How Does Go\'s Compiler Handle Function Inlining, and How Can I Influence It?

Inlining Optimization in Go

Unlike certain languages, Go does not allow explicit function inlining. Instead, the compiler dynamically assesses functions for possible inlining. This mechanism is aided by a debug option that allows users to observe potential inlining scenarios. However, the exact logic behind the Go compiler's inlining decisions remains largely undocumented.

Consider the following code, where a loop iteratively encrypts data using the Encrypt function:

func Encrypt(password []byte) ([]byte, error) {
    return bcrypt.GenerateFromPassword(password, 13)
}

for id, data := range someDataSet {
    newPassword, _ := Encrypt([]byte("generatedSomething"))
    data["password"] = newPassword
    someSaveCall(id, data)
}

To effectively inline the Encrypt function, the following considerations should be made:

• Until performance becomes an issue, inlining optimizations are unnecessary. The inlined and non-inlined functions will yield the same output.
• When performance is critical, avoid separate functions. Instead, directly include the function's body without defining it independently.
• Modify the 'l' debug flag for additional control. This flag adjusts the aggressiveness of the inlining process. Refer to the inl.go file in $GOROOT/src/cmd/compile/internal/inline/ for rules and guidelines.
• Enable diagnostic output using the 'm' flag. This provides insights into which calls are inlined or excluded. Higher values of 'm' may assist in debugging but are not guaranteed to persist in future Go versions.
• Consult Dave Cheney's blog post (2014): "Five things that make Go fast" for further information on inlining.
• Follow the discussion in GitHub issue #17566 for ongoing discussions related to inlining improvements in Go.

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