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go/constant: switch to floating-point representation when fractions b…
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…ecome too large

Use two internal representations for Float values (similar to what is done
for Int values). Transparently switch to a big.Float representation when
big.Rat values become unwieldy. This is almost never needed for real-world
programs but it is trivial to create test cases that cannot be handled with
rational arithmetic alone.

As a consequence, the go/constant API semantics changes slightly: Until now,
a value could always be represented in its "smallest" form (e.g., float values
that happened to be integers would be represented as integers). Now, constant
Kind depends on how the value was created, rather than its actual value. (The
reason why we cannot automatically "normalize" values to their smallest form
anymore is because floating-point numbers are not exact in general; and thus
normalization is often not possible in the first place, or would throw away
precision when it is not desired.) This has repercussions as to how constant
Values are used go/types and required corresponding adjustments.

Details of the changes:

go/constant package:
- use big.Rat and big.Float values to represent floating-point values
  (internal change)
- changed semantic of Value.Kind accordingly
- String now returns a short, human-readable form of a value
  (this leads to better error messages in go/types)
- added ToInt, ToFloat, and ToComplex conversion functions
- added ExactString to obtain an exact string form of a value

go/types:
- adjusted and simplified implementation of representableConst
- adjusted various places where Value.Kind was expected to be "smallest"
  by calling the respective ToInt/Float/Complex conversion functions
- enabled 5 disabled tests in stdlib_test.go that now work

api checker:
- print all constant values in a short human-readable form (floats are
  printed in floating-point form), but also print an exact form if it
  is different from the short form
- adjusted test golden file and go.1.1.text reference file

Fixes #11327.

Change-Id: I492b704aae5b0238e5b7cee13e18ffce61193587
Reviewed-on: https://go-review.googlesource.com/17360
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
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griesemer committed Dec 14, 2015
1 parent e568a01 commit d0c1746
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Showing 17 changed files with 753 additions and 328 deletions.
30 changes: 15 additions & 15 deletions api/go1.1.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -1983,13 +1983,13 @@ pkg log/syslog (openbsd-amd64-cgo), const LOG_SYSLOG = 40
pkg log/syslog (openbsd-amd64-cgo), const LOG_USER = 8
pkg log/syslog (openbsd-amd64-cgo), const LOG_UUCP = 64
pkg log/syslog (openbsd-amd64-cgo), const LOG_WARNING = 4
pkg math, const E = 271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000
pkg math, const Ln10 = 23025850929940456840179914546843642076011014886287729760333279/10000000000000000000000000000000000000000000000000000000000000
pkg math, const Ln2 = 693147180559945309417232121458176568075500134360255254120680009/1000000000000000000000000000000000000000000000000000000000000000
pkg math, const Log10E = 10000000000000000000000000000000000000000000000000000000000000/23025850929940456840179914546843642076011014886287729760333279
pkg math, const Log2E = 1000000000000000000000000000000000000000000000000000000000000000/693147180559945309417232121458176568075500134360255254120680009
pkg math, const MaxFloat32 = 340282346638528859811704183484516925440
pkg math, const MaxFloat64 = 179769313486231570814527423731704356798100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const E = 2.71828 // 271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000
pkg math, const Ln10 = 2.30259 // 23025850929940456840179914546843642076011014886287729760333279/10000000000000000000000000000000000000000000000000000000000000
pkg math, const Ln2 = 0.693147 // 693147180559945309417232121458176568075500134360255254120680009/1000000000000000000000000000000000000000000000000000000000000000
pkg math, const Log10E = 0.434294 // 10000000000000000000000000000000000000000000000000000000000000/23025850929940456840179914546843642076011014886287729760333279
pkg math, const Log2E = 1.4427 // 1000000000000000000000000000000000000000000000000000000000000000/693147180559945309417232121458176568075500134360255254120680009
pkg math, const MaxFloat32 = 3.40282e+38 // 340282346638528859811704183484516925440
pkg math, const MaxFloat64 = 1.79769e+308 // 179769313486231570814527423731704356798100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const MaxInt16 = 32767
pkg math, const MaxInt32 = 2147483647
pkg math, const MaxInt64 = 9223372036854775807
Expand All @@ -2002,14 +2002,14 @@ pkg math, const MinInt16 = -32768
pkg math, const MinInt32 = -2147483648
pkg math, const MinInt64 = -9223372036854775808
pkg math, const MinInt8 = -128
pkg math, const Phi = 80901699437494742410229341718281905886015458990288143106772431/50000000000000000000000000000000000000000000000000000000000000
pkg math, const Pi = 314159265358979323846264338327950288419716939937510582097494459/100000000000000000000000000000000000000000000000000000000000000
pkg math, const SmallestNonzeroFloat32 = 17516230804060213386546619791123951641/12500000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const SmallestNonzeroFloat64 = 4940656458412465441765687928682213723651/1000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const Sqrt2 = 70710678118654752440084436210484903928483593768847403658833987/50000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtE = 164872127070012814684865078781416357165377610071014801157507931/100000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtPhi = 63600982475703448212621123086874574585780402092004812430832019/50000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtPi = 177245385090551602729816748334114518279754945612238712821380779/100000000000000000000000000000000000000000000000000000000000000
pkg math, const Phi = 1.61803 // 80901699437494742410229341718281905886015458990288143106772431/50000000000000000000000000000000000000000000000000000000000000
pkg math, const Pi = 3.14159 // 314159265358979323846264338327950288419716939937510582097494459/100000000000000000000000000000000000000000000000000000000000000
pkg math, const SmallestNonzeroFloat32 = 1.4013e-45 // 17516230804060213386546619791123951641/12500000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const SmallestNonzeroFloat64 = 4.94066e-324 // 4940656458412465441765687928682213723651/1000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
pkg math, const Sqrt2 = 1.41421 // 70710678118654752440084436210484903928483593768847403658833987/50000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtE = 1.64872 // 164872127070012814684865078781416357165377610071014801157507931/100000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtPhi = 1.27202 // 63600982475703448212621123086874574585780402092004812430832019/50000000000000000000000000000000000000000000000000000000000000
pkg math, const SqrtPi = 1.77245 // 177245385090551602729816748334114518279754945612238712821380779/100000000000000000000000000000000000000000000000000000000000000
pkg math/big, const MaxBase = 36
pkg math/big, method (*Int) MarshalJSON() ([]uint8, error)
pkg math/big, method (*Int) SetUint64(uint64) *Int
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9 changes: 8 additions & 1 deletion src/cmd/api/goapi.go
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Original file line number Diff line number Diff line change
Expand Up @@ -680,7 +680,14 @@ func (w *Walker) emitObj(obj types.Object) {
switch obj := obj.(type) {
case *types.Const:
w.emitf("const %s %s", obj.Name(), w.typeString(obj.Type()))
w.emitf("const %s = %s", obj.Name(), obj.Val())
x := obj.Val()
short := x.String()
exact := x.ExactString()
if short == exact {
w.emitf("const %s = %s", obj.Name(), short)
} else {
w.emitf("const %s = %s // %s", obj.Name(), short, exact)
}
case *types.Var:
w.emitf("var %s %s", obj.Name(), w.typeString(obj.Type()))
case *types.TypeName:
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2 changes: 1 addition & 1 deletion src/cmd/api/testdata/src/pkg/p1/golden.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@ pkg p1, const ConstChase2 = 11
pkg p1, const ConstChase2 ideal-int
pkg p1, const ConversionConst = 5
pkg p1, const ConversionConst MyInt
pkg p1, const FloatConst = 3/2
pkg p1, const FloatConst = 1.5 // 3/2
pkg p1, const FloatConst ideal-float
pkg p1, const StrConst = "foo"
pkg p1, const StrConst ideal-string
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