MOVL n+24(FP), BX	// i = n
+	MOVQ $0, AX		// c = 0
+	JMP E9_
+
+L9_:	MOVQ (R8)(BX*8), DX
 	MOVQ DX, R12
 	SHRQ CX, DX:AX
 	MOVQ DX, (R10)(BX*8)
 	MOVQ R12, AX
 
-E9:	SUBL $1, BX         // i--
-	JGE L9
+E9_:	SUBL $1, BX		// i--
+	JGE L9_
 
 	MOVQ $0, DX
 	SHRQ CX, DX:AX
@@ -156,9 +202,9 @@ TEXT ·mulAddVWW(SB),7,$0
 	MOVQ z+0(FP), R10
 	MOVQ x+8(FP), R8
 	MOVQ y+16(FP), R9
-	MOVQ r+24(FP), CX   // c = r
+	MOVQ r+24(FP), CX	// c = r
 	MOVL n+32(FP), R11
-	MOVQ $0, BX         // i = 0
+	MOVQ $0, BX		// i = 0
 	JMP E5
 
 L5:	MOVQ (R8)(BX*8), AX
@@ -167,9 +213,9 @@ L5:	MOVQ (R8)(BX*8), AX
 	ADCQ $0, DX
 	MOVQ AX, (R10)(BX*8)
 	MOVQ DX, CX
-	ADDL $1, BX         // i++
+	ADDL $1, BX		// i++
 
-E5:	CMPQ BX, R11        // i < n
+E5:	CMPQ BX, R11		// i < n
 	JL L5
 
 	MOVQ CX, c+40(FP)
@@ -182,8 +228,8 @@ TEXT ·addMulVVW(SB),7,$0
 	MOVQ x+8(FP), R8
 	MOVQ y+16(FP), R9
 	MOVL n+24(FP), R11
-	MOVQ $0, BX         // i = 0
-	MOVQ $0, CX         // c = 0
+	MOVQ $0, BX		// i = 0
+	MOVQ $0, CX		// c = 0
 	JMP E6
 
 L6:	MOVQ (R8)(BX*8), AX
@@ -194,9 +240,9 @@ L6:	MOVQ (R8)(BX*8), AX
 	ADCQ $0, DX
 	MOVQ AX, (R10)(BX*8)
 	MOVQ DX, CX
-	ADDL $1, BX         // i++
+	ADDL $1, BX		// i++
 
-E6:	CMPQ BX, R11        // i < n
+E6:	CMPQ BX, R11		// i < n
 	JL L6
 
 	MOVQ CX, c+32(FP)
@@ -206,18 +252,18 @@ E6:	CMPQ BX, R11        // i < n
 // divWVW(z* Word, xn Word, x *Word, y Word, n int) (r Word)
 TEXT ·divWVW(SB),7,$0
 	MOVQ z+0(FP), R10
-	MOVQ xn+8(FP), DX   // r = xn
+	MOVQ xn+8(FP), DX	// r = xn
 	MOVQ x+16(FP), R8
 	MOVQ y+24(FP), R9
-	MOVL n+32(FP), BX   // i = n
+	MOVL n+32(FP), BX	// i = n
 	JMP E7
 
 L7:	MOVQ (R8)(BX*8), AX
 	DIVQ R9
 	MOVQ AX, (R10)(BX*8)
 
-E7:	SUBL $1, BX         // i--
-	JGE L7              // i >= 0
+E7:	SUBL $1, BX		// i--
+	JGE L7			// i >= 0
 
 	MOVQ DX, r+40(FP)
 	RET
diff --git a/src/pkg/big/int.go b/src/pkg/big/int.go
index 238292478..3e3d677e3 100755
--- a/src/pkg/big/int.go
+++ b/src/pkg/big/int.go
@@ -126,7 +126,7 @@ func (z *Int) Rem(x, y *Int) *Int {
 // QuoRem implements T-division and modulus (like Go):
 //
 //	q = x/y      with the result truncated to zero
-//      r = x - y*q
+//	r = x - y*q
 //
 // (See Daan Leijen, ``Division and Modulus for Computer Scientists''.)
 //
@@ -183,7 +183,7 @@ func (z *Int) Mod(x, y *Int) *Int {
 // DivMod implements Euclidian division and modulus (unlike Go):
 //
 //	q = x div y  such that
-//      m = x - y*q  with 0 <= m < |q|
+//	m = x - y*q  with 0 <= m < |q|
 //
 // (See Raymond T. Boute, ``The Euclidian definition of the functions
 // div and mod''. ACM Transactions on Programming Languages and
diff --git a/src/pkg/big/nat.go b/src/pkg/big/nat.go
index 1cad23777..023501cac 100755
--- a/src/pkg/big/nat.go
+++ b/src/pkg/big/nat.go
@@ -60,6 +60,11 @@ func (z nat) norm() nat {
 }
 
 
+// TODO(gri) Consider changing "make" such that is does not reserve space
+//           for a potential carry; instead callers must provide the correct
+//           m (+1). Should lead to clearer code and shorter allocations on
+//           average.
+
 func (z nat) make(m int) nat {
 	if cap(z) > m {
 		return z[0:m] // reuse z - has at least one extra word for a carry, if any
@@ -219,11 +224,7 @@ func (z nat) mulAddWW(x nat, y, r Word) nat {
 // basicMul multiplies x and y and leaves the result in z.
 // The (non-normalized) result is placed in z[0 : len(x) + len(y)].
 func basicMul(z, x, y nat) {
-	// initialize z
-	for i := range z[0 : len(x)+len(y)] {
-		z[i] = 0
-	}
-	// multiply
+	z[0 : len(x)+len(y)].clear() // initialize z
 	for i, d := range y {
 		if d != 0 {
 			z[len(x)+i] = addMulVVW(&z[i], &x[0], d, len(x))
@@ -534,28 +535,26 @@ func (z nat) div(z2, u, v nat) (q, r nat) {
 
 
 // q = (uIn-r)/v, with 0 <= r < y
+// Uses z as storage for q, and u as storage for r if possible.
 // See Knuth, Volume 2, section 4.3.1, Algorithm D.
 // Preconditions:
 //    len(v) >= 2
 //    len(uIn) >= len(v)
-func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
+func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
 	n := len(v)
-	m := len(uIn) - len(v)
+	m := len(uIn) - n
 
-	var u nat
-	if z2 == nil || &z2[0] == &uIn[0] {
-		u = u.make(len(uIn) + 1).clear() // uIn is an alias for z2
-	} else {
-		u = z2.make(len(uIn) + 1).clear()
-	}
-	qhatv := make(nat, len(v)+1)
 	q = z.make(m + 1)
+	qhatv := make(nat, n+1)
+	if alias(u, uIn) {
+		u = nil // u is an alias for uIn - cannot reuse
+	}
+	u = u.make(len(uIn) + 1).clear()
 
 	// D1.
-	shift := leadingZeros(v[n-1])
-	v.shiftLeftDeprecated(v, shift)
-	u.shiftLeftDeprecated(uIn, shift)
-	u[len(uIn)] = uIn[len(uIn)-1] >> (_W - shift)
+	shift := Word(leadingZeros(v[n-1]))
+	shlVW(&v[0], &v[0], shift, n)
+	u[len(uIn)] = shlVW(&u[0], &uIn[0], shift, len(uIn))
 
 	// D2.
 	for j := m; j >= 0; j-- {
@@ -583,12 +582,12 @@ func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
 		}
 
 		// D4.
-		qhatv[len(v)] = mulAddVWW(&qhatv[0], &v[0], qhat, 0, len(v))
+		qhatv[n] = mulAddVWW(&qhatv[0], &v[0], qhat, 0, n)
 
 		c := subVV(&u[j], &u[j], &qhatv[0], len(qhatv))
 		if c != 0 {
-			c := addVV(&u[j], &u[j], &v[0], len(v))
-			u[j+len(v)] += c
+			c := addVV(&u[j], &u[j], &v[0], n)
+			u[j+n] += c
 			qhat--
 		}
 
@@ -596,8 +595,8 @@ func (z nat) divLarge(z2, uIn, v nat) (q, r nat) {
 	}
 
 	q = q.norm()
-	u.shiftRightDeprecated(u, shift)
-	v.shiftRightDeprecated(v, shift)
+	shrVW(&u[0], &u[0], shift, len(u))
+	shrVW(&v[0], &v[0], shift, n)
 	r = u.norm()
 
 	return q, r
@@ -755,15 +754,10 @@ func (z nat) shl(x nat, s uint) nat {
 	}
 	// m > 0
 
-	// determine if z can be reused
-	// TODO(gri) change shlVW so we don't need this
-	if alias(z, x) {
-		z = nil // z is an alias for x - cannot reuse
-	}
-
 	n := m + int(s/_W)
 	z = z.make(n + 1)
 	z[n] = shlVW(&z[n-m], &x[0], Word(s%_W), m)
+	z[0 : n-m].clear()
 
 	return z.norm()
 }
@@ -778,12 +772,6 @@ func (z nat) shr(x nat, s uint) nat {
 	}
 	// n > 0
 
-	// determine if z can be reused
-	// TODO(gri) change shrVW so we don't need this
-	if alias(z, x) {
-		z = nil // z is an alias for x - cannot reuse
-	}
-
 	z = z.make(n)
 	shrVW(&z[0], &x[m-n], Word(s%_W), n)
 
@@ -791,48 +779,6 @@ func (z nat) shr(x nat, s uint) nat {
 }
 
 
-// TODO(gri) Remove these shift functions once shlVW and shrVW can be
-//           used directly in divLarge and powersOfTwoDecompose
-//
-// To avoid losing the top n bits, z should be sized so that
-// len(z) == len(x) + 1.
-func (z nat) shiftLeftDeprecated(x nat, n uint) nat {
-	if len(x) == 0 {
-		return x
-	}
-
-	ñ := _W - n
-	m := x[len(x)-1]
-	if len(z) > len(x) {
-		z[len(x)] = m >> ñ
-	}
-	for i := len(x) - 1; i >= 1; i-- {
-		y := x[i-1]
-		z[i] = m<<n | y>>ñ
-		m = y
-	}
-	z[0] = m << n
-	return z
-}
-
-
-func (z nat) shiftRightDeprecated(x nat, n uint) nat {
-	if len(x) == 0 {
-		return x
-	}
-
-	ñ := _W - n
-	m := x[0]
-	for i := 0; i < len(x)-1; i++ {
-		y := x[i+1]
-		z[i] = m>>n | y<<ñ
-		m = y
-	}
-	z[len(x)-1] = m >> n
-	return z
-}
-
-
 func (z nat) and(x, y nat) nat {
 	m := len(x)
 	n := len(y)
@@ -936,7 +882,7 @@ func (n nat) powersOfTwoDecompose() (q nat, k Word) {
 	x := trailingZeroBits(n[zeroWords])
 
 	q = q.make(len(n) - zeroWords)
-	q.shiftRightDeprecated(n[zeroWords:], uint(x))
+	shrVW(&q[0], &n[zeroWords], Word(x), len(q))
 	q = q.norm()
 
 	k = Word(_W*zeroWords + x)