How does struct field ordering affect memory layout and performance in Go?

CPU architectures require data to be aligned — an 8-byte integer must start at an address divisible by 8, a 4-byte integer divisible by 4, etc. The Go compiler adds invisible padding bytes between struct fields to satisfy alignment requirements. Poor field ordering wastes memory; reordering fields can eliminate padding.

// Poorly ordered — wastes 7 bytes of padding
type Wasteful struct {
    a bool    // 1 byte
              // 7 bytes PADDING (for b's 8-byte alignment)
    b int64   // 8 bytes
    c bool    // 1 byte
              // 7 bytes PADDING (to align next field / end of struct)
}  // Total: 24 bytes (wastes 14 bytes)

// Well ordered — no padding (fields largest to smallest)
type Compact struct {
    b int64   // 8 bytes
    a bool    // 1 byte
    c bool    // 1 byte
              // 6 bytes padding to align to 8-byte boundary at struct end
}  // Total: 16 bytes (saves 8 bytes vs Wasteful)

// Verify with unsafe
fmt.Println(unsafe.Sizeof(Wasteful{})) // 24
fmt.Println(unsafe.Sizeof(Compact{}))  // 16

// Performance impact:
// Larger structs → more cache lines → more cache misses in hot loops
// A 50% size reduction can be a 2x performance improvement in array iteration

// Tool: fieldalignment (golang.org/x/tools/go/analysis/passes/fieldalignment)
// go install golang.org/x/tools/cmd/fieldalignment@latest
// fieldalignment ./...     — reports suboptimal struct layouts
// fieldalignment -fix ./.. — rewrites structs (review before committing!)

// go vet includes a similar check with: -structtag flag

The rule of thumb: order struct fields from largest to smallest alignment requirement. In practice: int64/float64/uintptr (8 bytes) first, then int32/float32 (4 bytes), then int16 (2 bytes), then bool/int8/byte (1 byte) last. This is particularly important for structs that appear in large arrays or slices processed in hot paths.

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