src/cmd/compile/internal/gc: re-vendor math/big, manually adjust mpar…

…ith3.go

The only unreviewed change is in mparith3.go.

Change-Id: Iec0885e7688981cbaed04c152dc9b1c7032677e6
Reviewed-on: https://go-review.googlesource.com/10665
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>

src/cmd/compile/internal/big/arith.go

@@ -107,19 +107,34 @@ func log2(x Word) int {
 	return bitLen(x) - 1
 }
 
-// Number of leading zeros in x.
-func leadingZeros(x Word) uint {
+// nlz returns the number of leading zeros in x.
+func nlz(x Word) uint {
 	return uint(_W - bitLen(x))
 }
 
+// nlz64 returns the number of leading zeros in x.
+func nlz64(x uint64) uint {
+	switch _W {
+	case 32:
+		w := x >> 32
+		if w == 0 {
+			return 32 + nlz(Word(x))
+		}
+		return nlz(Word(w))
+	case 64:
+		return nlz(Word(x))
+	}
+	panic("unreachable")
+}
+
 // q = (u1<<_W + u0 - r)/y
 // Adapted from Warren, Hacker's Delight, p. 152.
 func divWW_g(u1, u0, v Word) (q, r Word) {
 	if u1 >= v {
 		return 1<<_W - 1, 1<<_W - 1
 	}
 
-	s := leadingZeros(v)
+	s := nlz(v)
 	v <<= s
 
 	vn1 := v >> _W2

src/cmd/compile/internal/big/bits_test.go

@@ -203,21 +203,21 @@ func TestFromBits(t *testing.T) {
 	}{
 		// all different bit numbers
 		{nil, "0"},
-		{Bits{0}, "0x.8p1"},
-		{Bits{1}, "0x.8p2"},
-		{Bits{-1}, "0x.8p0"},
-		{Bits{63}, "0x.8p64"},
-		{Bits{33, -30}, "0x.8000000000000001p34"},
-		{Bits{255, 0}, "0x.8000000000000000000000000000000000000000000000000000000000000001p256"},
+		{Bits{0}, "0x.8p+1"},
+		{Bits{1}, "0x.8p+2"},
+		{Bits{-1}, "0x.8p+0"},
+		{Bits{63}, "0x.8p+64"},
+		{Bits{33, -30}, "0x.8000000000000001p+34"},
+		{Bits{255, 0}, "0x.8000000000000000000000000000000000000000000000000000000000000001p+256"},
 
 		// multiple equal bit numbers
-		{Bits{0, 0}, "0x.8p2"},
-		{Bits{0, 0, 0, 0}, "0x.8p3"},
-		{Bits{0, 1, 0}, "0x.8p3"},
-		{append(Bits{2, 1, 0} /* 7 */, Bits{3, 1} /* 10 */ ...), "0x.88p5" /* 17 */},
+		{Bits{0, 0}, "0x.8p+2"},
+		{Bits{0, 0, 0, 0}, "0x.8p+3"},
+		{Bits{0, 1, 0}, "0x.8p+3"},
+		{append(Bits{2, 1, 0} /* 7 */, Bits{3, 1} /* 10 */ ...), "0x.88p+5" /* 17 */},
 	} {
 		f := test.bits.Float()
-		if got := f.Format('p', 0); got != test.want {
+		if got := f.Text('p', 0); got != test.want {
 			t.Errorf("setBits(%v) = %s; want %s", test.bits, got, test.want)
 		}
 	}

src/cmd/compile/internal/big/decimal.go

@@ -19,12 +19,14 @@
 
 package big
 
-// A decimal represents a floating-point number in decimal representation.
-// The value of a decimal x is x.mant * 10 ** x.exp with 0.5 <= x.mant < 1,
-// with the most-significant mantissa digit at index 0.
+// 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,
+// 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.
 type decimal struct {
 	mant []byte // mantissa ASCII digits, big-endian
-	exp  int    // exponent, valid if len(mant) > 0
+	exp  int    // exponent
 }
 
 // Maximum shift amount that can be done in one pass without overflow.
@@ -46,6 +48,7 @@ func (x *decimal) init(m nat, shift int) {
 	// special case 0
 	if len(m) == 0 {
 		x.mant = x.mant[:0]
+		x.exp = 0
 		return
 	}
 
@@ -255,4 +258,7 @@ func trim(x *decimal) {
 		i--
 	}
 	x.mant = x.mant[:i]
+	if i == 0 {
+		x.exp = 0
+	}
 }

src/cmd/compile/internal/big/float.go

@@ -16,7 +16,7 @@ import (
 	"math"
 )
 
-const debugFloat = true // enable for debugging
+const debugFloat = false // enable for debugging
 
 // A nonzero finite Float represents a multi-precision floating point number
 //
@@ -363,7 +363,7 @@ func (x *Float) validate() {
 	}
 	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)))
+		panic(fmt.Sprintf("msb not set in last word %#x of %s", x.mant[m-1], x.Text('p', 0)))
 	}
 	if x.prec == 0 {
 		panic("zero precision finite number")
@@ -381,14 +381,11 @@ 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
-	if z.form == zero {
+	if z.form != finite {
+		// ±0 or ±Inf => nothing left to do
 		return
 	}
 	// z.form == finite && len(z.mant) > 0
@@ -525,25 +522,6 @@ func (z *Float) round(sbit uint) {
 	return
 }
 
-// nlz returns the number of leading zero bits in x.
-func nlz(x Word) uint {
-	return _W - uint(bitLen(x))
-}
-
-func nlz64(x uint64) uint {
-	// TODO(gri) this can be done more nicely
-	if _W == 32 {
-		if x>>32 == 0 {
-			return 32 + nlz(Word(x))
-		}
-		return nlz(Word(x >> 32))
-	}
-	if _W == 64 {
-		return nlz(Word(x))
-	}
-	panic("unreachable")
-}
-
 func (z *Float) setBits64(neg bool, x uint64) *Float {
 	if z.prec == 0 {
 		z.prec = 64
@@ -732,25 +710,44 @@ func (z *Float) Copy(x *Float) *Float {
 	return z
 }
 
-func high32(x nat) uint32 {
-	// TODO(gri) This can be done more efficiently on 32bit platforms.
-	return uint32(high64(x) >> 32)
+// msb32 returns the 32 most significant bits of x.
+func msb32(x nat) uint32 {
+	i := len(x) - 1
+	if i < 0 {
+		return 0
+	}
+	if debugFloat && x[i]&(1<<(_W-1)) == 0 {
+		panic("x not normalized")
+	}
+	switch _W {
+	case 32:
+		return uint32(x[i])
+	case 64:
+		return uint32(x[i] >> 32)
+	}
+	panic("unreachable")
 }
 
-func high64(x nat) uint64 {
-	i := len(x)
-	if i == 0 {
+// msb64 returns the 64 most significant bits of x.
+func msb64(x nat) uint64 {
+	i := len(x) - 1
+	if i < 0 {
 		return 0
 	}
-	// i > 0
-	v := uint64(x[i-1])
-	if _W == 32 {
-		v <<= 32
-		if i > 1 {
-			v |= uint64(x[i-2])
+	if debugFloat && x[i]&(1<<(_W-1)) == 0 {
+		panic("x not normalized")
+	}
+	switch _W {
+	case 32:
+		v := uint64(x[i]) << 32
+		if i > 0 {
+			v |= uint64(x[i-1])
 		}
+		return v
+	case 64:
+		return uint64(x[i])
 	}
-	return v
+	panic("unreachable")
 }
 
 // Uint64 returns the unsigned integer resulting from truncating x
@@ -776,7 +773,7 @@ func (x *Float) Uint64() (uint64, Accuracy) {
 		// 1 <= x < Inf
 		if x.exp <= 64 {
 			// u = trunc(x) fits into a uint64
-			u := high64(x.mant) >> (64 - uint32(x.exp))
+			u := msb64(x.mant) >> (64 - uint32(x.exp))
 			if x.MinPrec() <= 64 {
 				return u, Exact
 			}
@@ -821,7 +818,7 @@ func (x *Float) Int64() (int64, Accuracy) {
 		// 1 <= |x| < +Inf
 		if x.exp <= 63 {
 			// i = trunc(x) fits into an int64 (excluding math.MinInt64)
-			i := int64(high64(x.mant) >> (64 - uint32(x.exp)))
+			i := int64(msb64(x.mant) >> (64 - uint32(x.exp)))
 			if x.neg {
 				i = -i
 			}
@@ -913,6 +910,7 @@ func (x *Float) Float32() (float32, Accuracy) {
 			}
 			return float32(math.Inf(+1)), Above
 		}
+		// e <= emax
 
 		// Determine sign, biased exponent, and mantissa.
 		var sign, bexp, mant uint32
@@ -933,11 +931,11 @@ func (x *Float) Float32() (float32, Accuracy) {
 				return 0.0, Below
 			}
 			// bexp = 0
-			mant = high32(r.mant) >> (fbits - r.prec)
+			mant = msb32(r.mant) >> (fbits - r.prec)
 		} else {
 			// normal number: emin <= e <= emax
 			bexp = uint32(e+bias) << mbits
-			mant = high32(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
+			mant = msb32(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
 		}
 
 		return math.Float32frombits(sign | bexp | mant), r.acc
@@ -1019,6 +1017,7 @@ func (x *Float) Float64() (float64, Accuracy) {
 			}
 			return math.Inf(+1), Above
 		}
+		// e <= emax
 
 		// Determine sign, biased exponent, and mantissa.
 		var sign, bexp, mant uint64
@@ -1039,11 +1038,11 @@ func (x *Float) Float64() (float64, Accuracy) {
 				return 0.0, Below
 			}
 			// bexp = 0
-			mant = high64(r.mant) >> (fbits - r.prec)
+			mant = msb64(r.mant) >> (fbits - r.prec)
 		} else {
 			// normal number: emin <= e <= emax
 			bexp = uint64(e+bias) << mbits
-			mant = high64(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
+			mant = msb64(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
 		}
 
 		return math.Float64frombits(sign | bexp | mant), r.acc