diff --git a/src/pkg/math/big/arith.go b/src/pkg/math/big/arith.go
index f30951ef0f..5a30d3cf3a 100644
--- a/src/pkg/math/big/arith.go
+++ b/src/pkg/math/big/arith.go
@@ -79,7 +79,7 @@ func mulAddWWW_g(x, y, c Word) (z1, z0 Word) {
 }
 
 // Length of x in bits.
-func bitLen(x Word) (n int) {
+func bitLen_g(x Word) (n int) {
 	for ; x >= 0x8000; x >>= 16 {
 		n += 16
 	}
diff --git a/src/pkg/math/big/arith_386.s b/src/pkg/math/big/arith_386.s
index 07c07b02cc..f1262c6514 100644
--- a/src/pkg/math/big/arith_386.s
+++ b/src/pkg/math/big/arith_386.s
@@ -245,7 +245,7 @@ E6:	CMPL BX, $0		// i < 0
 	RET
 
 
-// divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
+// func divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
 TEXT ·divWVW(SB),7,$0
 	MOVL z+0(FP), DI
 	MOVL xn+12(FP), DX	// r = xn
@@ -263,3 +263,14 @@ E7:	SUBL $1, BX		// i--
 
 	MOVL DX, r+32(FP)
 	RET
+
+// func bitLen(x Word) (n int)
+TEXT ·bitLen(SB),7,$0
+	BSRL x+0(FP), AX
+	JZ Z1
+	INCL AX
+	MOVL AX, n+4(FP)
+	RET
+
+Z1:	MOVL $0, n+4(FP)
+	RET
diff --git a/src/pkg/math/big/arith_amd64.s b/src/pkg/math/big/arith_amd64.s
index 89b65f38a1..088f724704 100644
--- a/src/pkg/math/big/arith_amd64.s
+++ b/src/pkg/math/big/arith_amd64.s
@@ -243,7 +243,7 @@ E6:	CMPQ BX, R11		// i < n
 	RET
 
 
-// divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
+// func divWVW(z []Word, xn Word, x []Word, y Word) (r Word)
 TEXT ·divWVW(SB),7,$0
 	MOVQ z+0(FP), R10
 	MOVQ xn+16(FP), DX	// r = xn
@@ -261,3 +261,14 @@ E7:	SUBL $1, BX		// i--
 
 	MOVQ DX, r+48(FP)
 	RET
+
+// func bitLen(x Word) (n int)
+TEXT ·bitLen(SB),7,$0
+	BSRQ x+0(FP), AX
+	JZ Z1
+	INCQ AX
+	MOVQ AX, n+8(FP)
+	RET
+
+Z1:	MOVQ $0, n+8(FP)
+	RET
diff --git a/src/pkg/math/big/arith_arm.s b/src/pkg/math/big/arith_arm.s
index 60abe6eaa9..dbf3360b58 100644
--- a/src/pkg/math/big/arith_arm.s
+++ b/src/pkg/math/big/arith_arm.s
@@ -290,7 +290,7 @@ E9:
 	RET
 
 
-// divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
+// func divWVW(z* Word, xn Word, x []Word, y Word) (r Word)
 TEXT ·divWVW(SB),7,$0
 	// ARM has no multiword division, so use portable code.
 	B ·divWVW_g(SB)
@@ -310,3 +310,12 @@ TEXT ·mulWW(SB),7,$0
 	MOVW	R4, z1+8(FP)
 	MOVW	R3, z0+12(FP)
 	RET
+
+// func bitLen(x Word) (n int)
+TEXT ·bitLen(SB),7,$0
+	MOVW	x+0(FP), R0
+	WORD	$0xe16f0f10 // CLZ R0, R0  (count leading zeros)
+	MOVW	$32, R1
+	SUB.S	R0, R1
+	MOVW	R1, n+4(FP)
+	RET
diff --git a/src/pkg/math/big/arith_decl.go b/src/pkg/math/big/arith_decl.go
index 95fcd8b94b..068cc8d938 100644
--- a/src/pkg/math/big/arith_decl.go
+++ b/src/pkg/math/big/arith_decl.go
@@ -16,3 +16,4 @@ 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)
+func bitLen(x Word) (n int)
diff --git a/src/pkg/math/big/arith_test.go b/src/pkg/math/big/arith_test.go
index 106cd92d85..cd02ba3674 100644
--- a/src/pkg/math/big/arith_test.go
+++ b/src/pkg/math/big/arith_test.go
@@ -334,6 +334,29 @@ func TestMulAddWWW(t *testing.T) {
 	}
 }
 
+func TestWordBitLen(t *testing.T) {
+	// Test every possible output of bitLen with the high bit set
+	// and then with all bits below max set
+	z := bitLen(0)
+	if z != 0 {
+		t.Errorf("0 got %d want 0", z)
+	}
+	x := Word(1) // Will be ...00010000...
+	y := Word(1) // Will be ...00011111...
+	for i := 1; i <= _W; i++ {
+		z = bitLen(x)
+		if z != i {
+			t.Errorf("%x got %d want %d", x, z, i)
+		}
+		z = bitLen(y)
+		if z != i {
+			t.Errorf("%x got %d want %d", y, z, i)
+		}
+		x <<= 1
+		y = (y << 1) | 0x1
+	}
+}
+
 // runs b.N iterations of bitLen called on a Word containing (1 << nbits)-1.
 func benchmarkBitLenN(b *testing.B, nbits uint) {
 	testword := Word((uint64(1) << nbits) - 1)