From: Robert Griesemer <gri@golang.org>
Date: Wed, 16 Dec 2015 18:11:26 +0000 (-0800)
Subject: cmd/compile: re-vendor math/big so we use latest version in compiler
X-Git-Tag: go1.6beta1~64
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=65cd1ba682a1588e7e6e6903695772216881b3f0;p=gostls13.git

cmd/compile: re-vendor math/big so we use latest version in compiler

This simply copies the current version of math/big into the
compiler directory. The change was created automatically by
running cmd/compile/internal/big/vendor.bash. No other manual
changes.

Change-Id: Ica225d196b3ac10dfd9d4dc1e4e4ef0b22812ff9
Reviewed-on: https://go-review.googlesource.com/17900
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---

diff --git a/src/cmd/compile/internal/big/decimal.go b/src/cmd/compile/internal/big/decimal.go
index 2595e5f8c1..2c0c9daebc 100644
--- a/src/cmd/compile/internal/big/decimal.go
+++ b/src/cmd/compile/internal/big/decimal.go
@@ -20,7 +20,7 @@
 package big
 
 // A decimal represents an unsigned floating-point number in decimal representation.
-// The value of a non-zero decimal x is x.mant * 10 ** x.exp with 0.5 <= x.mant < 1,
+// The value of a non-zero decimal d is d.mant * 10**d.exp with 0.5 <= d.mant < 1,
 // with the most-significant mantissa digit at index 0. For the zero decimal, the
 // mantissa length and exponent are 0.
 // The zero value for decimal represents a ready-to-use 0.0.
@@ -29,6 +29,14 @@ type decimal struct {
 	exp  int    // exponent
 }
 
+// at returns the i'th mantissa digit, starting with the most significant digit at 0.
+func (d *decimal) at(i int) byte {
+	if 0 <= i && i < len(d.mant) {
+		return d.mant[i]
+	}
+	return '0'
+}
+
 // Maximum shift amount that can be done in one pass without overflow.
 // A Word has _W bits and (1<<maxShift - 1)*10 + 9 must fit into Word.
 const maxShift = _W - 4
@@ -72,7 +80,7 @@ func (x *decimal) init(m nat, shift int) {
 	}
 
 	// Convert mantissa into decimal representation.
-	s := m.decimalString() // TODO(gri) avoid string conversion here
+	s := m.utoa(10)
 	n := len(s)
 	x.exp = n
 	// Trim trailing zeros; instead the exponent is tracking
@@ -92,12 +100,6 @@ func (x *decimal) init(m nat, shift int) {
 	}
 }
 
-// Possibly optimization: The current implementation of nat.string takes
-// a charset argument. When a right shift is needed, we could provide
-// "\x00\x01...\x09" instead of "012..9" (as in nat.decimalString) and
-// avoid the repeated +'0' and -'0' operations in decimal.shr (and do a
-// single +'0' pass at the end).
-
 // shr implements x >> s, for s <= maxShift.
 func shr(x *decimal, s uint) {
 	// Division by 1<<s using shift-and-subtract algorithm.
diff --git a/src/cmd/compile/internal/big/decimal_test.go b/src/cmd/compile/internal/big/decimal_test.go
index 81e022a47d..15bdb181e7 100644
--- a/src/cmd/compile/internal/big/decimal_test.go
+++ b/src/cmd/compile/internal/big/decimal_test.go
@@ -104,3 +104,13 @@ func TestDecimalRounding(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDecimalConversion(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for shift := -100; shift <= +100; shift++ {
+			var d decimal
+			d.init(natOne, shift)
+			d.String()
+		}
+	}
+}
diff --git a/src/cmd/compile/internal/big/float.go b/src/cmd/compile/internal/big/float.go
index 1f8edee5dd..b1c748c9a5 100644
--- a/src/cmd/compile/internal/big/float.go
+++ b/src/cmd/compile/internal/big/float.go
@@ -124,7 +124,7 @@ const (
 // rounding error is described by the Float's Accuracy.
 type RoundingMode byte
 
-// The following rounding modes are supported.
+// These constants define supported rounding modes.
 const (
 	ToNearestEven RoundingMode = iota // == IEEE 754-2008 roundTiesToEven
 	ToNearestAway                     // == IEEE 754-2008 roundTiesToAway
@@ -298,7 +298,7 @@ func (z *Float) setExpAndRound(exp int64, sbit uint) {
 // not require 0.5 <= |mant| < 1.0. Specifically:
 //
 //	mant := new(Float)
-//	new(Float).SetMantExp(mant, x.SetMantExp(mant)).Cmp(x).Eql() is true
+//	new(Float).SetMantExp(mant, x.MantExp(mant)).Cmp(x) == 0
 //
 // Special cases are:
 //
@@ -1123,7 +1123,7 @@ func (x *Float) Int(z *Int) (*Int, Accuracy) {
 
 // Rat returns the rational number corresponding to x;
 // or nil if x is an infinity.
-// The result is Exact is x is not an Inf.
+// The result is Exact if x is not an Inf.
 // If a non-nil *Rat argument z is provided, Rat stores
 // the result in z instead of allocating a new Rat.
 func (x *Float) Rat(z *Rat) (*Rat, Accuracy) {
diff --git a/src/cmd/compile/internal/big/floatconv.go b/src/cmd/compile/internal/big/floatconv.go
index 0e8b7b649e..37d5c06a6f 100644
--- a/src/cmd/compile/internal/big/floatconv.go
+++ b/src/cmd/compile/internal/big/floatconv.go
@@ -72,37 +72,46 @@ func (z *Float) scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	// ebase**exp. Finally, mantissa normalization (shift left) requires
 	// a correcting multiplication by 2**(-shiftcount). Multiplications
 	// are commutative, so we can apply them in any order as long as there
-	// is no loss of precision. We only have powers of 2 and 10; keep
-	// track via separate exponents exp2 and exp10.
+	// is no loss of precision. We only have powers of 2 and 10, and
+	// we split powers of 10 into the product of the same powers of
+	// 2 and 5. This reduces the size of the multiplication factor
+	// needed for base-10 exponents.
 
-	// normalize mantissa and get initial binary exponent
-	var exp2 = int64(len(z.mant))*_W - fnorm(z.mant)
+	// normalize mantissa and determine initial exponent contributions
+	exp2 := int64(len(z.mant))*_W - fnorm(z.mant)
+	exp5 := int64(0)
 
 	// determine binary or decimal exponent contribution of decimal point
-	var exp10 int64
 	if fcount < 0 {
 		// The mantissa has a "decimal" point ddd.dddd; and
 		// -fcount is the number of digits to the right of '.'.
 		// Adjust relevant exponent accodingly.
+		d := int64(fcount)
 		switch b {
-		case 16:
-			fcount *= 4 // hexadecimal digits are 4 bits each
-			fallthrough
+		case 10:
+			exp5 = d
+			fallthrough // 10**e == 5**e * 2**e
 		case 2:
-			exp2 += int64(fcount)
-		default: // b == 10
-			exp10 = int64(fcount)
+			exp2 += d
+		case 16:
+			exp2 += d * 4 // hexadecimal digits are 4 bits each
+		default:
+			panic("unexpected mantissa base")
 		}
-		// we don't need fcount anymore
+		// fcount consumed - not needed anymore
 	}
 
 	// take actual exponent into account
-	if ebase == 2 {
+	switch ebase {
+	case 10:
+		exp5 += exp
+		fallthrough
+	case 2:
 		exp2 += exp
-	} else { // ebase == 10
-		exp10 += exp
+	default:
+		panic("unexpected exponent base")
 	}
-	// we don't need exp anymore
+	// exp consumed - not needed anymore
 
 	// apply 2**exp2
 	if MinExp <= exp2 && exp2 <= MaxExp {
@@ -115,49 +124,76 @@ func (z *Float) scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 		return
 	}
 
-	if exp10 == 0 {
-		// no decimal exponent to consider
+	if exp5 == 0 {
+		// no decimal exponent contribution
 		z.round(0)
 		return
 	}
-	// exp10 != 0
+	// exp5 != 0
 
-	// apply 10**exp10
+	// apply 5**exp5
 	p := new(Float).SetPrec(z.Prec() + 64) // use more bits for p -- TODO(gri) what is the right number?
-	if exp10 < 0 {
-		z.Quo(z, p.pow10(-exp10))
+	if exp5 < 0 {
+		z.Quo(z, p.pow5(uint64(-exp5)))
 	} else {
-		z.Mul(z, p.pow10(exp10))
+		z.Mul(z, p.pow5(uint64(exp5)))
 	}
 
 	return
 }
 
-// These powers of 10 can be represented exactly as a float64.
-var pow10tab = [...]float64{
-	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9,
-	1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19,
+// These powers of 5 fit into a uint64.
+//
+//	for p, q := uint64(0), uint64(1); p < q; p, q = q, q*5 {
+//		fmt.Println(q)
+//	}
+//
+var pow5tab = [...]uint64{
+	1,
+	5,
+	25,
+	125,
+	625,
+	3125,
+	15625,
+	78125,
+	390625,
+	1953125,
+	9765625,
+	48828125,
+	244140625,
+	1220703125,
+	6103515625,
+	30517578125,
+	152587890625,
+	762939453125,
+	3814697265625,
+	19073486328125,
+	95367431640625,
+	476837158203125,
+	2384185791015625,
+	11920928955078125,
+	59604644775390625,
+	298023223876953125,
+	1490116119384765625,
+	7450580596923828125,
 }
 
-// pow10 sets z to 10**n and returns z.
+// pow5 sets z to 5**n and returns z.
 // n must not be negative.
-func (z *Float) pow10(n int64) *Float {
-	if n < 0 {
-		panic("pow10 called with negative argument")
-	}
-
-	const m = int64(len(pow10tab) - 1)
+func (z *Float) pow5(n uint64) *Float {
+	const m = uint64(len(pow5tab) - 1)
 	if n <= m {
-		return z.SetFloat64(pow10tab[n])
+		return z.SetUint64(pow5tab[n])
 	}
 	// n > m
 
-	z.SetFloat64(pow10tab[m])
+	z.SetUint64(pow5tab[m])
 	n -= m
 
 	// use more bits for f than for z
 	// TODO(gri) what is the right number?
-	f := new(Float).SetPrec(z.Prec() + 64).SetInt64(10)
+	f := new(Float).SetPrec(z.Prec() + 64).SetUint64(5)
 
 	for n > 0 {
 		if n&1 != 0 {
diff --git a/src/cmd/compile/internal/big/floatconv_test.go b/src/cmd/compile/internal/big/floatconv_test.go
index 156e1af300..b6f9993608 100644
--- a/src/cmd/compile/internal/big/floatconv_test.go
+++ b/src/cmd/compile/internal/big/floatconv_test.go
@@ -139,6 +139,8 @@ func TestFloatSetFloat64String(t *testing.T) {
 	}
 }
 
+func fdiv(a, b float64) float64 { return a / b }
+
 const (
 	below1e23 = 99999999999999974834176
 	above1e23 = 100000000000000008388608
@@ -187,11 +189,11 @@ func TestFloat64Text(t *testing.T) {
 		{1, 'e', 5, "1.00000e+00"},
 		{1, 'f', 5, "1.00000"},
 		{1, 'g', 5, "1"},
-		// {1, 'g', -1, "1"},
-		// {20, 'g', -1, "20"},
-		// {1234567.8, 'g', -1, "1.2345678e+06"},
-		// {200000, 'g', -1, "200000"},
-		// {2000000, 'g', -1, "2e+06"},
+		{1, 'g', -1, "1"},
+		{20, 'g', -1, "20"},
+		{1234567.8, 'g', -1, "1.2345678e+06"},
+		{200000, 'g', -1, "200000"},
+		{2000000, 'g', -1, "2e+06"},
 
 		// g conversion and zero suppression
 		{400, 'g', 2, "4e+02"},
@@ -207,22 +209,22 @@ func TestFloat64Text(t *testing.T) {
 		{0, 'e', 5, "0.00000e+00"},
 		{0, 'f', 5, "0.00000"},
 		{0, 'g', 5, "0"},
-		// {0, 'g', -1, "0"},
+		{0, 'g', -1, "0"},
 
 		{-1, 'e', 5, "-1.00000e+00"},
 		{-1, 'f', 5, "-1.00000"},
 		{-1, 'g', 5, "-1"},
-		// {-1, 'g', -1, "-1"},
+		{-1, 'g', -1, "-1"},
 
 		{12, 'e', 5, "1.20000e+01"},
 		{12, 'f', 5, "12.00000"},
 		{12, 'g', 5, "12"},
-		// {12, 'g', -1, "12"},
+		{12, 'g', -1, "12"},
 
 		{123456700, 'e', 5, "1.23457e+08"},
 		{123456700, 'f', 5, "123456700.00000"},
 		{123456700, 'g', 5, "1.2346e+08"},
-		// {123456700, 'g', -1, "1.234567e+08"},
+		{123456700, 'g', -1, "1.234567e+08"},
 
 		{1.2345e6, 'e', 5, "1.23450e+06"},
 		{1.2345e6, 'f', 5, "1234500.00000"},
@@ -232,36 +234,38 @@ func TestFloat64Text(t *testing.T) {
 		{1e23, 'f', 17, "99999999999999991611392.00000000000000000"},
 		{1e23, 'g', 17, "9.9999999999999992e+22"},
 
-		// {1e23, 'e', -1, "1e+23"},
-		// {1e23, 'f', -1, "100000000000000000000000"},
-		// {1e23, 'g', -1, "1e+23"},
+		{1e23, 'e', -1, "1e+23"},
+		{1e23, 'f', -1, "100000000000000000000000"},
+		{1e23, 'g', -1, "1e+23"},
 
 		{below1e23, 'e', 17, "9.99999999999999748e+22"},
 		{below1e23, 'f', 17, "99999999999999974834176.00000000000000000"},
 		{below1e23, 'g', 17, "9.9999999999999975e+22"},
 
-		// {below1e23, 'e', -1, "9.999999999999997e+22"},
-		// {below1e23, 'f', -1, "99999999999999970000000"},
-		// {below1e23, 'g', -1, "9.999999999999997e+22"},
+		{below1e23, 'e', -1, "9.999999999999997e+22"},
+		{below1e23, 'f', -1, "99999999999999970000000"},
+		{below1e23, 'g', -1, "9.999999999999997e+22"},
 
 		{above1e23, 'e', 17, "1.00000000000000008e+23"},
 		{above1e23, 'f', 17, "100000000000000008388608.00000000000000000"},
-		// {above1e23, 'g', 17, "1.0000000000000001e+23"},
+		{above1e23, 'g', 17, "1.0000000000000001e+23"},
 
-		// {above1e23, 'e', -1, "1.0000000000000001e+23"},
-		// {above1e23, 'f', -1, "100000000000000010000000"},
-		// {above1e23, 'g', -1, "1.0000000000000001e+23"},
+		{above1e23, 'e', -1, "1.0000000000000001e+23"},
+		{above1e23, 'f', -1, "100000000000000010000000"},
+		{above1e23, 'g', -1, "1.0000000000000001e+23"},
 
-		// {fdiv(5e-304, 1e20), 'g', -1, "5e-324"},
-		// {fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"},
+		{5e-304 / 1e20, 'g', -1, "5e-324"},
+		{-5e-304 / 1e20, 'g', -1, "-5e-324"},
+		{fdiv(5e-304, 1e20), 'g', -1, "5e-324"},   // avoid constant arithmetic
+		{fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"}, // avoid constant arithmetic
 
-		// {32, 'g', -1, "32"},
-		// {32, 'g', 0, "3e+01"},
+		{32, 'g', -1, "32"},
+		{32, 'g', 0, "3e+01"},
 
-		// {100, 'x', -1, "%x"},
+		{100, 'x', -1, "%x"},
 
-		// {math.NaN(), 'g', -1, "NaN"},
-		// {-math.NaN(), 'g', -1, "NaN"},
+		// {math.NaN(), 'g', -1, "NaN"},  // Float doesn't support NaNs
+		// {-math.NaN(), 'g', -1, "NaN"}, // Float doesn't support NaNs
 		{math.Inf(0), 'g', -1, "+Inf"},
 		{math.Inf(-1), 'g', -1, "-Inf"},
 		{-math.Inf(0), 'g', -1, "-Inf"},
@@ -279,18 +283,24 @@ func TestFloat64Text(t *testing.T) {
 		{1.5, 'f', 0, "2"},
 
 		// http://www.exploringbinary.com/java-hangs-when-converting-2-2250738585072012e-308/
-		// {2.2250738585072012e-308, 'g', -1, "2.2250738585072014e-308"},
+		{2.2250738585072012e-308, 'g', -1, "2.2250738585072014e-308"},
 		// http://www.exploringbinary.com/php-hangs-on-numeric-value-2-2250738585072011e-308/
-		// {2.2250738585072011e-308, 'g', -1, "2.225073858507201e-308"},
+		{2.2250738585072011e-308, 'g', -1, "2.225073858507201e-308"},
 
 		// Issue 2625.
 		{383260575764816448, 'f', 0, "383260575764816448"},
-		// {383260575764816448, 'g', -1, "3.8326057576481645e+17"},
+		{383260575764816448, 'g', -1, "3.8326057576481645e+17"},
 	} {
-		f := new(Float).SetFloat64(test.x)
+		// The test cases are from the strconv package which tests float64 values.
+		// When formatting values with prec = -1 (shortest representation),
+		// the actually available mantissa precision matters.
+		// For denormalized values, that precision is < 53 (SetFloat64 default).
+		// Compute and set the actual precision explicitly.
+		f := new(Float).SetPrec(actualPrec(test.x)).SetFloat64(test.x)
 		got := f.Text(test.format, test.prec)
 		if got != test.want {
 			t.Errorf("%v: got %s; want %s", test, got, test.want)
+			continue
 		}
 
 		if test.format == 'b' && test.x == 0 {
@@ -308,6 +318,15 @@ func TestFloat64Text(t *testing.T) {
 	}
 }
 
+// actualPrec returns the number of actually used mantissa bits.
+func actualPrec(x float64) uint {
+	if bits := math.Float64bits(x); x != 0 && bits&(0x7ff<<52) == 0 {
+		// x is denormalized
+		return 64 - nlz64(bits&(1<<52-1))
+	}
+	return 53
+}
+
 func TestFloatText(t *testing.T) {
 	for _, test := range []struct {
 		x      string
@@ -367,10 +386,20 @@ func TestFloatText(t *testing.T) {
 
 		// make sure "stupid" exponents don't stall the machine
 		{"1e1000000", 64, 'p', 0, "0x.88b3a28a05eade3ap+3321929"},
+		{"1e646456992", 64, 'p', 0, "0x.e883a0c5c8c7c42ap+2147483644"},
+		{"1e646456993", 64, 'p', 0, "+Inf"},
 		{"1e1000000000", 64, 'p', 0, "+Inf"},
 		{"1e-1000000", 64, 'p', 0, "0x.efb4542cc8ca418ap-3321928"},
+		{"1e-646456993", 64, 'p', 0, "0x.e17c8956983d9d59p-2147483647"},
+		{"1e-646456994", 64, 'p', 0, "0"},
 		{"1e-1000000000", 64, 'p', 0, "0"},
 
+		// minimum and maximum values
+		{"1p2147483646", 64, 'p', 0, "0x.8p+2147483647"},
+		{"0x.8p2147483647", 64, 'p', 0, "0x.8p+2147483647"},
+		{"0x.8p-2147483647", 64, 'p', 0, "0x.8p-2147483647"},
+		{"1p-2147483649", 64, 'p', 0, "0x.8p-2147483648"},
+
 		// TODO(gri) need tests for actual large Floats
 
 		{"0", 53, 'b', 0, "0"},
@@ -438,9 +467,6 @@ func TestFloatFormat(t *testing.T) {
 		value  interface{} // float32, float64, or string (== 512bit *Float)
 		want   string
 	}{
-		// TODO(gri) uncomment the disabled 'g'/'G' formats
-		// 	     below once (*Float).Text supports prec < 0
-
 		// from fmt/fmt_test.go
 		{"%+.3e", 0.0, "+0.000e+00"},
 		{"%+.3e", 1.0, "+1.000e+00"},
@@ -471,9 +497,9 @@ func TestFloatFormat(t *testing.T) {
 		{"%f", 1234.5678e-8, "0.000012"},
 		{"%f", -7.0, "-7.000000"},
 		{"%f", -1e-9, "-0.000000"},
-		// {"%g", 1234.5678e3, "1.2345678e+06"},
-		// {"%g", float32(1234.5678e3), "1.2345678e+06"},
-		// {"%g", 1234.5678e-8, "1.2345678e-05"},
+		{"%g", 1234.5678e3, "1.2345678e+06"},
+		{"%g", float32(1234.5678e3), "1.2345678e+06"},
+		{"%g", 1234.5678e-8, "1.2345678e-05"},
 		{"%g", -7.0, "-7"},
 		{"%g", -1e-9, "-1e-09"},
 		{"%g", float32(-1e-9), "-1e-09"},
@@ -482,9 +508,9 @@ func TestFloatFormat(t *testing.T) {
 		{"%E", 1234.5678e-8, "1.234568E-05"},
 		{"%E", -7.0, "-7.000000E+00"},
 		{"%E", -1e-9, "-1.000000E-09"},
-		// {"%G", 1234.5678e3, "1.2345678E+06"},
-		// {"%G", float32(1234.5678e3), "1.2345678E+06"},
-		// {"%G", 1234.5678e-8, "1.2345678E-05"},
+		{"%G", 1234.5678e3, "1.2345678E+06"},
+		{"%G", float32(1234.5678e3), "1.2345678E+06"},
+		{"%G", 1234.5678e-8, "1.2345678E-05"},
 		{"%G", -7.0, "-7"},
 		{"%G", -1e-9, "-1E-09"},
 		{"%G", float32(-1e-9), "-1E-09"},
@@ -500,9 +526,9 @@ func TestFloatFormat(t *testing.T) {
 		{"%-20f", 1.23456789e3, "1234.567890         "},
 		{"%20.8f", 1.23456789e3, "       1234.56789000"},
 		{"%20.8f", 1.23456789e-3, "          0.00123457"},
-		// {"%g", 1.23456789e3, "1234.56789"},
-		// {"%g", 1.23456789e-3, "0.00123456789"},
-		// {"%g", 1.23456789e20, "1.23456789e+20"},
+		{"%g", 1.23456789e3, "1234.56789"},
+		{"%g", 1.23456789e-3, "0.00123456789"},
+		{"%g", 1.23456789e20, "1.23456789e+20"},
 		{"%20e", math.Inf(1), "                +Inf"},
 		{"%-20f", math.Inf(-1), "-Inf                "},
 
@@ -541,7 +567,6 @@ func TestFloatFormat(t *testing.T) {
 		{"%v", -1e-9, "-1e-09"},
 		{"%v", float32(-1e-9), "-1e-09"},
 		{"%010v", 0.0, "0000000000"},
-		{"%010v", 0.0, "0000000000"},
 
 		// *Float cases
 		{"%.20f", "1e-20", "0.00000000000000000001"},
@@ -571,3 +596,67 @@ func TestFloatFormat(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkParseFloatSmallExp(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, s := range []string{
+			"1e0",
+			"1e-1",
+			"1e-2",
+			"1e-3",
+			"1e-4",
+			"1e-5",
+			"1e-10",
+			"1e-20",
+			"1e-50",
+			"1e1",
+			"1e2",
+			"1e3",
+			"1e4",
+			"1e5",
+			"1e10",
+			"1e20",
+			"1e50",
+		} {
+			var x Float
+			_, _, err := x.Parse(s, 0)
+			if err != nil {
+				b.Fatalf("%s: %v", s, err)
+			}
+		}
+	}
+}
+
+func BenchmarkParseFloatLargeExp(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, s := range []string{
+			"1e0",
+			"1e-10",
+			"1e-20",
+			"1e-30",
+			"1e-40",
+			"1e-50",
+			"1e-100",
+			"1e-500",
+			"1e-1000",
+			"1e-5000",
+			"1e-10000",
+			"1e10",
+			"1e20",
+			"1e30",
+			"1e40",
+			"1e50",
+			"1e100",
+			"1e500",
+			"1e1000",
+			"1e5000",
+			"1e10000",
+		} {
+			var x Float
+			_, _, err := x.Parse(s, 0)
+			if err != nil {
+				b.Fatalf("%s: %v", s, err)
+			}
+		}
+	}
+}
diff --git a/src/cmd/compile/internal/big/floatexample_test.go b/src/cmd/compile/internal/big/floatexample_test.go
index 6fd291c9c2..83c6bdacae 100644
--- a/src/cmd/compile/internal/big/floatexample_test.go
+++ b/src/cmd/compile/internal/big/floatexample_test.go
@@ -109,3 +109,33 @@ func ExampleFloat_Cmp() {
 	// +Inf   1.2    1
 	// +Inf  +Inf    0
 }
+
+func ExampleRoundingMode() {
+	operands := []float64{2.6, 2.5, 2.1, -2.1, -2.5, -2.6}
+
+	fmt.Print("   x")
+	for mode := big.ToNearestEven; mode <= big.ToPositiveInf; mode++ {
+		fmt.Printf("  %s", mode)
+	}
+	fmt.Println()
+
+	for _, f64 := range operands {
+		fmt.Printf("%4g", f64)
+		for mode := big.ToNearestEven; mode <= big.ToPositiveInf; mode++ {
+			// sample operands above require 2 bits to represent mantissa
+			// set binary precision to 2 to round them to integer values
+			f := new(big.Float).SetPrec(2).SetMode(mode).SetFloat64(f64)
+			fmt.Printf("  %*g", len(mode.String()), f)
+		}
+		fmt.Println()
+	}
+
+	// Output:
+	//    x  ToNearestEven  ToNearestAway  ToZero  AwayFromZero  ToNegativeInf  ToPositiveInf
+	//  2.6              3              3       2             3              2              3
+	//  2.5              2              3       2             3              2              3
+	//  2.1              2              2       2             3              2              3
+	// -2.1             -2             -2      -2            -3             -3             -2
+	// -2.5             -2             -3      -2            -3             -3             -2
+	// -2.6             -3             -3      -2            -3             -3             -2
+}
diff --git a/src/cmd/compile/internal/big/ftoa.go b/src/cmd/compile/internal/big/ftoa.go
index 5c5f2cea46..c5cdb5eb70 100644
--- a/src/cmd/compile/internal/big/ftoa.go
+++ b/src/cmd/compile/internal/big/ftoa.go
@@ -9,9 +9,9 @@
 package big
 
 import (
+	"bytes"
 	"fmt"
 	"strconv"
-	"strings"
 )
 
 // Text converts the floating-point number x to a string according
@@ -37,16 +37,16 @@ import (
 // printed by the 'e', 'E', 'f', 'g', and 'G' formats. For 'e', 'E', and 'f'
 // it is the number of digits after the decimal point. For 'g' and 'G' it is
 // the total number of digits. A negative precision selects the smallest
-// number of digits necessary to identify the value x uniquely.
+// number of decimal digits necessary to identify the value x uniquely using
+// x.Prec() mantissa bits.
 // The prec value is ignored for the 'b' or 'p' format.
-//
-// BUG(gri) Float.Text does not accept negative precisions (issue #10991).
 func (x *Float) Text(format byte, prec int) string {
 	const extra = 10 // TODO(gri) determine a good/better value here
 	return string(x.Append(make([]byte, 0, prec+extra), format, prec))
 }
 
 // String formats x like x.Text('g', 10).
+// (String must be called explicitly, Float.Format does not support %s verb.)
 func (x *Float) String() string {
 	return x.Text('g', 10)
 }
@@ -83,6 +83,7 @@ func (x *Float) Append(buf []byte, fmt byte, prec int) []byte {
 	// 1) convert Float to multiprecision decimal
 	var d decimal // == 0.0
 	if x.form == finite {
+		// x != 0
 		d.init(x.mant, int(x.exp)-x.mant.bitLen())
 	}
 
@@ -90,9 +91,7 @@ func (x *Float) Append(buf []byte, fmt byte, prec int) []byte {
 	shortest := false
 	if prec < 0 {
 		shortest = true
-		panic("unimplemented")
-		// TODO(gri) complete this
-		// roundShortest(&d, f.mant, int(f.exp))
+		roundShortest(&d, x)
 		// Precision for shortest representation mode.
 		switch fmt {
 		case 'e', 'E':
@@ -158,6 +157,80 @@ func (x *Float) Append(buf []byte, fmt byte, prec int) []byte {
 	return append(buf, '%', fmt)
 }
 
+func roundShortest(d *decimal, x *Float) {
+	// if the mantissa is zero, the number is zero - stop now
+	if len(d.mant) == 0 {
+		return
+	}
+
+	// Approach: All numbers in the interval [x - 1/2ulp, x + 1/2ulp]
+	// (possibly exclusive) round to x for the given precision of x.
+	// Compute the lower and upper bound in decimal form and find the
+	// shortest decimal number d such that lower <= d <= upper.
+
+	// TODO(gri) strconv/ftoa.do describes a shortcut in some cases.
+	// See if we can use it (in adjusted form) here as well.
+
+	// 1) Compute normalized mantissa mant and exponent exp for x such
+	// that the lsb of mant corresponds to 1/2 ulp for the precision of
+	// x (i.e., for mant we want x.prec + 1 bits).
+	mant := nat(nil).set(x.mant)
+	exp := int(x.exp) - mant.bitLen()
+	s := mant.bitLen() - int(x.prec+1)
+	switch {
+	case s < 0:
+		mant = mant.shl(mant, uint(-s))
+	case s > 0:
+		mant = mant.shr(mant, uint(+s))
+	}
+	exp += s
+	// x = mant * 2**exp with lsb(mant) == 1/2 ulp of x.prec
+
+	// 2) Compute lower bound by subtracting 1/2 ulp.
+	var lower decimal
+	var tmp nat
+	lower.init(tmp.sub(mant, natOne), exp)
+
+	// 3) Compute upper bound by adding 1/2 ulp.
+	var upper decimal
+	upper.init(tmp.add(mant, natOne), exp)
+
+	// The upper and lower bounds are possible outputs only if
+	// the original mantissa is even, so that ToNearestEven rounding
+	// would round to the original mantissa and not the neighbors.
+	inclusive := mant[0]&2 == 0 // test bit 1 since original mantissa was shifted by 1
+
+	// Now we can figure out the minimum number of digits required.
+	// Walk along until d has distinguished itself from upper and lower.
+	for i, m := range d.mant {
+		l := lower.at(i)
+		u := upper.at(i)
+
+		// Okay to round down (truncate) if lower has a different digit
+		// or if lower is inclusive and is exactly the result of rounding
+		// down (i.e., and we have reached the final digit of lower).
+		okdown := l != m || inclusive && i+1 == len(lower.mant)
+
+		// Okay to round up if upper has a different digit and either upper
+		// is inclusive or upper is bigger than the result of rounding up.
+		okup := m != u && (inclusive || m+1 < u || i+1 < len(upper.mant))
+
+		// If it's okay to do either, then round to the nearest one.
+		// If it's okay to do only one, do it.
+		switch {
+		case okdown && okup:
+			d.round(i + 1)
+			return
+		case okdown:
+			d.roundDown(i + 1)
+			return
+		case okup:
+			d.roundUp(i + 1)
+			return
+		}
+	}
+}
+
 // %e: d.ddddde±dd
 func fmtE(buf []byte, fmt byte, prec int, d decimal) []byte {
 	// first digit
@@ -219,11 +292,7 @@ func fmtF(buf []byte, prec int, d decimal) []byte {
 	if prec > 0 {
 		buf = append(buf, '.')
 		for i := 0; i < prec; i++ {
-			ch := byte('0')
-			if j := d.exp + i; 0 <= j && j < len(d.mant) {
-				ch = d.mant[j]
-			}
-			buf = append(buf, ch)
+			buf = append(buf, d.at(d.exp+i))
 		}
 	}
 
@@ -255,7 +324,7 @@ func (x *Float) fmtB(buf []byte) []byte {
 		m = nat(nil).shr(m, uint(w-x.prec))
 	}
 
-	buf = append(buf, m.decimalString()...)
+	buf = append(buf, m.utoa(10)...)
 	buf = append(buf, 'p')
 	e := int64(x.exp) - int64(x.prec)
 	if e >= 0 {
@@ -289,7 +358,7 @@ func (x *Float) fmtP(buf []byte) []byte {
 	m = m[i:]
 
 	buf = append(buf, "0x."...)
-	buf = append(buf, strings.TrimRight(m.hexString(), "0")...)
+	buf = append(buf, bytes.TrimRight(m.utoa(16), "0")...)
 	buf = append(buf, 'p')
 	if x.exp >= 0 {
 		buf = append(buf, '+')
@@ -314,10 +383,6 @@ func min(x, y int) int {
 // '+' and ' ' for sign control, '0' for space or zero padding,
 // and '-' for left or right justification. See the fmt package
 // for details.
-//
-// BUG(gri) A missing precision for the 'g' format, or a negative
-//          (via '*') precision is not yet supported. Instead the
-//          default precision (6) is used in that case (issue #10991).
 func (x *Float) Format(s fmt.State, format rune) {
 	prec, hasPrec := s.Precision()
 	if !hasPrec {
@@ -336,8 +401,7 @@ func (x *Float) Format(s fmt.State, format rune) {
 		fallthrough
 	case 'g', 'G':
 		if !hasPrec {
-			// TODO(gri) uncomment once (*Float).Text handles prec < 0
-			// prec = -1 // default precision for 'g', 'G'
+			prec = -1 // default precision for 'g', 'G'
 		}
 	default:
 		fmt.Fprintf(s, "%%!%c(*big.Float=%s)", format, x.String())
diff --git a/src/cmd/compile/internal/big/int.go b/src/cmd/compile/internal/big/int.go
index 65334e0ef5..16b7cd131b 100644
--- a/src/cmd/compile/internal/big/int.go
+++ b/src/cmd/compile/internal/big/int.go
@@ -551,8 +551,11 @@ func (z *Int) binaryGCD(a, b *Int) *Int {
 }
 
 // ProbablyPrime performs n Miller-Rabin tests to check whether x is prime.
-// If it returns true, x is prime with probability 1 - 1/4^n.
-// If it returns false, x is not prime. n must be > 0.
+// If x is prime, it returns true.
+// If x is not prime, it returns false with probability at least 1 - ¼ⁿ.
+//
+// It is not suitable for judging primes that an adversary may have crafted
+// to fool this test.
 func (x *Int) ProbablyPrime(n int) bool {
 	if n <= 0 {
 		panic("non-positive n for ProbablyPrime")
@@ -640,23 +643,23 @@ func Jacobi(x, y *Int) int {
 	}
 }
 
-// ModSqrt sets z to a square root of x mod p if such a square root exists, and
-// returns z. The modulus p must be an odd prime. If x is not a square mod p,
-// ModSqrt leaves z unchanged and returns nil. This function panics if p is
-// not an odd integer.
-func (z *Int) ModSqrt(x, p *Int) *Int {
-	switch Jacobi(x, p) {
-	case -1:
-		return nil // x is not a square mod p
-	case 0:
-		return z.SetInt64(0) // sqrt(0) mod p = 0
-	case 1:
-		break
-	}
-	if x.neg || x.Cmp(p) >= 0 { // ensure 0 <= x < p
-		x = new(Int).Mod(x, p)
-	}
+// modSqrt3Mod4 uses the identity
+//      (a^((p+1)/4))^2  mod p
+//   == u^(p+1)          mod p
+//   == u^2              mod p
+// to calculate the square root of any quadratic residue mod p quickly for 3
+// mod 4 primes.
+func (z *Int) modSqrt3Mod4Prime(x, p *Int) *Int {
+	z.Set(p)         // z = p
+	z.Add(z, intOne) // z = p + 1
+	z.Rsh(z, 2)      // z = (p + 1) / 4
+	z.Exp(x, z, p)   // z = x^z mod p
+	return z
+}
 
+// modSqrtTonelliShanks uses the Tonelli-Shanks algorithm to find the square
+// root of a quadratic residue modulo any prime.
+func (z *Int) modSqrtTonelliShanks(x, p *Int) *Int {
 	// Break p-1 into s*2^e such that s is odd.
 	var s Int
 	s.Sub(p, intOne)
@@ -703,6 +706,31 @@ func (z *Int) ModSqrt(x, p *Int) *Int {
 	}
 }
 
+// ModSqrt sets z to a square root of x mod p if such a square root exists, and
+// returns z. The modulus p must be an odd prime. If x is not a square mod p,
+// ModSqrt leaves z unchanged and returns nil. This function panics if p is
+// not an odd integer.
+func (z *Int) ModSqrt(x, p *Int) *Int {
+	switch Jacobi(x, p) {
+	case -1:
+		return nil // x is not a square mod p
+	case 0:
+		return z.SetInt64(0) // sqrt(0) mod p = 0
+	case 1:
+		break
+	}
+	if x.neg || x.Cmp(p) >= 0 { // ensure 0 <= x < p
+		x = new(Int).Mod(x, p)
+	}
+
+	// Check whether p is 3 mod 4, and if so, use the faster algorithm.
+	if len(p.abs) > 0 && p.abs[0]%4 == 3 {
+		return z.modSqrt3Mod4Prime(x, p)
+	}
+	// Otherwise, use Tonelli-Shanks.
+	return z.modSqrtTonelliShanks(x, p)
+}
+
 // Lsh sets z = x << n and returns z.
 func (z *Int) Lsh(x *Int, n uint) *Int {
 	z.abs = z.abs.shl(x.abs, n)
@@ -904,65 +932,3 @@ func (z *Int) Not(x *Int) *Int {
 	z.neg = true // z cannot be zero if x is positive
 	return z
 }
-
-// Gob codec version. Permits backward-compatible changes to the encoding.
-const intGobVersion byte = 1
-
-// GobEncode implements the gob.GobEncoder interface.
-func (x *Int) GobEncode() ([]byte, error) {
-	if x == nil {
-		return nil, nil
-	}
-	buf := make([]byte, 1+len(x.abs)*_S) // extra byte for version and sign bit
-	i := x.abs.bytes(buf) - 1            // i >= 0
-	b := intGobVersion << 1              // make space for sign bit
-	if x.neg {
-		b |= 1
-	}
-	buf[i] = b
-	return buf[i:], nil
-}
-
-// GobDecode implements the gob.GobDecoder interface.
-func (z *Int) GobDecode(buf []byte) error {
-	if len(buf) == 0 {
-		// Other side sent a nil or default value.
-		*z = Int{}
-		return nil
-	}
-	b := buf[0]
-	if b>>1 != intGobVersion {
-		return fmt.Errorf("Int.GobDecode: encoding version %d not supported", b>>1)
-	}
-	z.neg = b&1 != 0
-	z.abs = z.abs.setBytes(buf[1:])
-	return nil
-}
-
-// MarshalJSON implements the json.Marshaler interface.
-func (z *Int) MarshalJSON() ([]byte, error) {
-	// TODO(gri): get rid of the []byte/string conversions
-	return []byte(z.String()), nil
-}
-
-// UnmarshalJSON implements the json.Unmarshaler interface.
-func (z *Int) UnmarshalJSON(text []byte) error {
-	// TODO(gri): get rid of the []byte/string conversions
-	if _, ok := z.SetString(string(text), 0); !ok {
-		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Int", text)
-	}
-	return nil
-}
-
-// MarshalText implements the encoding.TextMarshaler interface.
-func (z *Int) MarshalText() (text []byte, err error) {
-	return []byte(z.String()), nil
-}
-
-// UnmarshalText implements the encoding.TextUnmarshaler interface.
-func (z *Int) UnmarshalText(text []byte) error {
-	if _, ok := z.SetString(string(text), 0); !ok {
-		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Int", text)
-	}
-	return nil
-}
diff --git a/src/cmd/compile/internal/big/int_test.go b/src/cmd/compile/internal/big/int_test.go
index bf3b832980..5d65217c61 100644
--- a/src/cmd/compile/internal/big/int_test.go
+++ b/src/cmd/compile/internal/big/int_test.go
@@ -6,10 +6,7 @@ package big
 
 import (
 	"bytes"
-	"encoding/gob"
 	"encoding/hex"
-	"encoding/json"
-	"encoding/xml"
 	"fmt"
 	"math/rand"
 	"testing"
@@ -1187,6 +1184,53 @@ func BenchmarkBitsetNegOrig(b *testing.B) {
 	}
 }
 
+// tri generates the trinomial 2**(n*2) - 2**n - 1, which is always 3 mod 4 and
+// 7 mod 8, so that 2 is always a quadratic residue.
+func tri(n uint) *Int {
+	x := NewInt(1)
+	x.Lsh(x, n)
+	x2 := new(Int).Lsh(x, n)
+	x2.Sub(x2, x)
+	x2.Sub(x2, intOne)
+	return x2
+}
+
+func BenchmarkModSqrt225_Tonelli(b *testing.B) {
+	p := tri(225)
+	x := NewInt(2)
+	for i := 0; i < b.N; i++ {
+		x.SetUint64(2)
+		x.modSqrtTonelliShanks(x, p)
+	}
+}
+
+func BenchmarkModSqrt224_3Mod4(b *testing.B) {
+	p := tri(225)
+	x := new(Int).SetUint64(2)
+	for i := 0; i < b.N; i++ {
+		x.SetUint64(2)
+		x.modSqrt3Mod4Prime(x, p)
+	}
+}
+
+func BenchmarkModSqrt5430_Tonelli(b *testing.B) {
+	p := tri(5430)
+	x := new(Int).SetUint64(2)
+	for i := 0; i < b.N; i++ {
+		x.SetUint64(2)
+		x.modSqrtTonelliShanks(x, p)
+	}
+}
+
+func BenchmarkModSqrt5430_3Mod4(b *testing.B) {
+	p := tri(5430)
+	x := new(Int).SetUint64(2)
+	for i := 0; i < b.N; i++ {
+		x.SetUint64(2)
+		x.modSqrt3Mod4Prime(x, p)
+	}
+}
+
 func TestBitwise(t *testing.T) {
 	x := new(Int)
 	y := new(Int)
@@ -1408,138 +1452,6 @@ func TestJacobiPanic(t *testing.T) {
 	panic(failureMsg)
 }
 
-var encodingTests = []string{
-	"-539345864568634858364538753846587364875430589374589",
-	"-678645873",
-	"-100",
-	"-2",
-	"-1",
-	"0",
-	"1",
-	"2",
-	"10",
-	"42",
-	"1234567890",
-	"298472983472983471903246121093472394872319615612417471234712061",
-}
-
-func TestIntGobEncoding(t *testing.T) {
-	var medium bytes.Buffer
-	enc := gob.NewEncoder(&medium)
-	dec := gob.NewDecoder(&medium)
-	for _, test := range encodingTests {
-		medium.Reset() // empty buffer for each test case (in case of failures)
-		var tx Int
-		tx.SetString(test, 10)
-		if err := enc.Encode(&tx); err != nil {
-			t.Errorf("encoding of %s failed: %s", &tx, err)
-		}
-		var rx Int
-		if err := dec.Decode(&rx); err != nil {
-			t.Errorf("decoding of %s failed: %s", &tx, err)
-		}
-		if rx.Cmp(&tx) != 0 {
-			t.Errorf("transmission of %s failed: got %s want %s", &tx, &rx, &tx)
-		}
-	}
-}
-
-// Sending a nil Int pointer (inside a slice) on a round trip through gob should yield a zero.
-// TODO: top-level nils.
-func TestGobEncodingNilIntInSlice(t *testing.T) {
-	buf := new(bytes.Buffer)
-	enc := gob.NewEncoder(buf)
-	dec := gob.NewDecoder(buf)
-
-	var in = make([]*Int, 1)
-	err := enc.Encode(&in)
-	if err != nil {
-		t.Errorf("gob encode failed: %q", err)
-	}
-	var out []*Int
-	err = dec.Decode(&out)
-	if err != nil {
-		t.Fatalf("gob decode failed: %q", err)
-	}
-	if len(out) != 1 {
-		t.Fatalf("wrong len; want 1 got %d", len(out))
-	}
-	var zero Int
-	if out[0].Cmp(&zero) != 0 {
-		t.Errorf("transmission of (*Int)(nill) failed: got %s want 0", out)
-	}
-}
-
-func TestIntJSONEncoding(t *testing.T) {
-	for _, test := range encodingTests {
-		var tx Int
-		tx.SetString(test, 10)
-		b, err := json.Marshal(&tx)
-		if err != nil {
-			t.Errorf("marshaling of %s failed: %s", &tx, err)
-		}
-		var rx Int
-		if err := json.Unmarshal(b, &rx); err != nil {
-			t.Errorf("unmarshaling of %s failed: %s", &tx, err)
-		}
-		if rx.Cmp(&tx) != 0 {
-			t.Errorf("JSON encoding of %s failed: got %s want %s", &tx, &rx, &tx)
-		}
-	}
-}
-
-var intVals = []string{
-	"-141592653589793238462643383279502884197169399375105820974944592307816406286",
-	"-1415926535897932384626433832795028841971",
-	"-141592653589793",
-	"-1",
-	"0",
-	"1",
-	"141592653589793",
-	"1415926535897932384626433832795028841971",
-	"141592653589793238462643383279502884197169399375105820974944592307816406286",
-}
-
-func TestIntJSONEncodingTextMarshaller(t *testing.T) {
-	for _, num := range intVals {
-		var tx Int
-		tx.SetString(num, 0)
-		b, err := json.Marshal(&tx)
-		if err != nil {
-			t.Errorf("marshaling of %s failed: %s", &tx, err)
-			continue
-		}
-		var rx Int
-		if err := json.Unmarshal(b, &rx); err != nil {
-			t.Errorf("unmarshaling of %s failed: %s", &tx, err)
-			continue
-		}
-		if rx.Cmp(&tx) != 0 {
-			t.Errorf("JSON encoding of %s failed: got %s want %s", &tx, &rx, &tx)
-		}
-	}
-}
-
-func TestIntXMLEncodingTextMarshaller(t *testing.T) {
-	for _, num := range intVals {
-		var tx Int
-		tx.SetString(num, 0)
-		b, err := xml.Marshal(&tx)
-		if err != nil {
-			t.Errorf("marshaling of %s failed: %s", &tx, err)
-			continue
-		}
-		var rx Int
-		if err := xml.Unmarshal(b, &rx); err != nil {
-			t.Errorf("unmarshaling of %s failed: %s", &tx, err)
-			continue
-		}
-		if rx.Cmp(&tx) != 0 {
-			t.Errorf("XML encoding of %s failed: got %s want %s", &tx, &rx, &tx)
-		}
-	}
-}
-
 func TestIssue2607(t *testing.T) {
 	// This code sequence used to hang.
 	n := NewInt(10)
diff --git a/src/cmd/compile/internal/big/intconv.go b/src/cmd/compile/internal/big/intconv.go
index 737d176cb8..56a75f87ae 100644
--- a/src/cmd/compile/internal/big/intconv.go
+++ b/src/cmd/compile/internal/big/intconv.go
@@ -12,30 +12,34 @@ import (
 	"io"
 )
 
-func (x *Int) String() string {
-	switch {
-	case x == nil:
+// TODO(gri) Should rename itoa to utoa (there's no sign). That
+// would permit the introduction of itoa which is like utoa but
+// reserves a byte for a possible sign that's passed in. That
+// would permit Int.Text to be implemented w/o the need for
+// string copy if the number is negative.
+
+// Text returns the string representation of x in the given base.
+// Base must be between 2 and 36, inclusive. The result uses the
+// lower-case letters 'a' to 'z' for digit values >= 10. No base
+// prefix (such as "0x") is added to the string.
+func (x *Int) Text(base int) string {
+	if x == nil {
 		return "<nil>"
-	case x.neg:
-		return "-" + x.abs.decimalString()
 	}
-	return x.abs.decimalString()
+	return string(x.abs.itoa(x.neg, base))
 }
 
-func charset(ch rune) string {
-	switch ch {
-	case 'b':
-		return lowercaseDigits[0:2]
-	case 'o':
-		return lowercaseDigits[0:8]
-	case 'd', 's', 'v':
-		return lowercaseDigits[0:10]
-	case 'x':
-		return lowercaseDigits[0:16]
-	case 'X':
-		return uppercaseDigits[0:16]
+// Append appends the string representation of x, as generated by
+// x.Text(base), to buf and returns the extended buffer.
+func (x *Int) Append(buf []byte, base int) []byte {
+	if x == nil {
+		return append(buf, "<nil>"...)
 	}
-	return "" // unknown format
+	return append(buf, x.abs.itoa(x.neg, base)...)
+}
+
+func (x *Int) String() string {
+	return x.Text(10)
 }
 
 // write count copies of text to s
@@ -60,15 +64,24 @@ func writeMultiple(s fmt.State, text string, count int) {
 // right justification.
 //
 func (x *Int) Format(s fmt.State, ch rune) {
-	cs := charset(ch)
-
-	// special cases
-	switch {
-	case cs == "":
+	// determine base
+	var base int
+	switch ch {
+	case 'b':
+		base = 2
+	case 'o':
+		base = 8
+	case 'd', 's', 'v':
+		base = 10
+	case 'x', 'X':
+		base = 16
+	default:
 		// unknown format
 		fmt.Fprintf(s, "%%!%c(big.Int=%s)", ch, x.String())
 		return
-	case x == nil:
+	}
+
+	if x == nil {
 		fmt.Fprint(s, "<nil>")
 		return
 	}
@@ -97,8 +110,15 @@ func (x *Int) Format(s fmt.State, ch rune) {
 		}
 	}
 
-	// determine digits with base set by len(cs) and digit characters from cs
-	digits := x.abs.string(cs)
+	digits := x.abs.utoa(base)
+	if ch == 'X' {
+		// faster than bytes.ToUpper
+		for i, d := range digits {
+			if 'a' <= d && d <= 'z' {
+				digits[i] = 'A' + (d - 'a')
+			}
+		}
+	}
 
 	// number of characters for the three classes of number padding
 	var left int  // space characters to left of digits for right justification ("%8d")
@@ -111,7 +131,7 @@ func (x *Int) Format(s fmt.State, ch rune) {
 		switch {
 		case len(digits) < precision:
 			zeros = precision - len(digits) // count of zero padding
-		case digits == "0" && precision == 0:
+		case len(digits) == 1 && digits[0] == '0' && precision == 0:
 			return // print nothing if zero value (x == 0) and zero precision ("." or ".0")
 		}
 	}
@@ -137,7 +157,7 @@ func (x *Int) Format(s fmt.State, ch rune) {
 	writeMultiple(s, sign, 1)
 	writeMultiple(s, prefix, 1)
 	writeMultiple(s, "0", zeros)
-	writeMultiple(s, digits, 1)
+	s.Write(digits)
 	writeMultiple(s, " ", right)
 }
 
diff --git a/src/cmd/compile/internal/big/intconv_test.go b/src/cmd/compile/internal/big/intconv_test.go
index 2deb84b48f..514208145f 100644
--- a/src/cmd/compile/internal/big/intconv_test.go
+++ b/src/cmd/compile/internal/big/intconv_test.go
@@ -17,19 +17,19 @@ var stringTests = []struct {
 	val  int64
 	ok   bool
 }{
-	{in: "", ok: false},
-	{in: "a", ok: false},
-	{in: "z", ok: false},
-	{in: "+", ok: false},
-	{in: "-", ok: false},
-	{in: "0b", ok: false},
-	{in: "0x", ok: false},
-	{in: "2", base: 2, ok: false},
-	{in: "0b2", base: 0, ok: false},
-	{in: "08", ok: false},
-	{in: "8", base: 8, ok: false},
-	{in: "0xg", base: 0, ok: false},
-	{in: "g", base: 16, ok: false},
+	{in: ""},
+	{in: "a"},
+	{in: "z"},
+	{in: "+"},
+	{in: "-"},
+	{in: "0b"},
+	{in: "0x"},
+	{in: "2", base: 2},
+	{in: "0b2", base: 0},
+	{in: "08"},
+	{in: "8", base: 8},
+	{in: "0xg", base: 0},
+	{in: "g", base: 16},
 	{"0", "0", 0, 0, true},
 	{"0", "0", 10, 0, true},
 	{"0", "0", 16, 0, true},
@@ -41,7 +41,7 @@ var stringTests = []struct {
 	{"-10", "-10", 16, -16, true},
 	{"+10", "10", 16, 16, true},
 	{"0x10", "16", 0, 16, true},
-	{in: "0x10", base: 16, ok: false},
+	{in: "0x10", base: 16},
 	{"-0x10", "-16", 0, -16, true},
 	{"+0x10", "16", 0, 16, true},
 	{"00", "0", 0, 0, true},
@@ -58,6 +58,57 @@ var stringTests = []struct {
 	{"1001010111", "1001010111", 2, 0x257, true},
 }
 
+func TestIntText(t *testing.T) {
+	z := new(Int)
+	for _, test := range stringTests {
+		if !test.ok {
+			continue
+		}
+
+		_, ok := z.SetString(test.in, test.base)
+		if !ok {
+			t.Errorf("%v: failed to parse", test)
+			continue
+		}
+
+		base := test.base
+		if base == 0 {
+			base = 10
+		}
+
+		if got := z.Text(base); got != test.out {
+			t.Errorf("%v: got %s; want %s", test, got, test.out)
+		}
+	}
+}
+
+func TestAppendText(t *testing.T) {
+	z := new(Int)
+	var buf []byte
+	for _, test := range stringTests {
+		if !test.ok {
+			continue
+		}
+
+		_, ok := z.SetString(test.in, test.base)
+		if !ok {
+			t.Errorf("%v: failed to parse", test)
+			continue
+		}
+
+		base := test.base
+		if base == 0 {
+			base = 10
+		}
+
+		i := len(buf)
+		buf = z.Append(buf, base)
+		if got := string(buf[i:]); got != test.out {
+			t.Errorf("%v: got %s; want %s", test, got, test.out)
+		}
+	}
+}
+
 func format(base int) string {
 	switch base {
 	case 2:
@@ -79,15 +130,13 @@ func TestGetString(t *testing.T) {
 		z.SetInt64(test.val)
 
 		if test.base == 10 {
-			s := z.String()
-			if s != test.out {
-				t.Errorf("#%da got %s; want %s", i, s, test.out)
+			if got := z.String(); got != test.out {
+				t.Errorf("#%da got %s; want %s", i, got, test.out)
 			}
 		}
 
-		s := fmt.Sprintf(format(test.base), z)
-		if s != test.out {
-			t.Errorf("#%db got %s; want %s", i, s, test.out)
+		if got := fmt.Sprintf(format(test.base), z); got != test.out {
+			t.Errorf("#%db got %s; want %s", i, got, test.out)
 		}
 	}
 }
diff --git a/src/cmd/compile/internal/big/nat.go b/src/cmd/compile/internal/big/nat.go
index 6545bc17ed..e60318dc88 100644
--- a/src/cmd/compile/internal/big/nat.go
+++ b/src/cmd/compile/internal/big/nat.go
@@ -2,31 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package big implements multi-precision arithmetic (big numbers).
-// The following numeric types are supported:
-//
-//   Int    signed integers
-//   Rat    rational numbers
-//   Float  floating-point numbers
-//
-// Methods are typically of the form:
-//
-//   func (z *T) Unary(x *T) *T        // z = op x
-//   func (z *T) Binary(x, y *T) *T    // z = x op y
-//   func (x *T) M() T1                // v = x.M()
-//
-// with T one of Int, Rat, or Float. For unary and binary operations, the
-// result is the receiver (usually named z in that case); if it is one of
-// the operands x or y it may be overwritten (and its memory reused).
-// To enable chaining of operations, the result is also returned. Methods
-// returning a result other than *Int, *Rat, or *Float take an operand as
-// the receiver (usually named x in that case).
-//
-package big
+// This file implements unsigned multi-precision integers (natural
+// numbers). They are the building blocks for the implementation
+// of signed integers, rationals, and floating-point numbers.
 
-// This file contains operations on unsigned multi-precision integers.
-// These are the building blocks for the operations on signed integers
-// and rationals.
+package big
 
 import "math/rand"
 
@@ -216,23 +196,36 @@ func basicMul(z, x, y nat) {
 	}
 }
 
-// montgomery computes x*y*2^(-n*_W) mod m,
-// assuming k = -1/m mod 2^_W.
+// montgomery computes z mod m = x*y*2**(-n*_W) mod m,
+// assuming k = -1/m mod 2**_W.
 // z is used for storing the result which is returned;
 // z must not alias x, y or m.
+// See Gueron, "Efficient Software Implementations of Modular Exponentiation".
+// https://eprint.iacr.org/2011/239.pdf
+// In the terminology of that paper, this is an "Almost Montgomery Multiplication":
+// x and y are required to satisfy 0 <= z < 2**(n*_W) and then the result
+// z is guaranteed to satisfy 0 <= z < 2**(n*_W), but it may not be < m.
 func (z nat) montgomery(x, y, m nat, k Word, n int) nat {
-	var c1, c2 Word
+	// This code assumes x, y, m are all the same length, n.
+	// (required by addMulVVW and the for loop).
+	// It also assumes that x, y are already reduced mod m,
+	// or else the result will not be properly reduced.
+	if len(x) != n || len(y) != n || len(m) != n {
+		panic("math/big: mismatched montgomery number lengths")
+	}
+	var c1, c2, c3 Word
 	z = z.make(n)
 	z.clear()
 	for i := 0; i < n; i++ {
 		d := y[i]
-		c1 += addMulVVW(z, x, d)
+		c2 = addMulVVW(z, x, d)
 		t := z[0] * k
-		c2 = addMulVVW(z, m, t)
-
+		c3 = addMulVVW(z, m, t)
 		copy(z, z[1:])
-		z[n-1] = c1 + c2
-		if z[n-1] < c1 {
+		cx := c1 + c2
+		cy := cx + c3
+		z[n-1] = cy
+		if cx < c2 || cy < c3 {
 			c1 = 1
 		} else {
 			c1 = 0
@@ -1082,7 +1075,7 @@ func (z nat) expNNMontgomery(x, y, m nat) nat {
 	x = rr
 
 	// Ideally the precomputations would be performed outside, and reused
-	// k0 = -mˆ-1 mod 2ˆ_W. Algorithm from: Dumas, J.G. "On Newton–Raphson
+	// k0 = -m**-1 mod 2**_W. Algorithm from: Dumas, J.G. "On Newton–Raphson
 	// Iteration for Multiplicative Inverses Modulo Prime Powers".
 	k0 := 2 - m[0]
 	t := m[0] - 1
@@ -1092,7 +1085,7 @@ func (z nat) expNNMontgomery(x, y, m nat) nat {
 	}
 	k0 = -k0
 
-	// RR = 2ˆ(2*_W*len(m)) mod m
+	// RR = 2**(2*_W*len(m)) mod m
 	RR = RR.setWord(1)
 	zz = zz.shl(RR, uint(2*numWords*_W))
 	_, RR = RR.div(RR, zz, m)
@@ -1141,9 +1134,12 @@ func (z nat) expNNMontgomery(x, y, m nat) nat {
 	return zz.norm()
 }
 
-// probablyPrime performs reps Miller-Rabin tests to check whether n is prime.
-// If it returns true, n is prime with probability 1 - 1/4^reps.
-// If it returns false, n is not prime.
+// probablyPrime performs n Miller-Rabin tests to check whether x is prime.
+// If x is prime, it returns true.
+// If x is not prime, it returns false with probability at least 1 - ¼ⁿ.
+//
+// It is not suitable for judging primes that an adversary may have crafted
+// to fool this test.
 func (n nat) probablyPrime(reps int) bool {
 	if len(n) == 0 {
 		return false
diff --git a/src/cmd/compile/internal/big/nat_test.go b/src/cmd/compile/internal/big/nat_test.go
index 7ac3cb8a84..56b62d24d6 100644
--- a/src/cmd/compile/internal/big/nat_test.go
+++ b/src/cmd/compile/internal/big/nat_test.go
@@ -158,7 +158,7 @@ var mulRangesN = []struct {
 
 func TestMulRangeN(t *testing.T) {
 	for i, r := range mulRangesN {
-		prod := nat(nil).mulRange(r.a, r.b).decimalString()
+		prod := string(nat(nil).mulRange(r.a, r.b).utoa(10))
 		if prod != r.prod {
 			t.Errorf("#%d: got %s; want %s", i, prod, r.prod)
 		}
@@ -326,7 +326,7 @@ func TestTrailingZeroBits(t *testing.T) {
 	for i := uint(0); i <= 3*_W; i++ {
 		n := y.trailingZeroBits()
 		if n != i {
-			t.Errorf("got 0x%s.trailingZeroBits() = %d; want %d", y.hexString(), n, i)
+			t.Errorf("got 0x%s.trailingZeroBits() = %d; want %d", y.utoa(16), n, i)
 		}
 		y = y.shl(y, 1)
 	}
@@ -341,25 +341,57 @@ var montgomeryTests = []struct {
 		"0xffffffffffffffffffffffffffffffffffffffffffffffffe",
 		"0xffffffffffffffffffffffffffffffffffffffffffffffffe",
 		"0xfffffffffffffffffffffffffffffffffffffffffffffffff",
-		0x0000000000000000,
-		"0xffffffffffffffffffffffffffffffffffffffffff",
-		"0xffffffffffffffffffffffffffffffffff",
+		1,
+		"0x1000000000000000000000000000000000000000000",
+		"0x10000000000000000000000000000000000",
 	},
 	{
-		"0x0000000080000000",
-		"0x00000000ffffffff",
+		"0x000000000ffffff5",
+		"0x000000000ffffff0",
 		"0x0000000010000001",
 		0xff0000000fffffff,
-		"0x0000000088000000",
-		"0x0000000007800001",
+		"0x000000000bfffff4",
+		"0x0000000003400001",
+	},
+	{
+		"0x0000000080000000",
+		"0x00000000ffffffff",
+		"0x1000000000000001",
+		0xfffffffffffffff,
+		"0x0800000008000001",
+		"0x0800000008000001",
+	},
+	{
+		"0x0000000080000000",
+		"0x0000000080000000",
+		"0xffffffff00000001",
+		0xfffffffeffffffff,
+		"0xbfffffff40000001",
+		"0xbfffffff40000001",
+	},
+	{
+		"0x0000000080000000",
+		"0x0000000080000000",
+		"0x00ffffff00000001",
+		0xfffffeffffffff,
+		"0xbfffff40000001",
+		"0xbfffff40000001",
 	},
 	{
-		"0xffffffffffffffffffffffffffffffff00000000000022222223333333333444444444",
-		"0xffffffffffffffffffffffffffffffff999999999999999aaabbbbbbbbcccccccccccc",
+		"0x0000000080000000",
+		"0x0000000080000000",
+		"0x0000ffff00000001",
+		0xfffeffffffff,
+		"0xbfff40000001",
+		"0xbfff40000001",
+	},
+	{
+		"0x3321ffffffffffffffffffffffffffff00000000000022222623333333332bbbb888c0",
+		"0x3321ffffffffffffffffffffffffffff00000000000022222623333333332bbbb888c0",
 		"0x33377fffffffffffffffffffffffffffffffffffffffffffff0000000000022222eee1",
 		0xdecc8f1249812adf,
-		"0x22bb05b6d95eaaeca2bb7c05e51f807bce9064b5fbad177161695e4558f9474e91cd79",
-		"0x14beb58d230f85b6d95eaaeca2bb7c05e51f807bce9064b5fb45669afa695f228e48cd",
+		"0x04eb0e11d72329dc0915f86784820fc403275bf2f6620a20e0dd344c5cd0875e50deb5",
+		"0x0d7144739a7d8e11d72329dc0915f86784820fc403275bf2f61ed96f35dd34dbb3d6a0",
 	},
 	{
 		"0x10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000ffffffffffffffffffffffffffffffff00000000000022222223333333333444444444",
@@ -372,10 +404,27 @@ var montgomeryTests = []struct {
 }
 
 func TestMontgomery(t *testing.T) {
+	one := NewInt(1)
+	_B := new(Int).Lsh(one, _W)
 	for i, test := range montgomeryTests {
 		x := natFromString(test.x)
 		y := natFromString(test.y)
 		m := natFromString(test.m)
+		for len(x) < len(m) {
+			x = append(x, 0)
+		}
+		for len(y) < len(m) {
+			y = append(y, 0)
+		}
+
+		if x.cmp(m) > 0 {
+			_, r := nat(nil).div(nil, x, m)
+			t.Errorf("#%d: x > m (0x%s > 0x%s; use 0x%s)", i, x.utoa(16), m.utoa(16), r.utoa(16))
+		}
+		if y.cmp(m) > 0 {
+			_, r := nat(nil).div(nil, x, m)
+			t.Errorf("#%d: y > m (0x%s > 0x%s; use 0x%s)", i, y.utoa(16), m.utoa(16), r.utoa(16))
+		}
 
 		var out nat
 		if _W == 32 {
@@ -384,11 +433,31 @@ func TestMontgomery(t *testing.T) {
 			out = natFromString(test.out64)
 		}
 
-		k0 := Word(test.k0 & _M) // mask k0 to ensure that it fits for 32-bit systems.
+		// t.Logf("#%d: len=%d\n", i, len(m))
+
+		// check output in table
+		xi := &Int{abs: x}
+		yi := &Int{abs: y}
+		mi := &Int{abs: m}
+		p := new(Int).Mod(new(Int).Mul(xi, new(Int).Mul(yi, new(Int).ModInverse(new(Int).Lsh(one, uint(len(m))*_W), mi))), mi)
+		if out.cmp(p.abs.norm()) != 0 {
+			t.Errorf("#%d: out in table=0x%s, computed=0x%s", i, out.utoa(16), p.abs.norm().utoa(16))
+		}
+
+		// check k0 in table
+		k := new(Int).Mod(&Int{abs: m}, _B)
+		k = new(Int).Sub(_B, k)
+		k = new(Int).Mod(k, _B)
+		k0 := Word(new(Int).ModInverse(k, _B).Uint64())
+		if k0 != Word(test.k0) {
+			t.Errorf("#%d: k0 in table=%#x, computed=%#x\n", i, test.k0, k0)
+		}
+
+		// check montgomery with correct k0 produces correct output
 		z := nat(nil).montgomery(x, y, m, k0, len(m))
 		z = z.norm()
 		if z.cmp(out) != 0 {
-			t.Errorf("#%d got %s want %s", i, z.decimalString(), out.decimalString())
+			t.Errorf("#%d: got 0x%s want 0x%s", i, z.utoa(16), out.utoa(16))
 		}
 	}
 }
@@ -429,7 +498,7 @@ func TestExpNN(t *testing.T) {
 
 		z := nat(nil).expNN(x, y, m)
 		if z.cmp(out) != 0 {
-			t.Errorf("#%d got %s want %s", i, z.decimalString(), out.decimalString())
+			t.Errorf("#%d got %s want %s", i, z.utoa(10), out.utoa(10))
 		}
 	}
 }
@@ -486,7 +555,7 @@ var fiboNums = []string{
 func TestFibo(t *testing.T) {
 	for i, want := range fiboNums {
 		n := i * 10
-		got := fibo(n).decimalString()
+		got := string(fibo(n).utoa(10))
 		if got != want {
 			t.Errorf("fibo(%d) failed: got %s want %s", n, got, want)
 		}
diff --git a/src/cmd/compile/internal/big/natconv.go b/src/cmd/compile/internal/big/natconv.go
index 80da307147..d2ce667fb6 100644
--- a/src/cmd/compile/internal/big/natconv.go
+++ b/src/cmd/compile/internal/big/natconv.go
@@ -14,6 +14,11 @@ import (
 	"sync"
 )
 
+const digits = "0123456789abcdefghijklmnopqrstuvwxyz"
+
+// Note: MaxBase = len(digits), but it must remain a rune constant
+//       for API compatibility.
+
 // MaxBase is the largest number base accepted for string conversions.
 const MaxBase = 'z' - 'a' + 10 + 1
 
@@ -229,45 +234,33 @@ func (z nat) scan(r io.ByteScanner, base int, fracOk bool) (res nat, b, count in
 	return
 }
 
-// Character sets for string conversion.
-const (
-	lowercaseDigits = "0123456789abcdefghijklmnopqrstuvwxyz"
-	uppercaseDigits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-)
-
-// decimalString returns a decimal representation of x.
-// It calls x.string with the charset "0123456789".
-func (x nat) decimalString() string {
-	return x.string(lowercaseDigits[:10])
-}
-
-// hexString returns a hexadecimal representation of x.
-// It calls x.string with the charset "0123456789abcdef".
-func (x nat) hexString() string {
-	return x.string(lowercaseDigits[:16])
+// utoa converts x to an ASCII representation in the given base;
+// base must be between 2 and MaxBase, inclusive.
+func (x nat) utoa(base int) []byte {
+	return x.itoa(false, base)
 }
 
-// string converts x to a string using digits from a charset; a digit with
-// value d is represented by charset[d]. The conversion base is determined
-// by len(charset), which must be >= 2 and <= 256.
-func (x nat) string(charset string) string {
-	b := Word(len(charset))
-	if b < 2 || b > 256 {
-		panic("invalid character set length")
+// itoa is like utoa but it prepends a '-' if neg && x != 0.
+func (x nat) itoa(neg bool, base int) []byte {
+	if base < 2 || base > MaxBase {
+		panic("invalid base")
 	}
 
 	// x == 0
 	if len(x) == 0 {
-		return string(charset[0])
+		return []byte("0")
 	}
 	// len(x) > 0
 
 	// allocate buffer for conversion
-	i := int(float64(x.bitLen())/math.Log2(float64(b))) + 1 // off by one at most
+	i := int(float64(x.bitLen())/math.Log2(float64(base))) + 1 // off by 1 at most
+	if neg {
+		i++
+	}
 	s := make([]byte, i)
 
 	// convert power of two and non power of two bases separately
-	if b == b&-b {
+	if b := Word(base); b == b&-b {
 		// shift is base b digit size in bits
 		shift := trailingZeroBits(b) // shift > 0 because b >= 2
 		mask := Word(1<<shift - 1)
@@ -279,7 +272,7 @@ func (x nat) string(charset string) string {
 			// convert full digits
 			for nbits >= shift {
 				i--
-				s[i] = charset[w&mask]
+				s[i] = digits[w&mask]
 				w >>= shift
 				nbits -= shift
 			}
@@ -293,7 +286,7 @@ func (x nat) string(charset string) string {
 				// partial digit in current word w (== x[k-1]) and next word x[k]
 				w |= x[k] << nbits
 				i--
-				s[i] = charset[w&mask]
+				s[i] = digits[w&mask]
 
 				// advance
 				w = x[k] >> (shift - nbits)
@@ -304,7 +297,7 @@ func (x nat) string(charset string) string {
 		// convert digits of most-significant word w (omit leading zeros)
 		for w != 0 {
 			i--
-			s[i] = charset[w&mask]
+			s[i] = digits[w&mask]
 			w >>= shift
 		}
 
@@ -319,18 +312,23 @@ func (x nat) string(charset string) string {
 		q := nat(nil).set(x)
 
 		// convert q to string s in base b
-		q.convertWords(s, charset, b, ndigits, bb, table)
+		q.convertWords(s, b, ndigits, bb, table)
 
 		// strip leading zeros
 		// (x != 0; thus s must contain at least one non-zero digit
 		// and the loop will terminate)
 		i = 0
-		for zero := charset[0]; s[i] == zero; {
+		for s[i] == '0' {
 			i++
 		}
 	}
 
-	return string(s[i:])
+	if neg {
+		i--
+		s[i] = '-'
+	}
+
+	return s[i:]
 }
 
 // Convert words of q to base b digits in s. If q is large, it is recursively "split in half"
@@ -349,7 +347,7 @@ func (x nat) string(charset string) string {
 // ~30x for 20000 digits. Use nat_test.go's BenchmarkLeafSize tests to optimize leafSize for
 // specific hardware.
 //
-func (q nat) convertWords(s []byte, charset string, b Word, ndigits int, bb Word, table []divisor) {
+func (q nat) convertWords(s []byte, b Word, ndigits int, bb Word, table []divisor) {
 	// split larger blocks recursively
 	if table != nil {
 		// len(q) > leafSize > 0
@@ -374,8 +372,8 @@ func (q nat) convertWords(s []byte, charset string, b Word, ndigits int, bb Word
 
 			// convert subblocks and collect results in s[:h] and s[h:]
 			h := len(s) - table[index].ndigits
-			r.convertWords(s[h:], charset, b, ndigits, bb, table[0:index])
-			s = s[:h] // == q.convertWords(s, charset, b, ndigits, bb, table[0:index+1])
+			r.convertWords(s[h:], b, ndigits, bb, table[0:index])
+			s = s[:h] // == q.convertWords(s, b, ndigits, bb, table[0:index+1])
 		}
 	}
 
@@ -393,7 +391,7 @@ func (q nat) convertWords(s []byte, charset string, b Word, ndigits int, bb Word
 				// this appears to be faster for BenchmarkString10000Base10
 				// and smaller strings (but a bit slower for larger ones)
 				t := r / 10
-				s[i] = charset[r-t<<3-t-t] // TODO(gri) replace w/ t*10 once compiler produces better code
+				s[i] = '0' + byte(r-t<<3-t-t) // TODO(gri) replace w/ t*10 once compiler produces better code
 				r = t
 			}
 		}
@@ -403,17 +401,16 @@ func (q nat) convertWords(s []byte, charset string, b Word, ndigits int, bb Word
 			q, r = q.divW(q, bb)
 			for j := 0; j < ndigits && i > 0; j++ {
 				i--
-				s[i] = charset[r%b]
+				s[i] = digits[r%b]
 				r /= b
 			}
 		}
 	}
 
 	// prepend high-order zeros
-	zero := charset[0]
 	for i > 0 { // while need more leading zeros
 		i--
-		s[i] = zero
+		s[i] = '0'
 	}
 }
 
diff --git a/src/cmd/compile/internal/big/natconv_test.go b/src/cmd/compile/internal/big/natconv_test.go
index f321fbc2df..028e5a858e 100644
--- a/src/cmd/compile/internal/big/natconv_test.go
+++ b/src/cmd/compile/internal/big/natconv_test.go
@@ -5,20 +5,19 @@
 package big
 
 import (
+	"bytes"
 	"io"
 	"strings"
 	"testing"
 )
 
-func toString(x nat, charset string) string {
-	base := len(charset)
-
+func itoa(x nat, base int) []byte {
 	// special cases
 	switch {
 	case base < 2:
 		panic("illegal base")
 	case len(x) == 0:
-		return string(charset[0])
+		return []byte("0")
 	}
 
 	// allocate buffer for conversion
@@ -33,54 +32,53 @@ func toString(x nat, charset string) string {
 		i--
 		var r Word
 		q, r = q.divW(q, Word(base))
-		s[i] = charset[r]
+		s[i] = digits[r]
 	}
 
-	return string(s[i:])
+	return s[i:]
 }
 
 var strTests = []struct {
 	x nat    // nat value to be converted
-	c string // conversion charset
+	b int    // conversion base
 	s string // expected result
 }{
-	{nil, "01", "0"},
-	{nat{1}, "01", "1"},
-	{nat{0xc5}, "01", "11000101"},
-	{nat{03271}, lowercaseDigits[:8], "3271"},
-	{nat{10}, lowercaseDigits[:10], "10"},
-	{nat{1234567890}, uppercaseDigits[:10], "1234567890"},
-	{nat{0xdeadbeef}, lowercaseDigits[:16], "deadbeef"},
-	{nat{0xdeadbeef}, uppercaseDigits[:16], "DEADBEEF"},
-	{nat{0x229be7}, lowercaseDigits[:17], "1a2b3c"},
-	{nat{0x309663e6}, uppercaseDigits[:32], "O9COV6"},
+	{nil, 2, "0"},
+	{nat{1}, 2, "1"},
+	{nat{0xc5}, 2, "11000101"},
+	{nat{03271}, 8, "3271"},
+	{nat{10}, 10, "10"},
+	{nat{1234567890}, 10, "1234567890"},
+	{nat{0xdeadbeef}, 16, "deadbeef"},
+	{nat{0x229be7}, 17, "1a2b3c"},
+	{nat{0x309663e6}, 32, "o9cov6"},
 }
 
 func TestString(t *testing.T) {
-	// test invalid character set explicitly
+	// test invalid base explicitly
 	var panicStr string
 	func() {
 		defer func() {
 			panicStr = recover().(string)
 		}()
-		natOne.string("0")
+		natOne.utoa(1)
 	}()
-	if panicStr != "invalid character set length" {
-		t.Errorf("expected panic for invalid character set")
+	if panicStr != "invalid base" {
+		t.Errorf("expected panic for invalid base")
 	}
 
 	for _, a := range strTests {
-		s := a.x.string(a.c)
+		s := string(a.x.utoa(a.b))
 		if s != a.s {
 			t.Errorf("string%+v\n\tgot s = %s; want %s", a, s, a.s)
 		}
 
-		x, b, _, err := nat(nil).scan(strings.NewReader(a.s), len(a.c), false)
+		x, b, _, err := nat(nil).scan(strings.NewReader(a.s), a.b, false)
 		if x.cmp(a.x) != 0 {
 			t.Errorf("scan%+v\n\tgot z = %v; want %v", a, x, a.x)
 		}
-		if b != len(a.c) {
-			t.Errorf("scan%+v\n\tgot b = %d; want %d", a, b, len(a.c))
+		if b != a.b {
+			t.Errorf("scan%+v\n\tgot b = %d; want %d", a, b, a.b)
 		}
 		if err != nil {
 			t.Errorf("scan%+v\n\tgot error = %s", a, err)
@@ -236,7 +234,7 @@ func TestScanPi(t *testing.T) {
 	if err != nil {
 		t.Errorf("scanning pi: %s", err)
 	}
-	if s := z.decimalString(); s != pi {
+	if s := string(z.utoa(10)); s != pi {
 		t.Errorf("scanning pi: got %s", s)
 	}
 }
@@ -265,12 +263,12 @@ func BenchmarkScanPi(b *testing.B) {
 func BenchmarkStringPiParallel(b *testing.B) {
 	var x nat
 	x, _, _, _ = x.scan(strings.NewReader(pi), 0, false)
-	if x.decimalString() != pi {
+	if string(x.utoa(10)) != pi {
 		panic("benchmark incorrect: conversion failed")
 	}
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {
-			x.decimalString()
+			x.utoa(10)
 		}
 	})
 }
@@ -304,15 +302,14 @@ func ScanHelper(b *testing.B, base int, x, y Word) {
 	var z nat
 	z = z.expWW(x, y)
 
-	var s string
-	s = z.string(lowercaseDigits[:base])
-	if t := toString(z, lowercaseDigits[:base]); t != s {
+	s := z.utoa(base)
+	if t := itoa(z, base); !bytes.Equal(s, t) {
 		b.Fatalf("scanning: got %s; want %s", s, t)
 	}
 	b.StartTimer()
 
 	for i := 0; i < b.N; i++ {
-		z.scan(strings.NewReader(s), base, false)
+		z.scan(bytes.NewReader(s), base, false)
 	}
 }
 
@@ -344,11 +341,11 @@ func StringHelper(b *testing.B, base int, x, y Word) {
 	b.StopTimer()
 	var z nat
 	z = z.expWW(x, y)
-	z.string(lowercaseDigits[:base]) // warm divisor cache
+	z.utoa(base) // warm divisor cache
 	b.StartTimer()
 
 	for i := 0; i < b.N; i++ {
-		_ = z.string(lowercaseDigits[:base])
+		_ = z.utoa(base)
 	}
 }
 
@@ -372,7 +369,7 @@ func BenchmarkLeafSize16(b *testing.B) { LeafSizeHelper(b, 10, 16) }
 func BenchmarkLeafSize32(b *testing.B) { LeafSizeHelper(b, 10, 32) } // try some large lengths
 func BenchmarkLeafSize64(b *testing.B) { LeafSizeHelper(b, 10, 64) }
 
-func LeafSizeHelper(b *testing.B, base Word, size int) {
+func LeafSizeHelper(b *testing.B, base, size int) {
 	b.StopTimer()
 	originalLeafSize := leafSize
 	resetTable(cacheBase10.table[:])
@@ -382,12 +379,12 @@ func LeafSizeHelper(b *testing.B, base Word, size int) {
 	for d := 1; d <= 10000; d *= 10 {
 		b.StopTimer()
 		var z nat
-		z = z.expWW(base, Word(d))           // build target number
-		_ = z.string(lowercaseDigits[:base]) // warm divisor cache
+		z = z.expWW(Word(base), Word(d)) // build target number
+		_ = z.utoa(base)                 // warm divisor cache
 		b.StartTimer()
 
 		for i := 0; i < b.N; i++ {
-			_ = z.string(lowercaseDigits[:base])
+			_ = z.utoa(base)
 		}
 	}
 
@@ -408,13 +405,13 @@ func resetTable(table []divisor) {
 }
 
 func TestStringPowers(t *testing.T) {
-	var b, p Word
-	for b = 2; b <= 16; b++ {
+	var p Word
+	for b := 2; b <= 16; b++ {
 		for p = 0; p <= 512; p++ {
-			x := nat(nil).expWW(b, p)
-			xs := x.string(lowercaseDigits[:b])
-			xs2 := toString(x, lowercaseDigits[:b])
-			if xs != xs2 {
+			x := nat(nil).expWW(Word(b), p)
+			xs := x.utoa(b)
+			xs2 := itoa(x, b)
+			if !bytes.Equal(xs, xs2) {
 				t.Errorf("failed at %d ** %d in base %d: %s != %s", b, p, b, xs, xs2)
 			}
 		}
diff --git a/src/cmd/compile/internal/big/rat.go b/src/cmd/compile/internal/big/rat.go
index fb16f18a96..2cd9ed0938 100644
--- a/src/cmd/compile/internal/big/rat.go
+++ b/src/cmd/compile/internal/big/rat.go
@@ -7,8 +7,6 @@
 package big
 
 import (
-	"encoding/binary"
-	"errors"
 	"fmt"
 	"math"
 )
@@ -510,61 +508,3 @@ func (z *Rat) Quo(x, y *Rat) *Rat {
 	z.a.neg = a.neg != b.neg
 	return z.norm()
 }
-
-// Gob codec version. Permits backward-compatible changes to the encoding.
-const ratGobVersion byte = 1
-
-// GobEncode implements the gob.GobEncoder interface.
-func (x *Rat) GobEncode() ([]byte, error) {
-	if x == nil {
-		return nil, nil
-	}
-	buf := make([]byte, 1+4+(len(x.a.abs)+len(x.b.abs))*_S) // extra bytes for version and sign bit (1), and numerator length (4)
-	i := x.b.abs.bytes(buf)
-	j := x.a.abs.bytes(buf[:i])
-	n := i - j
-	if int(uint32(n)) != n {
-		// this should never happen
-		return nil, errors.New("Rat.GobEncode: numerator too large")
-	}
-	binary.BigEndian.PutUint32(buf[j-4:j], uint32(n))
-	j -= 1 + 4
-	b := ratGobVersion << 1 // make space for sign bit
-	if x.a.neg {
-		b |= 1
-	}
-	buf[j] = b
-	return buf[j:], nil
-}
-
-// GobDecode implements the gob.GobDecoder interface.
-func (z *Rat) GobDecode(buf []byte) error {
-	if len(buf) == 0 {
-		// Other side sent a nil or default value.
-		*z = Rat{}
-		return nil
-	}
-	b := buf[0]
-	if b>>1 != ratGobVersion {
-		return fmt.Errorf("Rat.GobDecode: encoding version %d not supported", b>>1)
-	}
-	const j = 1 + 4
-	i := j + binary.BigEndian.Uint32(buf[j-4:j])
-	z.a.neg = b&1 != 0
-	z.a.abs = z.a.abs.setBytes(buf[j:i])
-	z.b.abs = z.b.abs.setBytes(buf[i:])
-	return nil
-}
-
-// MarshalText implements the encoding.TextMarshaler interface.
-func (r *Rat) MarshalText() (text []byte, err error) {
-	return []byte(r.RatString()), nil
-}
-
-// UnmarshalText implements the encoding.TextUnmarshaler interface.
-func (r *Rat) UnmarshalText(text []byte) error {
-	if _, ok := r.SetString(string(text)); !ok {
-		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Rat", text)
-	}
-	return nil
-}
diff --git a/src/cmd/compile/internal/big/rat_test.go b/src/cmd/compile/internal/big/rat_test.go
index 012d0c47ec..3a06fca3c3 100644
--- a/src/cmd/compile/internal/big/rat_test.go
+++ b/src/cmd/compile/internal/big/rat_test.go
@@ -5,10 +5,6 @@
 package big
 
 import (
-	"bytes"
-	"encoding/gob"
-	"encoding/json"
-	"encoding/xml"
 	"math"
 	"testing"
 )
@@ -280,116 +276,6 @@ func TestRatSetFrac64Rat(t *testing.T) {
 	}
 }
 
-func TestRatGobEncoding(t *testing.T) {
-	var medium bytes.Buffer
-	enc := gob.NewEncoder(&medium)
-	dec := gob.NewDecoder(&medium)
-	for _, test := range encodingTests {
-		medium.Reset() // empty buffer for each test case (in case of failures)
-		var tx Rat
-		tx.SetString(test + ".14159265")
-		if err := enc.Encode(&tx); err != nil {
-			t.Errorf("encoding of %s failed: %s", &tx, err)
-		}
-		var rx Rat
-		if err := dec.Decode(&rx); err != nil {
-			t.Errorf("decoding of %s failed: %s", &tx, err)
-		}
-		if rx.Cmp(&tx) != 0 {
-			t.Errorf("transmission of %s failed: got %s want %s", &tx, &rx, &tx)
-		}
-	}
-}
-
-// Sending a nil Rat pointer (inside a slice) on a round trip through gob should yield a zero.
-// TODO: top-level nils.
-func TestGobEncodingNilRatInSlice(t *testing.T) {
-	buf := new(bytes.Buffer)
-	enc := gob.NewEncoder(buf)
-	dec := gob.NewDecoder(buf)
-
-	var in = make([]*Rat, 1)
-	err := enc.Encode(&in)
-	if err != nil {
-		t.Errorf("gob encode failed: %q", err)
-	}
-	var out []*Rat
-	err = dec.Decode(&out)
-	if err != nil {
-		t.Fatalf("gob decode failed: %q", err)
-	}
-	if len(out) != 1 {
-		t.Fatalf("wrong len; want 1 got %d", len(out))
-	}
-	var zero Rat
-	if out[0].Cmp(&zero) != 0 {
-		t.Errorf("transmission of (*Int)(nill) failed: got %s want 0", out)
-	}
-}
-
-var ratNums = []string{
-	"-141592653589793238462643383279502884197169399375105820974944592307816406286",
-	"-1415926535897932384626433832795028841971",
-	"-141592653589793",
-	"-1",
-	"0",
-	"1",
-	"141592653589793",
-	"1415926535897932384626433832795028841971",
-	"141592653589793238462643383279502884197169399375105820974944592307816406286",
-}
-
-var ratDenoms = []string{
-	"1",
-	"718281828459045",
-	"7182818284590452353602874713526624977572",
-	"718281828459045235360287471352662497757247093699959574966967627724076630353",
-}
-
-func TestRatJSONEncoding(t *testing.T) {
-	for _, num := range ratNums {
-		for _, denom := range ratDenoms {
-			var tx Rat
-			tx.SetString(num + "/" + denom)
-			b, err := json.Marshal(&tx)
-			if err != nil {
-				t.Errorf("marshaling of %s failed: %s", &tx, err)
-				continue
-			}
-			var rx Rat
-			if err := json.Unmarshal(b, &rx); err != nil {
-				t.Errorf("unmarshaling of %s failed: %s", &tx, err)
-				continue
-			}
-			if rx.Cmp(&tx) != 0 {
-				t.Errorf("JSON encoding of %s failed: got %s want %s", &tx, &rx, &tx)
-			}
-		}
-	}
-}
-
-func TestRatXMLEncoding(t *testing.T) {
-	for _, num := range ratNums {
-		for _, denom := range ratDenoms {
-			var tx Rat
-			tx.SetString(num + "/" + denom)
-			b, err := xml.Marshal(&tx)
-			if err != nil {
-				t.Errorf("marshaling of %s failed: %s", &tx, err)
-				continue
-			}
-			var rx Rat
-			if err := xml.Unmarshal(b, &rx); err != nil {
-				t.Errorf("unmarshaling of %s failed: %s", &tx, err)
-				continue
-			}
-			if rx.Cmp(&tx) != 0 {
-				t.Errorf("XML encoding of %s failed: got %s want %s", &tx, &rx, &tx)
-			}
-		}
-	}
-}
-
 func TestIssue2379(t *testing.T) {
 	// 1) no aliasing
 	q := NewRat(3, 2)
diff --git a/src/cmd/compile/internal/big/ratconv.go b/src/cmd/compile/internal/big/ratconv.go
index 961ff649a5..4566ff4e39 100644
--- a/src/cmd/compile/internal/big/ratconv.go
+++ b/src/cmd/compile/internal/big/ratconv.go
@@ -188,11 +188,15 @@ func scanExponent(r io.ByteScanner, binExpOk bool) (exp int64, base int, err err
 
 // String returns a string representation of x in the form "a/b" (even if b == 1).
 func (x *Rat) String() string {
-	s := "/1"
+	var buf []byte
+	buf = x.a.Append(buf, 10)
+	buf = append(buf, '/')
 	if len(x.b.abs) != 0 {
-		s = "/" + x.b.abs.decimalString()
+		buf = x.b.Append(buf, 10)
+	} else {
+		buf = append(buf, '1')
 	}
-	return x.a.String() + s
+	return string(buf)
 }
 
 // RatString returns a string representation of x in the form "a/b" if b != 1,
@@ -208,12 +212,17 @@ func (x *Rat) RatString() string {
 // digits of precision after the decimal point. The last digit is rounded to
 // nearest, with halves rounded away from zero.
 func (x *Rat) FloatString(prec int) string {
+	var buf []byte
+
 	if x.IsInt() {
-		s := x.a.String()
+		buf = x.a.Append(buf, 10)
 		if prec > 0 {
-			s += "." + strings.Repeat("0", prec)
+			buf = append(buf, '.')
+			for i := prec; i > 0; i-- {
+				buf = append(buf, '0')
+			}
 		}
-		return s
+		return string(buf)
 	}
 	// x.b.abs != 0
 
@@ -237,16 +246,19 @@ func (x *Rat) FloatString(prec int) string {
 		}
 	}
 
-	s := q.decimalString()
 	if x.a.neg {
-		s = "-" + s
+		buf = append(buf, '-')
 	}
+	buf = append(buf, q.utoa(10)...) // itoa ignores sign if q == 0
 
 	if prec > 0 {
-		rs := r.decimalString()
-		leadingZeros := prec - len(rs)
-		s += "." + strings.Repeat("0", leadingZeros) + rs
+		buf = append(buf, '.')
+		rs := r.utoa(10)
+		for i := prec - len(rs); i > 0; i-- {
+			buf = append(buf, '0')
+		}
+		buf = append(buf, rs...)
 	}
 
-	return s
+	return string(buf)
 }