How does defer work internally in Go and what are its performance implications?

defer schedules a function call to run when the surrounding function returns — whether normally or via panic. The deferred call's arguments are evaluated immediately when the defer statement is executed, not when the deferred function runs.

// Arguments evaluated immediately at defer statement
x := 10
defer fmt.Println(x) // prints 10 even if x is later changed
x = 20
// Output: 10

// Named return values + defer — modify return before it reaches caller
func divide(a, b float64) (result float64, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("panic: %v", r)
        }
    }()
    result = a / b
    return // named return
}

// LIFO order — multiple defers run last-in first-out
func cleanup() {
    defer fmt.Println("first defer")
    defer fmt.Println("second defer")
    defer fmt.Println("third defer")
    // Output: third, second, first
}

// Performance: defer has overhead (allocates defer record pre-Go 1.14)
// Go 1.14+ inlines simple defers — near-zero overhead for non-looping, non-panic paths

// Avoid defer in tight loops — call cleanup explicitly
func processFiles(files []string) {
    for _, f := range files {
        func() { // wrap in closure so defer fires per file
            fh, _ := os.Open(f)
            defer fh.Close() // OK inside the per-file closure
            // process...
        }()
    }
}

Internal representation: in Go 1.14+, the compiler classifies defers as open-coded (inlined when statically determinable), stack-allocated, or heap-allocated. Open-coded defers have near-zero overhead — the compiler emits the deferred code at each return site. Only defers inside loops or under conditions that can vary at runtime fall back to the slower heap-allocated path.

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