math/big: added (internal) Float.form field for easier case distinctions

This is a fairly significant _internal_ representation change. Instead
of encoding 0, finite, infinite, and NaN values with special mantissa
and exponent values, a new (1 byte) 'form' field is used (without making
the Float struct bigger). The form field permits simpler and faster
case distinctions. As a side benefit, for zero and non-finite floats,
fewer fields need to be set. Also, the exponent range is not the full
int32 range (in the old format, infExp and nanExp were used to represent
Inf and NaN values and tests for those values sometimes didn't test
for the empty mantissa, so the range was reduced by 2 values).

The correspondence between the old and new fields is as follows.
Old representation:

x                 neg      mant         exp
---------------------------------------------------------------
+/-0              sign     empty        0
0 < |x| < +Inf    sign     mantissa     exponent
+/-Inf            sign     empty        infExp
NaN               false    empty        nanExp

New representation (- stands for ignored fields):

x                 neg      mant         exp         form
---------------------------------------------------------------
+/-0              sign     -            -           zero
0 < |x| < +Inf    sign     mantissa     exponent    finite
+/-Inf            sign     -            -           inf
NaN               -        -            -           nan

Client should not be affected by this change.

Change-Id: I7e355894d602ceb23f9ec01da755fe6e0386b101
Reviewed-on: https://go-review.googlesource.com/6870Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/big/decimal.go

@@ -37,6 +37,9 @@ const maxShift = _W - 4
 // precision argument and keeping track of when a number was truncated early
 // (equivalent of "sticky bit" in binary rounding).
 
+// TODO(gri) Along the same lines, enforce some limit to shift magnitudes
+// to avoid "infinitely" long running conversions (until we run out of space).
+
 // Init initializes x to the decimal representation of m << shift (for
 // shift >= 0), or m >> -shift (for shift < 0).
 func (x *decimal) init(m nat, shift int) {

src/math/big/float.go

@@ -24,7 +24,9 @@ const debugFloat = true // enable for debugging
 //
 // with 0.5 <= mantissa < 1.0, and MinExp <= exponent <= MaxExp.
 // A Float may also be zero (+0, -0), infinite (+Inf, -Inf) or
-// not-a-number (NaN).
+// not-a-number (NaN). Except for NaNs, all Floats are ordered,
+// and the ordering of two Floats x and y is defined by x.Cmp(y).
+// NaNs are always different from any other Float value.
 //
 // Each Float value also has a precision, rounding mode, and accuracy.
 // The precision is the maximum number of mantissa bits available to
@@ -57,29 +59,49 @@ type Float struct {
 	prec uint32
 	mode RoundingMode
 	acc  Accuracy
+	form form
 	neg  bool
 	mant nat
 	exp  int32
 }
 
-// Internal representation: The mantissa bits x.mant of a Float x are stored
-// in a nat slice long enough to hold up to x.prec bits; the slice may (but
-// doesn't have to) be shorter if the mantissa contains trailing 0 bits.
-// Unless x is a zero, infinity, or NaN, x.mant is normalized such that the
-// msb of x.mant == 1 (i.e., the msb is shifted all the way "to the left").
-// Thus, if the mantissa has trailing 0 bits or x.prec is not a multiple
-// of the the Word size _W, x.mant[0] has trailing zero bits. Zero, Inf, and
-// NaN values have an empty mantissa and a 0, infExp, or NanExp exponent,
-// respectively.
-
+// Exponent and precision limits.
 const (
 	MaxExp  = math.MaxInt32  // largest supported exponent
-	MinExp  = math.MinInt32 + 2 // smallest supported exponent
-	infExp  = math.MinInt32 + 1 // exponent for Inf values
-	nanExp  = math.MinInt32 + 0 // exponent for NaN values
+	MinExp  = math.MinInt32  // smallest supported exponent
 	MaxPrec = math.MaxUint32 // largest (theoretically) supported precision; likely memory-limited
 )
 
+// Internal representation: The mantissa bits x.mant of a nonzero finite
+// Float x are stored in a nat slice long enough to hold up to x.prec bits;
+// the slice may (but doesn't have to) be shorter if the mantissa contains
+// trailing 0 bits. x.mant is normalized if the msb of x.mant == 1 (i.e.,
+// the msb is shifted all the way "to the left"). Thus, if the mantissa has
+// trailing 0 bits or x.prec is not a multiple of the the Word size _W,
+// x.mant[0] has trailing zero bits. The msb of the mantissa corresponds
+// to the value 0.5; the exponent x.exp shifts the binary point as needed.
+//
+// A zero or non-finite Float x ignores x.mant and x.exp. A NaN x ignores
+// the sign x.neg.
+//
+// x                 form      neg      mant         exp
+// ----------------------------------------------------------
+// ±0                zero      sign     -            -
+// 0 < |x| < +Inf    finite    sign     mantissa     exponent
+// ±Inf              inf       sign     -            -
+// NaN               nan       -        -            -
+
+// A form value describes the internal representation.
+type form byte
+
+// The form value order is relevant - do not change!
+const (
+	zero form = iota
+	finite
+	inf
+	nan
+)
+
 // RoundingMode determines how a Float value is rounded to the
 // desired precision. Rounding may change the Float value; the
 // rounding error is described by the Float's Accuracy.
@@ -113,6 +135,13 @@ const (
 
 //go:generate stringer -type=Accuracy
 
+func (x *Float) cmpZero() Accuracy {
+	if x.neg {
+		return Above
+	}
+	return Below
+}
+
 // SetPrec sets z's precision to prec and returns the (possibly) rounded
 // value of z. Rounding occurs according to z's rounding mode if the mantissa
 // cannot be represented in prec bits without loss of precision.
@@ -124,14 +153,10 @@ func (z *Float) SetPrec(prec uint) *Float {
 	// special case
 	if prec == 0 {
 		z.prec = 0
-		if len(z.mant) != 0 {
-			// truncate and compute accuracy
-			z.setZero()
-			acc := Below
-			if z.neg {
-				acc = Above
-			}
-			z.acc = acc
+		if z.form == finite {
+			// truncate z to 0
+			z.acc = z.cmpZero()
+			z.form = zero
 		}
 		return z
 	}
@@ -165,8 +190,11 @@ func (x *Float) Prec() uint {
 
 // MinPrec returns the minimum precision required to represent x exactly
 // (i.e., the smallest prec before x.SetPrec(prec) would start rounding x).
-// The result is 0 for ±0, ±Inf, and NaN.
+// The result is 0 if x is 0 or not finite.
 func (x *Float) MinPrec() uint {
+	if x.form != finite {
+		return 0
+	}
 	return uint(len(x.mant))*_W - x.mant.trailingZeroBits()
 }
 
@@ -190,7 +218,7 @@ func (x *Float) Sign() int {
 	if debugFloat {
 		x.validate()
 	}
-	if len(x.mant) == 0 && x.exp != infExp {
+	if x.form == zero || x.form == nan {
 		return 0
 	}
 	if x.neg {
@@ -219,18 +247,36 @@ func (x *Float) MantExp(mant *Float) (exp int) {
 	if debugFloat {
 		x.validate()
 	}
-	if len(x.mant) != 0 {
+	if x.form == finite {
 		exp = int(x.exp)
 	}
 	if mant != nil {
 		mant.Copy(x)
-		if x.exp >= MinExp {
+		if mant.form == finite {
 			mant.exp = 0
 		}
 	}
 	return
 }
 
+// setExp sets the exponent for z.
+// If e < MinExp, z becomes ±0; if e > MaxExp, z becomes ±Inf.
+func (z *Float) setExp(e int64) {
+	if debugFloat && z.form != finite {
+		panic("setExp called for non-finite Float")
+	}
+	switch {
+	case e < MinExp:
+		// TODO(gri) check that accuracy is adjusted if necessary
+		z.form = zero // underflow
+	default:
+		z.exp = int32(e)
+	case e > MaxExp:
+		// TODO(gri) check that accuracy is adjusted if necessary
+		z.form = inf // overflow
+	}
+}
+
 // SetMantExp sets z to mant × 2**exp and and returns z.
 // The result z has the same precision and rounding mode
 // as mant. SetMantExp is an inverse of MantExp but does
@@ -253,7 +299,7 @@ func (z *Float) SetMantExp(mant *Float, exp int) *Float {
 		mant.validate()
 	}
 	z.Copy(mant)
-	if len(z.mant) == 0 {
+	if z.form != finite {
 		return z
 	}
 	z.setExp(int64(z.exp) + int64(exp))
@@ -263,28 +309,28 @@ func (z *Float) SetMantExp(mant *Float, exp int) *Float {
 // IsNeg reports whether x is negative.
 // A NaN value is not negative.
 func (x *Float) IsNeg() bool {
-	return x.neg && x.exp != nanExp
+	return x.neg && x.form != nan
 }
 
 // IsZero reports whether x is +0 or -0.
 func (x *Float) IsZero() bool {
-	return len(x.mant) == 0 && x.exp == 0
+	return x.form == zero
 }
 
 // IsFinite reports whether -Inf < x < Inf.
 // A NaN value is not finite.
 func (x *Float) IsFinite() bool {
-	return len(x.mant) != 0 || x.exp == 0
+	return x.form <= finite
 }
 
 // IsInf reports whether x is +Inf or -Inf.
 func (x *Float) IsInf() bool {
-	return x.exp == infExp
+	return x.form == inf
 }
 
 // IsNaN reports whether x is a NaN value.
 func (x *Float) IsNaN() bool {
-	return x.exp == nanExp
+	return x.form == nan
 }
 
 // IsInt reports whether x is an integer.
@@ -293,56 +339,32 @@ func (x *Float) IsInt() bool {
 	if debugFloat {
 		x.validate()
 	}
-	// pick off easy cases
+	// special cases
+	if x.form != finite {
+		return x.form == zero
+	}
+	// x.form == finite
 	if x.exp <= 0 {
-		// |x| < 1 || |x| == Inf || x is NaN
-		return len(x.mant) == 0 && x.exp == 0 // x == 0
+		return false
 	}
 	// x.exp > 0
 	return x.prec <= uint32(x.exp) || x.MinPrec() <= uint(x.exp) // not enough bits for fractional mantissa
 }
 
-func (z *Float) setZero() {
-	z.mant = z.mant[:0]
-	z.exp = 0
-}
-
-func (z *Float) setInf() {
-	z.mant = z.mant[:0]
-	z.exp = infExp
-}
-
-// setExp sets the exponent for z.
-// If e < MinExp, z becomes ±0; if e > MaxExp, z becomes ±Inf.
-func (z *Float) setExp(e int64) {
-	switch {
-	case e < MinExp:
-		z.setZero()
-	default:
-		if len(z.mant) == 0 {
-			e = 0
-		}
-		z.exp = int32(e)
-	case e > MaxExp:
-		z.setInf()
-	}
-}
-
 // debugging support
 func (x *Float) validate() {
 	if !debugFloat {
 		// avoid performance bugs
 		panic("validate called but debugFloat is not set")
 	}
-	const msb = 1 << (_W - 1)
+	if x.form != finite {
+		return
+	}
 	m := len(x.mant)
 	if m == 0 {
-		// 0.0, Inf, or NaN
-		if x.exp != 0 && x.exp >= MinExp {
-			panic(fmt.Sprintf("empty matissa with invalid exponent %d", x.exp))
-		}
-		return
+		panic("nonzero finite x with empty mantissa")
 	}
+	const msb = 1 << (_W - 1)
 	if x.mant[m-1]&msb == 0 {
 		panic(fmt.Sprintf("msb not set in last word %#x of %s", x.mant[m-1], x.Format('p', 0)))
 	}
@@ -362,20 +384,20 @@ func (x *Float) validate() {
 func (z *Float) round(sbit uint) {
 	if debugFloat {
 		z.validate()
+		if z.form > finite {
+			panic(fmt.Sprintf("round called for non-finite value %s", z))
+		}
 	}
+	// z.form <= finite
 
 	z.acc = Exact
-
-	// handle zero, Inf, and NaN
-	m := uint32(len(z.mant)) // present mantissa length in words
-	if m == 0 {
-		if z.exp == nanExp {
-			z.acc = Undef
-		}
+	if z.form == zero {
 		return
 	}
+	// z.form == finite && len(z.mant) > 0
 	// m > 0 implies z.prec > 0 (checked by validate)
 
+	m := uint32(len(z.mant)) // present mantissa length in words
 	bits := m * _W           // present mantissa bits
 	if bits <= z.prec {
 		// mantissa fits => nothing to do
@@ -487,6 +509,8 @@ func (z *Float) round(sbit uint) {
 	// zero out trailing bits in least-significant word
 	z.mant[0] &^= lsb - 1
 
+	// TODO(gri) can z.mant be all 0s at this point?
+
 	// update accuracy
 	if z.acc != Exact && z.neg {
 		z.acc ^= Below | Above
@@ -525,10 +549,11 @@ func (z *Float) setBits64(neg bool, x uint64) *Float {
 	z.acc = Exact
 	z.neg = neg
 	if x == 0 {
-		z.setZero()
+		z.form = zero
 		return z
 	}
 	// x != 0
+	z.form = finite
 	s := nlz64(x)
 	z.mant = z.mant.setUint64(x << s)
 	z.exp = int32(64 - s) // always fits
@@ -566,20 +591,20 @@ func (z *Float) SetFloat64(x float64) *Float {
 		z.prec = 53
 	}
 	if math.IsNaN(x) {
-		z.SetNaN()
-		return z
+		return z.SetNaN()
 	}
 	z.acc = Exact
 	z.neg = math.Signbit(x) // handle -0, -Inf correctly
-	if math.IsInf(x, 0) {
-		z.setInf()
+	if x == 0 {
+		z.form = zero
 		return z
 	}
-	if x == 0 {
-		z.setZero()
+	if math.IsInf(x, 0) {
+		z.form = inf
 		return z
 	}
 	// normalized x != 0
+	z.form = finite
 	fmant, exp := math.Frexp(x) // get normalized mantissa
 	z.mant = z.mant.setUint64(1<<63 | math.Float64bits(fmant)<<11)
 	z.exp = int32(exp) // always fits
@@ -620,11 +645,11 @@ func (z *Float) SetInt(x *Int) *Float {
 	z.acc = Exact
 	z.neg = x.neg
 	if len(x.abs) == 0 {
-		z.mant = z.mant[:0]
-		z.exp = 0
+		z.form = zero
 		return z
 	}
 	// x != 0
+	z.form = finite
 	z.mant = z.mant.set(x.abs)
 	fnorm(z.mant)
 	z.setExp(int64(bits))
@@ -655,18 +680,16 @@ func (z *Float) SetRat(x *Rat) *Float {
 // z is unchanged and the result is always Exact.
 func (z *Float) SetInf(sign int) *Float {
 	z.acc = Exact
+	z.form = inf
 	z.neg = sign < 0
-	z.setInf()
 	return z
 }
 
 // SetNaN sets z to a NaN value, and returns z.
-// The precision of z is unchanged and the result is always Exact.
+// The precision of z is unchanged and the result is always Undef.
 func (z *Float) SetNaN() *Float {
-	z.acc = Exact
-	z.neg = false
-	z.mant = z.mant[:0]
-	z.exp = nanExp
+	z.acc = Undef
+	z.form = nan
 	return z
 }
 
@@ -685,6 +708,7 @@ func (z *Float) Set(x *Float) *Float {
 		if z.prec == 0 {
 			z.prec = x.prec
 		}
+		z.form = x.form
 		z.neg = x.neg
 		z.exp = x.exp
 		z.mant = z.mant.set(x.mant)
@@ -706,6 +730,7 @@ func (z *Float) Copy(x *Float) *Float {
 		z.prec = x.prec
 		z.mode = x.mode
 		z.acc = x.acc
+		z.form = x.form
 		z.neg = x.neg
 		z.mant = z.mant.set(x.mant)
 		z.exp = x.exp
@@ -739,31 +764,17 @@ func (x *Float) Uint64() (uint64, Accuracy) {
 		x.validate()
 	}
 
-	// special cases
-	if len(x.mant) == 0 {
-		switch x.exp {
-		case 0:
-			return 0, Exact // ±0
-		case infExp:
-			if x.neg {
-				return 0, Above // -Inf
-			}
-			return math.MaxUint64, Below // +Inf
-		case nanExp:
-			return 0, Undef // NaN
-		}
-		panic("unreachable")
-	}
-
+	switch x.form {
+	case finite:
 		if x.neg {
 			return 0, Above
 		}
-	// x > 0
+		// 0 < x < +Inf
 		if x.exp <= 0 {
 			// 0 < x < 1
 			return 0, Below
 		}
-	// 1 <= x
+		// 1 <= x < Inf
 		if x.exp <= 64 {
 			// u = trunc(x) fits into a uint64
 			u := high64(x.mant) >> (64 - uint32(x.exp))
@@ -774,6 +785,21 @@ func (x *Float) Uint64() (uint64, Accuracy) {
 		}
 		// x too large
 		return math.MaxUint64, Below
+
+	case zero:
+		return 0, Exact
+
+	case inf:
+		if x.neg {
+			return 0, Above
+		}
+		return math.MaxUint64, Below
+
+	case nan:
+		return 0, Undef
+	}
+
+	panic("unreachable")
 }
 
 // Int64 returns the integer resulting from truncating x towards zero.
@@ -786,27 +812,10 @@ func (x *Float) Int64() (int64, Accuracy) {
 		x.validate()
 	}
 
-	// special cases
-	if len(x.mant) == 0 {
-		switch x.exp {
-		case 0:
-			return 0, Exact // ±0
-		case infExp:
-			if x.neg {
-				return math.MinInt64, Above // -Inf
-			}
-			return math.MaxInt64, Below // +Inf
-		case nanExp:
-			return 0, Undef // NaN
-		}
-		panic("unreachable")
-	}
-
+	switch x.form {
+	case finite:
 		// 0 < |x| < +Inf
-	acc := Below
-	if x.neg {
-		acc = Above
-	}
+		acc := x.cmpZero()
 		if x.exp <= 0 {
 			// 0 < |x| < 1
 			return 0, acc
@@ -832,8 +841,23 @@ func (x *Float) Int64() (int64, Accuracy) {
 			}
 			return math.MinInt64, acc
 		}
-	// x == +Inf
+		// x too large
+		return math.MaxInt64, Below
+
+	case zero:
+		return 0, Exact
+
+	case inf:
+		if x.neg {
+			return math.MinInt64, Above
+		}
 		return math.MaxInt64, Below
+
+	case nan:
+		return 0, Undef
+	}
+
+	panic("unreachable")
 }
 
 // Float64 returns the closest float64 value of x
@@ -844,27 +868,8 @@ func (x *Float) Float64() (float64, Accuracy) {
 		x.validate()
 	}
 
-	// special cases
-	if len(x.mant) == 0 {
-		switch x.exp {
-		case 0:
-			if x.neg {
-				var zero float64
-				return -zero, Exact
-			}
-			return 0, Exact
-		case infExp:
-			var sign int
-			if x.neg {
-				sign = -1
-			}
-			return math.Inf(sign), Exact
-		case nanExp:
-			return math.NaN(), Undef
-		}
-		panic("unreachable")
-	}
-
+	switch x.form {
+	case finite:
 		// 0 < |x| < +Inf
 		var r Float
 		r.prec = 53
@@ -876,6 +881,26 @@ func (x *Float) Float64() (float64, Accuracy) {
 		e := uint64(1022+r.exp) & 0x7ff // TODO(gri) check for overflow
 		m := high64(r.mant) >> 11 & (1<<52 - 1)
 		return math.Float64frombits(s | e<<52 | m), r.acc
+
+	case zero:
+		z := 0.0
+		if x.neg {
+			z = -z
+		}
+		return z, Exact
+
+	case inf:
+		sign := +1
+		if x.neg {
+			sign = -1
+		}
+		return math.Inf(sign), Exact
+
+	case nan:
+		return math.NaN(), Undef
+	}
+
+	panic("unreachable")
 }
 
 // Int returns the result of truncating x towards zero;
@@ -889,34 +914,14 @@ func (x *Float) Int(z *Int) (*Int, Accuracy) {
 		x.validate()
 	}
 
-	if z == nil {
-		// no need to do this for Inf and NaN
-		// but those are rare enough that we
-		// don't care
+	if z == nil && x.form <= finite {
 		z = new(Int)
 	}
 
-	// special cases
-	if len(x.mant) == 0 {
-		switch x.exp {
-		case 0:
-			return z.SetInt64(0), Exact // 0
-		case infExp:
-			if x.neg {
-				return nil, Above
-			}
-			return nil, Below
-		case nanExp:
-			return nil, Undef
-		}
-		panic("unreachable")
-	}
-
+	switch x.form {
+	case finite:
 		// 0 < |x| < +Inf
-	acc := Below
-	if x.neg {
-		acc = Above
-	}
+		acc := x.cmpZero()
 		if x.exp <= 0 {
 			// 0 < |x| < 1
 			return z.SetInt64(0), acc
@@ -944,6 +949,18 @@ func (x *Float) Int(z *Int) (*Int, Accuracy) {
 			z.abs = z.abs.shr(x.mant, allBits-exp)
 		}
 		return z, acc
+
+	case zero:
+		return z.SetInt64(0), Exact
+
+	case inf:
+		return nil, x.cmpZero()
+
+	case nan:
+		return nil, Undef
+	}
+
+	panic("unreachable")
 }
 
 // Rat returns the rational number corresponding to x;
@@ -956,30 +973,13 @@ func (x *Float) Rat(z *Rat) (*Rat, Accuracy) {
 		x.validate()
 	}
 
-	if z == nil {
-		// no need to do this for Inf and NaN
-		// but those are rare enough that we
-		// don't care
+	if z == nil && x.form <= finite {
 		z = new(Rat)
 	}
 
-	// special cases
-	if len(x.mant) == 0 {
-		switch x.exp {
-		case 0:
-			return z.SetInt64(0), Exact // 0
-		case infExp:
-			if x.neg {
-				return nil, Above
-			}
-			return nil, Below
-		case nanExp:
-			return nil, Undef
-		}
-		panic("unreachable")
-	}
-
-	// 0 <= |x| < Inf
+	switch x.form {
+	case finite:
+		// 0 < |x| < +Inf
 		allBits := int32(len(x.mant)) * _W
 		// build up numerator and denominator
 		z.a.neg = x.neg
@@ -999,6 +999,18 @@ func (x *Float) Rat(z *Rat) (*Rat, Accuracy) {
 			z.norm()
 		}
 		return z, Exact
+
+	case zero:
+		return z.SetInt64(0), Exact
+
+	case inf:
+		return nil, x.cmpZero()
+
+	case nan:
+		return nil, Undef
+	}
+
+	panic("unreachable")
 }
 
 // Abs sets z to the (possibly rounded) value |x| (the absolute value of x)
@@ -1031,7 +1043,7 @@ func (z *Float) uadd(x, y *Float) {
 	// http://www.vinc17.net/research/papers/rnc6.pdf
 
 	if debugFloat && (len(x.mant) == 0 || len(y.mant) == 0) {
-		panic("uadd called with 0 argument")
+		panic("uadd called with empty mantissa")
 	}
 
 	// compute exponents ex, ey for mantissa with "binary point"
@@ -1070,7 +1082,7 @@ func (z *Float) usub(x, y *Float) {
 	// by special-casing, and the code will diverge.
 
 	if debugFloat && (len(x.mant) == 0 || len(y.mant) == 0) {
-		panic("usub called with 0 argument")
+		panic("usub called with empty mantissa")
 	}
 
 	ex := int64(x.exp) - int64(len(x.mant))*_W
@@ -1094,7 +1106,7 @@ func (z *Float) usub(x, y *Float) {
 	// operands may have cancelled each other out
 	if len(z.mant) == 0 {
 		z.acc = Exact
-		z.setZero()
+		z.form = zero
 		return
 	}
 	// len(z.mant) > 0
@@ -1107,7 +1119,7 @@ func (z *Float) usub(x, y *Float) {
 // x and y must not be 0, Inf, or NaN.
 func (z *Float) umul(x, y *Float) {
 	if debugFloat && (len(x.mant) == 0 || len(y.mant) == 0) {
-		panic("umul called with 0 argument")
+		panic("umul called with empty mantissa")
 	}
 
 	// Note: This is doing too much work if the precision
@@ -1128,7 +1140,7 @@ func (z *Float) umul(x, y *Float) {
 // x and y must not be 0, Inf, or NaN.
 func (z *Float) uquo(x, y *Float) {
 	if debugFloat && (len(x.mant) == 0 || len(y.mant) == 0) {
-		panic("uquo called with 0 argument")
+		panic("uquo called with empty mantissa")
 	}
 
 	// mantissa length in words for desired result precision + 1
@@ -1176,7 +1188,7 @@ func (z *Float) uquo(x, y *Float) {
 // while ignoring the signs of x and y. x and y must not be 0, Inf, or NaN.
 func (x *Float) ucmp(y *Float) int {
 	if debugFloat && (len(x.mant) == 0 || len(y.mant) == 0) {
-		panic("ucmp called with 0 argument")
+		panic("ucmp called with empty mantissa")
 	}
 
 	switch {
@@ -1248,14 +1260,14 @@ func (z *Float) Add(x, y *Float) *Float {
 	}
 
 	// special cases
-	if len(x.mant) == 0 || len(y.mant) == 0 {
-		if x.exp <= infExp || y.exp <= infExp {
+	if x.form != finite || y.form != finite {
+		if x.form > finite || y.form > finite {
 			// TODO(gri) handle Inf separately
 			return z.SetNaN()
 		}
-		if len(x.mant) == 0 { // x == ±0
+		if x.form == zero {
 			z.Set(y)
-			if len(z.mant) == 0 && z.exp == 0 {
+			if z.form == zero {
 				z.neg = x.neg && y.neg // -0 + -0 == -0
 			}
 			return z
@@ -1265,6 +1277,7 @@ func (z *Float) Add(x, y *Float) *Float {
 	}
 
 	// x, y != 0
+	z.form = finite
 	z.neg = x.neg
 	if x.neg == y.neg {
 		// x + y == x + y
@@ -1282,7 +1295,7 @@ func (z *Float) Add(x, y *Float) *Float {
 	}
 
 	// -0 is only possible for -0 + -0
-	if len(z.mant) == 0 {
+	if z.form == zero {
 		z.neg = false
 	}
 
@@ -1303,14 +1316,14 @@ func (z *Float) Sub(x, y *Float) *Float {
 	}
 
 	// special cases
-	if len(x.mant) == 0 || len(y.mant) == 0 {
-		if x.exp <= infExp || y.exp <= infExp {
+	if x.form != finite || y.form != finite {
+		if x.form > finite || y.form > finite {
 			// TODO(gri) handle Inf separately
 			return z.SetNaN()
 		}
-		if len(x.mant) == 0 { // x == ±0
+		if x.form == zero {
 			z.Neg(y)
-			if len(z.mant) == 0 && z.exp == 0 {
+			if z.form == zero {
 				z.neg = x.neg && !y.neg // -0 - 0 == -0
 			}
 			return z
@@ -1320,6 +1333,7 @@ func (z *Float) Sub(x, y *Float) *Float {
 	}
 
 	// x, y != 0
+	z.form = finite
 	z.neg = x.neg
 	if x.neg != y.neg {
 		// x - (-y) == x + y
@@ -1337,7 +1351,7 @@ func (z *Float) Sub(x, y *Float) *Float {
 	}
 
 	// -0 is only possible for -0 - 0
-	if len(z.mant) == 0 {
+	if z.form == zero {
 		z.neg = false
 	}
 
@@ -1360,24 +1374,25 @@ func (z *Float) Mul(x, y *Float) *Float {
 	z.neg = x.neg != y.neg
 
 	// special cases
-	if len(x.mant) == 0 || len(y.mant) == 0 {
-		if x.exp <= infExp || y.exp <= infExp {
+	if x.form != finite || y.form != finite {
+		if x.form > finite || y.form > finite {
 			// TODO(gri) handle Inf separately
 			return z.SetNaN()
 		}
 		// x == ±0 || y == ±0
 		z.acc = Exact
-		z.setZero()
+		z.form = zero
 		return z
 	}
 
-	if len(x.mant) == 0 || len(y.mant) == 0 {
+	if x.form == zero || y.form == zero {
 		z.acc = Exact
-		z.setZero()
+		z.form = zero
 		return z
 	}
 
 	// x, y != 0
+	z.form = finite
 	z.umul(x, y)
 	return z
 }
@@ -1399,26 +1414,27 @@ func (z *Float) Quo(x, y *Float) *Float {
 
 	// special cases
 	z.acc = Exact
-	if len(x.mant) == 0 || len(y.mant) == 0 {
-		if x.exp <= infExp || y.exp <= infExp {
+	if x.form != finite || y.form != finite {
+		if x.form > finite || y.form > finite {
 			// TODO(gri) handle Inf separately
 			return z.SetNaN()
 		}
-		if len(x.mant) == 0 {
-			if len(y.mant) == 0 {
+		if x.form == zero {
+			if y.form == zero {
 				return z.SetNaN()
 			}
-			z.setZero()
+			z.form = zero
 			return z
 		}
 		// x != 0
-		if len(y.mant) == 0 {
-			z.setInf()
+		if y.form == zero {
+			z.form = inf
 			return z
 		}
 	}
 
 	// x, y != 0
+	z.form = finite
 	z.uquo(x, y)
 	return z
 }
@@ -1475,16 +1491,17 @@ func umax32(x, y uint32) uint32 {
 //	+2 if x == +Inf
 //
 func (x *Float) ord() int {
-	m := 1 // common case
-	if len(x.mant) == 0 {
-		m = 0
-		if x.exp == infExp {
+	var m int
+	switch x.form {
+	case finite:
+		m = 1
+	case zero:
+		return 0
+	case inf:
 		m = 2
-		}
-		if x.exp == nanExp {
+	case nan:
 		panic("unimplemented")
 	}
-	}
 	if x.neg {
 		m = -m
 	}

src/math/big/float_test.go

@@ -90,15 +90,20 @@ func TestFloatZeroValue(t *testing.T) {
 
 func makeFloat(s string) *Float {
 	var x Float
-	if s == "Inf" || s == "+Inf" {
+
+	switch s {
+	case "0":
+		return &x
+	case "-0":
+		return x.Neg(&x)
+	case "Inf", "+Inf":
 		return x.SetInf(+1)
-	}
-	if s == "-Inf" {
+	case "-Inf":
 		return x.SetInf(-1)
-	}
-	if s == "NaN" || s == "-NaN" {
+	case "NaN", "-NaN":
 		return x.SetNaN()
 	}
+
 	x.SetPrec(1000)
 	if _, ok := x.SetString(s); !ok {
 		panic(fmt.Sprintf("%q is not a valid float", s))
@@ -146,13 +151,6 @@ func TestFloatSetPrec(t *testing.T) {
 		if got, acc := x.String(), x.Acc(); got != test.want || acc != test.acc {
 			t.Errorf("%s.SetPrec(%d) = %s (%s); want %s (%s)", test.x, test.prec, got, acc, test.want, test.acc)
 		}
-		// look inside x and check correct value for x.exp
-		if len(x.mant) == 0 {
-			// ±0, ±Inf, or NaN
-			if x.exp != 0 && x.exp > MinExp {
-				t.Errorf("%s.SetPrec(%d): incorrect exponent %d", test.x, test.prec, x.exp)
-			}
-		}
 	}
 }
 
@@ -209,7 +207,7 @@ func feq(x, y *Float) bool {
 	if x.IsNaN() || y.IsNaN() {
 		return x.IsNaN() && y.IsNaN()
 	}
-	return x.Cmp(y) == 0 && x.neg == y.neg
+	return x.Cmp(y) == 0 && x.IsNeg() == y.IsNeg()
 }
 
 func TestFloatMantExp(t *testing.T) {
@@ -261,9 +259,9 @@ func TestFloatSetMantExp(t *testing.T) {
 		{"Inf", 1234, "+Inf"},
 		{"+Inf", -1234, "+Inf"},
 		{"-Inf", -1234, "-Inf"},
-		{"0", -MaxExp - 1, "0"},
-		{"0.5", -MaxExp - 1, "+0"},  // exponent underflow
-		{"-0.5", -MaxExp - 1, "-0"}, // exponent underflow
+		{"0", MinExp, "0"},
+		{"0.25", MinExp, "+0"},    // exponent underflow
+		{"-0.25", MinExp, "-0"},   // exponent underflow
 		{"1", MaxExp, "+Inf"},     // exponent overflow
 		{"2", MaxExp - 1, "+Inf"}, // exponent overflow
 		{"0.75", 1, "1.5"},

src/math/big/floatconv.go

@@ -96,11 +96,13 @@ func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	// special-case 0
 	if len(z.mant) == 0 {
 		z.acc = Exact
-		z.exp = 0
+		z.form = zero
 		return
 	}
 	// len(z.mant) > 0
 
+	z.form = finite
+
 	// The mantissa may have a decimal point (fcount <= 0) and there
 	// may be a nonzero exponent exp. The decimal point amounts to a
 	// division by b**(-fcount). An exponent means multiplication by
@@ -275,9 +277,13 @@ func (x *Float) bstring(buf []byte) []byte {
 	if x.neg {
 		buf = append(buf, '-')
 	}
-	if len(x.mant) == 0 {
+	if x.form == zero {
 		return append(buf, '0')
 	}
+
+	if debugFloat && x.form != finite {
+		panic("non-finite float")
+	}
 	// x != 0
 
 	// adjust mantissa to use exactly x.prec bits
@@ -306,9 +312,13 @@ func (x *Float) pstring(buf []byte) []byte {
 	if x.neg {
 		buf = append(buf, '-')
 	}
-	if len(x.mant) == 0 {
+	if x.form == zero {
 		return append(buf, '0')
 	}
+
+	if debugFloat && x.form != finite {
+		panic("non-finite float")
+	}
 	// x != 0
 
 	// remove trailing 0 words early

src/math/big/ftoa.go

@@ -19,11 +19,17 @@ import "strconv"
 
 // bigFtoa formats a float for the %e, %E, %f, %g, and %G formats.
 func (f *Float) bigFtoa(buf []byte, fmt byte, prec int) []byte {
-	// TODO(gri) handle Inf.
+	if debugFloat && !f.IsFinite() {
+		panic("non-finite float")
+	}
 
 	// 1) convert Float to multiprecision decimal
+	var mant nat
+	if f.form == finite {
+		mant = f.mant
+	}
 	var d decimal
-	d.init(f.mant, int(f.exp)-f.mant.bitLen())
+	d.init(mant, int(f.exp)-f.mant.bitLen())
 
 	// 2) round to desired precision
 	shortest := false