How Can I Explicitly Control Function Inlining in Go?

Inlining in Go: Techniques for Explicit Control

In Go, the compiler optimizes code execution by inlining functions, where the code of the called function is copied and directly inserted into the caller's code. While this improves performance, it can also lead to situations where function calls need to be explicitly prevented from inlining.

Disabling Inline Optimization

Inline pragma:

Go offers the //go:noinline pragma, which disables inlining for specific functions. Place this directive immediately before the function declaration to prevent its inline execution.

//go:noinline
func isPrime(p int) bool {
    // ...
}

Compiler flag:

Alternatively, you can disable all inline optimization using the -gcflags=-l compiler flag.

go build -gcflags=-l primes.go

Equivalency to GCC's -O0 Option

The -O0 option in GCC disables all code optimizations, including inlining. However, Go's inlining optimization is more fine-grained, allowing explicit control over which functions should be inlined or not.

Example: Preventing Inlining of Prime Checking Function

Consider the following code snippet from the primes example:

if isPrime(p) {
    fmt.Println(p)
}

func isPrime(p int) bool {
    for i := 2; i < p; i += 1 {
        for j := 2; j < p; j += 1 {
            if i * j == p {
                return false
            }
        }
    }
    return true
}

By default, the isPrime function would be inlined into the if statement, potentially slowing down the program due to the double loop. Adding the //go:noinline directive to the isPrime function ensures that it's called directly, improving performance.

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