How Does Go Handle Arithmetic Operations with Untyped Constants?

How are Arithmetic Operations Performed on Constants in Go?

Arithmetic operations involving constants in Go can be a perplexing aspect of the language. Despite the seemingly large values of some constants, they don't reside in memory at runtime. These constants, referred to as "untyped arbitrary-precision constants," exist solely during compilation.

Representation of Constants

Constants are not physically stored in executable binaries. Instead, function calls are recorded that operate on values of finite precision types. These types are determined by:

• The default type of the untyped constant
• The result's type after performing arithmetic operations

For example:

const Huge = 1e1000
fmt.Println(Huge / 1e999) // Prints 10.0

In this code, Huge has a default type of float64, and the result of the division is also a float64. Consequently, the executable contains neither 1e1000 nor 1e999, but only the value 10.0 of type float64.

Arbitrary Precision at Compile Time

Despite the absence of true arbitrary precision at runtime, the Go compiler must handle these constants at compile time. The language specification allows compilers flexibility in representing constants, but ensures that:

• Integer constants have at least 256 bits of precision.
• Floating-point constants have at least 256 bits of mantissa and 32 bits of exponent.

Implementation of Arithmetic Operations

Although the Go specification does not specify the implementation details of arithmetic operations on constants, the standard library provides packages for working with arbitrary precision values:

• math/big for representing large integers and rational numbers
• go/constant for evaluating constant expressions and representing their results

These packages implement arbitrary precision arithmetic by utilizing big integers and rational numbers, allowing for precise operations on values that exceed the precision limits of builtin types.

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