🚀 Go Struct Alignment: Why Field Order Matters for Performance

Did you know that the way you order fields in a Go struct can waste memory? Go automatically adds padding bytes to ensure proper memory alignment, and a poorly structured layout can increase memory usage by 30% or more!

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❌ Bad Struct: Wasting Memory

type BadStruct struct {
    A int8  // 1 byte
    B int64 // 8 bytes
    C int8  // 1 byte
}

Checking its size:

fmt.Println(unsafe.Sizeof(BadStruct{})) // Output: 24 bytes

Even though the actual data is just 10 bytes, the struct takes up 24 bytes due to padding.

🔍 Step-by-step memory layout (BadStruct)

Field	Type	Size (bytes)	Offset	Why?
A	int8	1	0	Starts at 0, perfectly aligned
(padding)	-	7	1-7	Added to align B to the next 8-byte boundary
B	int64	8	8	Must start at an 8-byte-aligned address
C	int8	1	16	Starts at the next available byte
(padding)	-	7	17-23	Ensures struct size is a multiple of 8

Total size:

1 (A) + 7 (padding) + 8 (B) + 1 (C) + 7 (padding) = 24 bytes

👉 Go adds 14 bytes of padding, making the struct much larger than necessary! 😱

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✅ Optimized Struct: Efficient Memory Use

type GoodStruct struct {
    B int64 // 8 bytes
    A int8  // 1 byte
    C int8  // 1 byte
}

Checking its size:

fmt.Println(unsafe.Sizeof(GoodStruct{})) // Output: 16 bytes

By simply reordering fields, we reduce the struct size by 33% (from 24B to 16B)! 🚀

🔍 Step-by-step memory layout (GoodStruct)

Field	Type	Size (bytes)	Offset	Why?
B	int64	8	0	Starts at 0, perfectly aligned
A	int8	1	8	Starts immediately after B
C	int8	1	9	Starts right after A
(padding)	-	6	10-15	Ensures struct size is a multiple of 8

Total size:

8 (B) + 1 (A) + 1 (C) + 6 (padding) = 16 bytes

🚀 By placing larger fields first, we eliminate unnecessary padding and optimize memory usage!

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🔍 Why Does This Happen?

Go aligns fields based on their natural size (e.g., int64 must be at an address divisible by 8).

• If a smaller field (int8) comes before a larger field (int64), Go inserts padding bytes to align memory correctly.
• By placing larger fields first, we reduce unnecessary padding, making the struct smaller and more efficient.

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🛠 Pro Tip

Use this to check field memory offsets:

reflect.TypeOf(struct{}).Field(i).Offset

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💬 Have you optimized struct layouts in your Go projects? Let's discuss in the comments! 🚀

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