Below code compiles:
package main
import "fmt"
const (
// Max integer value on 64 bit architecture.
maxInt = 9223372036854775807
// Much larger value than int64.
bigger = 9223372036854775808543522345
// Will NOT compile
// biggerInt int64 = 9223372036854775808543522345
)
func main() {
fmt.Println("Will Compile")
//fmt.Println(bigger) // error
}
Type is size in memory + representation of bits in that memory
What is the implicit type assigned to bigger at compile time? Because error constant 9223372036854775808543522345 overflows int for line fmt.Println(bigger)
Those are untyped constants. They have larger limits than typed constants:
https://golang.org/ref/spec#Constants
In particular:
Represent integer constants with at least 256 bits.
None, it's an untyped constant. Because you haven't assigned it to any variable or used it in any expression, it doesn't "need" to be given a representation as any concrete type yet. Numeric constants in Go have effectively unlimited precision (required by the language spec to be at least 256 bits for integers, and at least 256 mantissa bits for floating-point numbers, but I believe that the golang/go compiler uses the Go arbitrary-precision types internally which are only limited by memory). See the section about Constants in the language spec.
What is the use of a constant if you can't assign it to a variable of any type? Well, it can be part of a constant expression. Constant expressions are evaluated at arbitrary precision, and their results may be able to be assigned to a variable. In other words, it's allowed to use values that are too big to represent to reach an answer that is representable, as long as all of that computation happens at compile time.
From this comment:
my goal is to convertf bigger = 9223372036854775808543522345 to binary form
we find that your question is an XY problem.
Since we do know that the constant exceeds 64 bits, we'll need to take it apart into multiple 64-bit words, or store it in some sort of bigger-integer storage.
Go provides math/big for general purpose large-number operations, or in this case we can take advantage of the fact that it's easy to store up to 127-bit signed values (or 128-bit unsigned values) in a struct holding two 64-bit integers (at least one of which is unsigned).
This rather trivial program prints the result of converting to binary:
500000000 x 2-sup-64 + 543522345 as binary:
111011100110101100101000000000000000000000000000000000000000000100000011001010111111000101001
package main
import "fmt"
const (
// Much larger value than int64.
bigger = 9223372036854775808543522345
d64 = 1 << 64
)
type i128 struct {
Upper int64
Lower uint64
}
func main() {
x := i128{Upper: bigger / d64, Lower: bigger % d64}
fmt.Printf("%d x 2-sup-64 + %d as binary:\n%b%.64b\n", x.Upper, x.Lower, x.Upper, x.Lower)
}
Related
The Go language specification on the section about Constant expressions states:
A compiler may use rounding while computing untyped floating-point or complex constant expressions; see the implementation restriction in the section on constants. This rounding may cause a floating-point constant expression to be invalid in an integer context, even if it would be integral when calculated using infinite precision, and vice versa.
Does the sentence
This rounding may cause a floating-point constant expression to be invalid in an integer context
point to something like the following:
func main() {
a := 853784574674.23846278367
fmt.Println(int8(a)) // output: 0
}
The quoted part from the spec does not apply to your example, as a is not a constant expression but a variable, so int8(a) is converting a non-constant expression. This conversion is covered by Spec: Conversions, Conversions between numeric types:
When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero).
[...] In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.
Since you convert a non-constant expression a being 853784574674.23846278367 to an integer, the fraction part is discarded, and since the result does not fit into an int8, the result is not specified, it's implementation-dependent.
The quoted part means that while constants are represented with a lot higher precision than the builtin types (eg. float64 or int64), the precision that a compiler (have to) implement is not infinite (for practical reasons), and even if a floating point literal is representable precisely, performing operations on them may be carried out with intermediate roundings and may not give mathematically correct result.
The spec includes the minimum supportable precision:
Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:
Represent integer constants with at least 256 bits.
Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed binary exponent of at least 16 bits.
Give an error if unable to represent an integer constant precisely.
Give an error if unable to represent a floating-point or complex constant due to overflow.
Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.
For example:
const (
x = 1e100000 + 1
y = 1e100000
)
func main() {
fmt.Println(x - y)
}
This code should output 1 as x is being 1 larger than y. Running it on the Go Playground outputs 0 because the constant expression x - y is executed with roundings, and the +1 is lost as a result. Both x and y are integers (have no fraction part), so in integer context the result should be 1. But the number being 1e100000, representing it requires around ~333000 bits, which is not a valid requirement from a compiler (according to the spec, 256 bit mantissa is sufficient).
If we lower the constants, we get correct result:
const (
x = 1e1000 + 1
y = 1e1000
)
func main() {
fmt.Println(x - y)
}
This outputs the mathematically correct 1 result. Try it on the Go Playground. Representing the number 1e1000 requires around ~3333 bits which seems to be supported (and it's way above the minimum 256 bit requirement).
An int8 is a signed integer, and can have a value from -128 to 127. That's why you are seeing unexpected value with int8(a) conversion.
I've been reading this post on constants in Go, and I'm trying to understand how they are stored and used in memory. You can perform operations on very large constants in Go, and as long as the result fits in memory, you can coerce that result to a type. For example, this code prints 10, as you would expect:
const Huge = 1e1000
fmt.Println(Huge / 1e999)
How does this work under the hood? At some point, Go has to store 1e1000 and 1e999 in memory, in order to perform operations on them. So how are constants stored, and how does Go perform arithmetic on them?
Short summary (TL;DR) is at the end of the answer.
Untyped arbitrary-precision constants don't live at runtime, constants live only at compile time (during the compilation). That being said, Go does not have to represent constants with arbitrary precision at runtime, only when compiling your application.
Why? Because constants do not get compiled into the executable binaries. They don't have to be. Let's take your example:
const Huge = 1e1000
fmt.Println(Huge / 1e999)
There is a constant Huge in the source code (and will be in the package object), but it won't appear in your executable. Instead a function call to fmt.Println() will be recorded with a value passed to it, whose type will be float64. So in the executable only a float64 value being 10.0 will be recorded. There is no sign of any number being 1e1000 in the executable.
This float64 type is derived from the default type of the untyped constant Huge. 1e1000 is a floating-point literal. To verify it:
const Huge = 1e1000
x := Huge / 1e999
fmt.Printf("%T", x) // Prints float64
Back to the arbitrary precision:
Spec: Constants:
Numeric constants represent exact values of arbitrary precision and do not overflow.
So constants represent exact values of arbitrary precision. As we saw, there is no need to represent constants with arbitrary precision at runtime, but the compiler still has to do something at compile time. And it does!
Obviously "infinite" precision cannot be dealt with. But there is no need, as the source code itself is not "infinite" (size of the source is finite). Still, it's not practical to allow truly arbitrary precision. So the spec gives some freedom to compilers regarding to this:
Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:
Represent integer constants with at least 256 bits.
Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed exponent of at least 32 bits.
Give an error if unable to represent an integer constant precisely.
Give an error if unable to represent a floating-point or complex constant due to overflow.
Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.
These requirements apply both to literal constants and to the result of evaluating constant expressions.
However, also note that when all the above said, the standard package provides you the means to still represent and work with values (constants) with "arbitrary" precision, see package go/constant. You may look into its source to get an idea how it's implemented.
Implementation is in go/constant/value.go. Types representing such values:
// A Value represents the value of a Go constant.
type Value interface {
// Kind returns the value kind.
Kind() Kind
// String returns a short, human-readable form of the value.
// For numeric values, the result may be an approximation;
// for String values the result may be a shortened string.
// Use ExactString for a string representing a value exactly.
String() string
// ExactString returns an exact, printable form of the value.
ExactString() string
// Prevent external implementations.
implementsValue()
}
type (
unknownVal struct{}
boolVal bool
stringVal string
int64Val int64 // Int values representable as an int64
intVal struct{ val *big.Int } // Int values not representable as an int64
ratVal struct{ val *big.Rat } // Float values representable as a fraction
floatVal struct{ val *big.Float } // Float values not representable as a fraction
complexVal struct{ re, im Value }
)
As you can see, the math/big package is used to represent untyped arbitrary precision values. big.Int is for example (from math/big/int.go):
// An Int represents a signed multi-precision integer.
// The zero value for an Int represents the value 0.
type Int struct {
neg bool // sign
abs nat // absolute value of the integer
}
Where nat is (from math/big/nat.go):
// An unsigned integer x of the form
//
// x = x[n-1]*_B^(n-1) + x[n-2]*_B^(n-2) + ... + x[1]*_B + x[0]
//
// with 0 <= x[i] < _B and 0 <= i < n is stored in a slice of length n,
// with the digits x[i] as the slice elements.
//
// A number is normalized if the slice contains no leading 0 digits.
// During arithmetic operations, denormalized values may occur but are
// always normalized before returning the final result. The normalized
// representation of 0 is the empty or nil slice (length = 0).
//
type nat []Word
And finally Word is (from math/big/arith.go)
// A Word represents a single digit of a multi-precision unsigned integer.
type Word uintptr
Summary
At runtime: predefined types provide limited precision, but you can "mimic" arbitrary precision with certain packages, such as math/big and go/constant. At compile time: constants seemingly provide arbitrary precision, but in reality a compiler may not live up to this (doesn't have to); but still the spec provides minimal precision for constants that all compiler must support, e.g. integer constants must be represented with at least 256 bits which is 32 bytes (compared to int64 which is "only" 8 bytes).
When an executable binary is created, results of constant expressions (with arbitrary precision) have to be converted and represented with values of finite precision types – which may not be possible and thus may result in compile-time errors. Note that only results –not intermediate operands– have to be converted to finite precision, constant operations are carried out with arbitrary precision.
How this arbitrary or enhanced precision is implemented is not defined by the spec, math/big for example stores "digits" of the number in a slice (where digits is not a digit of the base 10 representation, but "digit" is an uintptr which is like base 4294967295 representation on 32-bit architectures, and even bigger on 64-bit architectures).
Go constants are not allocated to memory. They are used in context by the compiler. The blog post you refer to gives the example of Pi:
Pi = 3.14159265358979323846264338327950288419716939937510582097494459
If you assign Pi to a float32 it will lose precision to fit, but if you assign it to a float64, it will lose less precision, but the compiler will determine what type to use.
I'm writing some inner-loop type input parsing code, and need to compare the length of a buffer to a uint32. The ideal solution would be fast, concise, dumb (easy to see the correctness of), and work for all possible inputs including those in which an attacker maliciously manipulates the values. Integer overflow is a big deal in this context if it can be exploited to crash the program. This is what I have so far:
// Safely check if len(buf) <= size for all possible values of each
func sizeok(buf []MyType, size uint32) bool {
var n int = len(buf)
return n == int(uint32(n)) && uint32(n) <= size
}
That's a pain, and can't be abstracted over other slice types.
My questions: First, is this actually correct? (You can never be too careful defending against exploitable integer overflows.) Second, is there a simpler way to do it with a single comparison? Maybe uint(len(buf)) <= uint(size) if that could be guaranteed to work securely on all platforms and inputs? Or uint64(len(buf)) <= uint64(size) if that won't generate suboptimal code on 32-bit platforms?
The Go Programming Language Specification
Length and capacity
The built-in functions len and cap take arguments of various types and
return a result of type int. The implementation guarantees that the
result always fits into an int.
Call Argument type Result
len(s) string type string length in bytes
[n]T, *[n]T array length (== n)
[]T slice length
map[K]T map length (number of defined keys)
chan T number of elements queued in channel buffer
Numeric types
uint32 all unsigned 32-bit integers (0 to 4294967295)
int64 all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
There is also a set of predeclared numeric types with
implementation-specific sizes:
uint either 32 or 64 bits
int same size as uint
len(buf) is type int which is 32 or 64 bits depending on the implementation. For example, for any Go type with a built-in len function,
// Safely check if len(buf) <= size for all possible values of each
var size uint32
if int64(len(buf)) <= int64(size) {
// handle len(buf) <= size
}
I've been reading this post on constants in Go, and I'm trying to understand how they are stored and used in memory. You can perform operations on very large constants in Go, and as long as the result fits in memory, you can coerce that result to a type. For example, this code prints 10, as you would expect:
const Huge = 1e1000
fmt.Println(Huge / 1e999)
How does this work under the hood? At some point, Go has to store 1e1000 and 1e999 in memory, in order to perform operations on them. So how are constants stored, and how does Go perform arithmetic on them?
Short summary (TL;DR) is at the end of the answer.
Untyped arbitrary-precision constants don't live at runtime, constants live only at compile time (during the compilation). That being said, Go does not have to represent constants with arbitrary precision at runtime, only when compiling your application.
Why? Because constants do not get compiled into the executable binaries. They don't have to be. Let's take your example:
const Huge = 1e1000
fmt.Println(Huge / 1e999)
There is a constant Huge in the source code (and will be in the package object), but it won't appear in your executable. Instead a function call to fmt.Println() will be recorded with a value passed to it, whose type will be float64. So in the executable only a float64 value being 10.0 will be recorded. There is no sign of any number being 1e1000 in the executable.
This float64 type is derived from the default type of the untyped constant Huge. 1e1000 is a floating-point literal. To verify it:
const Huge = 1e1000
x := Huge / 1e999
fmt.Printf("%T", x) // Prints float64
Back to the arbitrary precision:
Spec: Constants:
Numeric constants represent exact values of arbitrary precision and do not overflow.
So constants represent exact values of arbitrary precision. As we saw, there is no need to represent constants with arbitrary precision at runtime, but the compiler still has to do something at compile time. And it does!
Obviously "infinite" precision cannot be dealt with. But there is no need, as the source code itself is not "infinite" (size of the source is finite). Still, it's not practical to allow truly arbitrary precision. So the spec gives some freedom to compilers regarding to this:
Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:
Represent integer constants with at least 256 bits.
Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed exponent of at least 32 bits.
Give an error if unable to represent an integer constant precisely.
Give an error if unable to represent a floating-point or complex constant due to overflow.
Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.
These requirements apply both to literal constants and to the result of evaluating constant expressions.
However, also note that when all the above said, the standard package provides you the means to still represent and work with values (constants) with "arbitrary" precision, see package go/constant. You may look into its source to get an idea how it's implemented.
Implementation is in go/constant/value.go. Types representing such values:
// A Value represents the value of a Go constant.
type Value interface {
// Kind returns the value kind.
Kind() Kind
// String returns a short, human-readable form of the value.
// For numeric values, the result may be an approximation;
// for String values the result may be a shortened string.
// Use ExactString for a string representing a value exactly.
String() string
// ExactString returns an exact, printable form of the value.
ExactString() string
// Prevent external implementations.
implementsValue()
}
type (
unknownVal struct{}
boolVal bool
stringVal string
int64Val int64 // Int values representable as an int64
intVal struct{ val *big.Int } // Int values not representable as an int64
ratVal struct{ val *big.Rat } // Float values representable as a fraction
floatVal struct{ val *big.Float } // Float values not representable as a fraction
complexVal struct{ re, im Value }
)
As you can see, the math/big package is used to represent untyped arbitrary precision values. big.Int is for example (from math/big/int.go):
// An Int represents a signed multi-precision integer.
// The zero value for an Int represents the value 0.
type Int struct {
neg bool // sign
abs nat // absolute value of the integer
}
Where nat is (from math/big/nat.go):
// An unsigned integer x of the form
//
// x = x[n-1]*_B^(n-1) + x[n-2]*_B^(n-2) + ... + x[1]*_B + x[0]
//
// with 0 <= x[i] < _B and 0 <= i < n is stored in a slice of length n,
// with the digits x[i] as the slice elements.
//
// A number is normalized if the slice contains no leading 0 digits.
// During arithmetic operations, denormalized values may occur but are
// always normalized before returning the final result. The normalized
// representation of 0 is the empty or nil slice (length = 0).
//
type nat []Word
And finally Word is (from math/big/arith.go)
// A Word represents a single digit of a multi-precision unsigned integer.
type Word uintptr
Summary
At runtime: predefined types provide limited precision, but you can "mimic" arbitrary precision with certain packages, such as math/big and go/constant. At compile time: constants seemingly provide arbitrary precision, but in reality a compiler may not live up to this (doesn't have to); but still the spec provides minimal precision for constants that all compiler must support, e.g. integer constants must be represented with at least 256 bits which is 32 bytes (compared to int64 which is "only" 8 bytes).
When an executable binary is created, results of constant expressions (with arbitrary precision) have to be converted and represented with values of finite precision types – which may not be possible and thus may result in compile-time errors. Note that only results –not intermediate operands– have to be converted to finite precision, constant operations are carried out with arbitrary precision.
How this arbitrary or enhanced precision is implemented is not defined by the spec, math/big for example stores "digits" of the number in a slice (where digits is not a digit of the base 10 representation, but "digit" is an uintptr which is like base 4294967295 representation on 32-bit architectures, and even bigger on 64-bit architectures).
Go constants are not allocated to memory. They are used in context by the compiler. The blog post you refer to gives the example of Pi:
Pi = 3.14159265358979323846264338327950288419716939937510582097494459
If you assign Pi to a float32 it will lose precision to fit, but if you assign it to a float64, it will lose less precision, but the compiler will determine what type to use.
At http://tour.golang.org/#14 they show an example where the number 1 is shifted by 64 bits. This of course would result in an overflow, but then it is subtracted by 1 and all is well. How does half of the expression result in a failure while the entire expression as whole work just fine?
Thoughts:
I would assume that the setting of the unsigned to a number larger than what it allows is what causes the explosion. It would seem that memory is allocated more loosely on the right hand side of the expression than on the left? Is this true?
The result of your expression is a (compile time) constant and the expression is therefore evaluated during compilation. The language specification mandates that
Constant expressions are always evaluated exactly; intermediate values
and the constants themselves may require precision significantly
larger than supported by any predeclared type in the language. The
following are legal declarations:
const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
const Four int8 = Huge >> 98 // Four == 4 (type int8)
https://golang.org/ref/spec#Constant_expressions
That is because the Go compiler handles constant expressions as numeric constants. Contrary to the data-types that have to obey the law of range, storage bits and side-effects like overflow, numeric constants never lose precision.
Numeric constants only get deduced down to a data-type with limited precision and range at the moment when you assign them to a variable (which has a known type and thus a numeric range and a defined way to store the number into bits). You can also force them to get deduced to a ordinary data-type by using them as part of a equation that contains non Numeric constant types.
Confused? So was I..
Here is a longer write-up on the data-types and how constants are handled: http://www.goinggo.net/2014/04/introduction-to-numeric-constants-in-go.html?m=1
I decided to try it. For reasons that are subtle, executing the expression as a constant expression (1 << 64 -1) or piece by piece at run time gives the same answer. This is because of 2 different mechanisms. A constant expression is fully evaluated with infinite precision before being assigned to the variable. The step by step execution explicitly allows overflows and underflows through addition, subtraction and shift operations, and thus the result is the same.
See https://golang.org/ref/spec#Integer_overflow for a description of how integers are supposed to overflow.
However, doing it in groups, ie 1<<64 and then -1 causes overflow errors!
You can make a variable overflow though arithmetic, but you can not assign an overflow to a variable.
Try it yourself. Paste the code below into http://try.golang.org/
This one works:
// You can edit this code!
// Click here and start typing.
package main
import "fmt"
func main() {
var MaxInt uint64 = 1
MaxInt = MaxInt << 64
MaxInt = MaxInt - 1
fmt.Println("%d",MaxInt)
}
This one doesn't work:
// You can edit this code!
// Click here and start typing.
package main
import "fmt"
func main() {
var MaxInt uint64 = 1 << 64
MaxInt = MaxInt - 1
fmt.Println("%d",MaxInt)
}
Actually 1 << 64 - 1 does not always result in a left shift of 64 and minus 1. The - operator is applied before the << operator in most languages, at least in any I know (like C++, Java, ...). Therefore 1 << 64 - 1 <=> 1 << 63.
But Go behaves different: https://golang.org/ref/spec#Operator_precedence
The - operator comes after the << operator.
The result of a 64 Bit left shift is based on the data type. It's just like adding 64 of 0 on the right, while cutting any Bit that extend the data type on the left side. In some languages an overflow may be valid, in some other not.
Compilers may also behave different based on the interpretion when your shift is greater or equal than the actual data type size. I know that the Java compiler will reduce the actual shift as often by size of the data type until it's smaller than the size of the data byte.
Sounds difficult, but here and easy example for a long data type with 64 Bit size.
so i << 64 <=> i << 0 <=> i
or i << 65 <=> i << 1
or i << 130 <=> i << 66 <=> i << 2.
As said, this may differ with different compilers / languages. There is never a solid answer without refering to a certain language.
For learning I would suggest a more common language than Go, maybe like something from the C family.
var MaxInt uint64 = 1<<64 - 1
BITSHIFTING
In binary, 1, with 64 0s after it (10000...).
Same as 2^64.
This maxes out 64-bit unsigned integer (positive numbers only).
So, we subtract 1 to prevent the error.
Therefore, this is the maximum value unsigned integer we can write.
You can see here go constant int actually is a bigInt, so 1 << 63 won't overflow. But var a int64 = 1 << 63 will overflow, because you are a assign a value bigger than int64.