mirror of
https://github.com/golang/go
synced 2024-11-23 10:20:03 -07:00
cab54408da
Noted by gri in CL 100660044 review but I missed them. TBR=gri CC=golang-codereviews https://golang.org/cl/97570049
165 lines
7.9 KiB
Go
165 lines
7.9 KiB
Go
// run
|
||
|
||
// Check conversion of constant to float32/float64 near min/max boundaries.
|
||
|
||
// Copyright 2014 The Go Authors. All rights reserved.
|
||
// Use of this source code is governed by a BSD-style
|
||
// license that can be found in the LICENSE file.
|
||
|
||
package main
|
||
|
||
import (
|
||
"fmt"
|
||
"math"
|
||
)
|
||
|
||
// The largest exact float32 is f₁ = (1+(1-2²³))×2¹²⁷ = (1-2²⁴)×2¹²⁸ = 2¹²⁸ - 2¹⁰⁴.
|
||
// The next float32 would be f₂ = (1+1)×2¹²⁷ = 1×2¹²⁸, except that exponent is out of range.
|
||
// Float32 conversion rounds to the nearest float32, rounding to even mantissa:
|
||
// between f₁ and f₂, values closer to f₁ round to f₁ and values closer to f₂ are rejected as out of range.
|
||
// f₁ is an odd mantissa, so the halfway point (f₁+f₂)/2 rounds to f₂ and is rejected.
|
||
// The halfway point is (f₁+f₂)/2 = 2¹²⁸ - 2¹⁰⁵.
|
||
//
|
||
// The same is true of float64, with different constants: s/24/53/ and s/128/1024/.
|
||
|
||
const (
|
||
two24 = 1.0 * (1 << 24)
|
||
two53 = 1.0 * (1 << 53)
|
||
two64 = 1.0 * (1 << 64)
|
||
two128 = two64 * two64
|
||
two256 = two128 * two128
|
||
two512 = two256 * two256
|
||
two768 = two512 * two256
|
||
two1024 = two512 * two512
|
||
|
||
ulp32 = two128 / two24
|
||
max32 = two128 - ulp32
|
||
|
||
ulp64 = two1024 / two53
|
||
max64 = two1024 - ulp64
|
||
)
|
||
|
||
var cvt = []struct {
|
||
bits uint64 // keep us honest
|
||
exact interface{}
|
||
approx interface{}
|
||
text string
|
||
}{
|
||
// 0
|
||
{0x7f7ffffe, float32(max32 - ulp32), float32(max32 - ulp32 - ulp32/2), "max32 - ulp32 - ulp32/2"},
|
||
{0x7f7ffffe, float32(max32 - ulp32), float32(max32 - ulp32), "max32 - ulp32"},
|
||
{0x7f7ffffe, float32(max32 - ulp32), float32(max32 - ulp32/2), "max32 - ulp32/2"},
|
||
{0x7f7ffffe, float32(max32 - ulp32), float32(max32 - ulp32 + ulp32/2), "max32 - ulp32 + ulp32/2"},
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32 + ulp32/2 + ulp32/two64), "max32 - ulp32 + ulp32/2 + ulp32/two64"},
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32/2 + ulp32/two64), "max32 - ulp32/2 + ulp32/two64"},
|
||
{0x7f7fffff, float32(max32), float32(max32), "max32"},
|
||
{0x7f7fffff, float32(max32), float32(max32 + ulp32/2 - ulp32/two64), "max32 + ulp32/2 - ulp32/two64"},
|
||
|
||
{0xff7ffffe, float32(-(max32 - ulp32)), float32(-(max32 - ulp32 - ulp32/2)), "-(max32 - ulp32 - ulp32/2)"},
|
||
{0xff7ffffe, float32(-(max32 - ulp32)), float32(-(max32 - ulp32)), "-(max32 - ulp32)"},
|
||
{0xff7ffffe, float32(-(max32 - ulp32)), float32(-(max32 - ulp32/2)), "-(max32 - ulp32/2)"},
|
||
{0xff7ffffe, float32(-(max32 - ulp32)), float32(-(max32 - ulp32 + ulp32/2)), "-(max32 - ulp32 + ulp32/2)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32 + ulp32/2 + ulp32/two64)), "-(max32 - ulp32 + ulp32/2 + ulp32/two64)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32/2 + ulp32/two64)), "-(max32 - ulp32/2 + ulp32/two64)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32)), "-(max32)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 + ulp32/2 - ulp32/two64)), "-(max32 + ulp32/2 - ulp32/two64)"},
|
||
|
||
// These are required to work: according to the Go spec, the internal float mantissa must be at least 256 bits,
|
||
// and these expressions can be represented exactly with a 256-bit mantissa.
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32 + ulp32/2 + 1), "max32 - ulp32 + ulp32/2 + 1"},
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32/2 + 1), "max32 - ulp32/2 + 1"},
|
||
{0x7f7fffff, float32(max32), float32(max32 + ulp32/2 - 1), "max32 + ulp32/2 - 1"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32 + ulp32/2 + 1)), "-(max32 - ulp32 + ulp32/2 + 1)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32/2 + 1)), "-(max32 - ulp32/2 + 1)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 + ulp32/2 - 1)), "-(max32 + ulp32/2 - 1)"},
|
||
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32 + ulp32/2 + 1/two128), "max32 - ulp32 + ulp32/2 + 1/two128"},
|
||
{0x7f7fffff, float32(max32), float32(max32 - ulp32/2 + 1/two128), "max32 - ulp32/2 + 1/two128"},
|
||
{0x7f7fffff, float32(max32), float32(max32 + ulp32/2 - 1/two128), "max32 + ulp32/2 - 1/two128"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32 + ulp32/2 + 1/two128)), "-(max32 - ulp32 + ulp32/2 + 1/two128)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 - ulp32/2 + 1/two128)), "-(max32 - ulp32/2 + 1/two128)"},
|
||
{0xff7fffff, float32(-(max32)), float32(-(max32 + ulp32/2 - 1/two128)), "-(max32 + ulp32/2 - 1/two128)"},
|
||
|
||
{0x7feffffffffffffe, float64(max64 - ulp64), float64(max64 - ulp64 - ulp64/2), "max64 - ulp64 - ulp64/2"},
|
||
{0x7feffffffffffffe, float64(max64 - ulp64), float64(max64 - ulp64), "max64 - ulp64"},
|
||
{0x7feffffffffffffe, float64(max64 - ulp64), float64(max64 - ulp64/2), "max64 - ulp64/2"},
|
||
{0x7feffffffffffffe, float64(max64 - ulp64), float64(max64 - ulp64 + ulp64/2), "max64 - ulp64 + ulp64/2"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 - ulp64 + ulp64/2 + ulp64/two64), "max64 - ulp64 + ulp64/2 + ulp64/two64"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 - ulp64/2 + ulp64/two64), "max64 - ulp64/2 + ulp64/two64"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64), "max64"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 + ulp64/2 - ulp64/two64), "max64 + ulp64/2 - ulp64/two64"},
|
||
|
||
{0xffeffffffffffffe, float64(-(max64 - ulp64)), float64(-(max64 - ulp64 - ulp64/2)), "-(max64 - ulp64 - ulp64/2)"},
|
||
{0xffeffffffffffffe, float64(-(max64 - ulp64)), float64(-(max64 - ulp64)), "-(max64 - ulp64)"},
|
||
{0xffeffffffffffffe, float64(-(max64 - ulp64)), float64(-(max64 - ulp64/2)), "-(max64 - ulp64/2)"},
|
||
{0xffeffffffffffffe, float64(-(max64 - ulp64)), float64(-(max64 - ulp64 + ulp64/2)), "-(max64 - ulp64 + ulp64/2)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 - ulp64 + ulp64/2 + ulp64/two64)), "-(max64 - ulp64 + ulp64/2 + ulp64/two64)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 - ulp64/2 + ulp64/two64)), "-(max64 - ulp64/2 + ulp64/two64)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64)), "-(max64)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 + ulp64/2 - ulp64/two64)), "-(max64 + ulp64/2 - ulp64/two64)"},
|
||
|
||
// These are required to work.
|
||
// The mantissas are exactly 256 bits.
|
||
// max64 is just below 2¹⁰²⁴ so the bottom bit we can use is 2⁷⁶⁸.
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 - ulp64 + ulp64/2 + two768), "max64 - ulp64 + ulp64/2 + two768"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 - ulp64/2 + two768), "max64 - ulp64/2 + two768"},
|
||
{0x7fefffffffffffff, float64(max64), float64(max64 + ulp64/2 - two768), "max64 + ulp64/2 - two768"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 - ulp64 + ulp64/2 + two768)), "-(max64 - ulp64 + ulp64/2 + two768)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 - ulp64/2 + two768)), "-(max64 - ulp64/2 + two768)"},
|
||
{0xffefffffffffffff, float64(-(max64)), float64(-(max64 + ulp64/2 - two768)), "-(max64 + ulp64/2 - two768)"},
|
||
}
|
||
|
||
var bugged = false
|
||
|
||
func bug() {
|
||
if !bugged {
|
||
bugged = true
|
||
fmt.Println("BUG")
|
||
}
|
||
}
|
||
|
||
func main() {
|
||
u64 := math.Float64frombits(0x7fefffffffffffff) - math.Float64frombits(0x7feffffffffffffe)
|
||
if ulp64 != u64 {
|
||
bug()
|
||
fmt.Printf("ulp64=%g, want %g", ulp64, u64)
|
||
}
|
||
|
||
u32 := math.Float32frombits(0x7f7fffff) - math.Float32frombits(0x7f7ffffe)
|
||
if ulp32 != u32 {
|
||
bug()
|
||
fmt.Printf("ulp32=%g, want %g", ulp32, u32)
|
||
}
|
||
|
||
for _, c := range cvt {
|
||
if bits(c.exact) != c.bits {
|
||
bug()
|
||
fmt.Printf("%s: inconsistent table: bits=%#x (%g) but exact=%g (%#x)\n", c.text, c.bits, fromBits(c.bits, c.exact), c.exact, bits(c.exact))
|
||
}
|
||
if c.approx != c.exact || bits(c.approx) != c.bits {
|
||
bug()
|
||
fmt.Printf("%s: have %g (%#x) want %g (%#x)\n", c.text, c.approx, bits(c.approx), c.exact, c.bits)
|
||
}
|
||
}
|
||
}
|
||
|
||
func bits(x interface{}) interface{} {
|
||
switch x := x.(type) {
|
||
case float32:
|
||
return uint64(math.Float32bits(x))
|
||
case float64:
|
||
return math.Float64bits(x)
|
||
}
|
||
return 0
|
||
}
|
||
|
||
func fromBits(b uint64, x interface{}) interface{} {
|
||
switch x.(type) {
|
||
case float32:
|
||
return math.Float32frombits(uint32(b))
|
||
case float64:
|
||
return math.Float64frombits(b)
|
||
}
|
||
return "?"
|
||
}
|