Fatima`s Blog

WHAT is Type alignment

• Type alignment is Golang is all about how data is arranged to ensure efficient access by the CPU. The starting memory address of a value of a certain type must be a multiple of an integer N, where N is the size of the type. That sounds confusing but what this means is if a value takes up 2 bytes (int16) then the starting memory address should be even, e.g., 100, 102, etc.
• This N is called the alignment guarantee of the type.

WHY Type Align

• Modern CPUs fetch data from memory in chunks. If data is not aligned properly, the CPU may need to read multiple memory locations instead of just one, causing performance issues.
• E.g., if a uint32 (4 bytes) starts at memory address 102, the CPU has to read extra bytes across boundaries (e.g., part from 102-103 and part from 104-105), which slows things down.

Field vs. General Alignment

General alignment: When used as a standalone variable. Field alignment: When used inside a struct.

• The alignment of a struct depends on the alignment of its fields. The largest alignment requirement among the fields determines the struct’s alignment.
• For example:

type S struct {

x int8    // 1-byte 

y int64   // 8-byte 

}

• x as a standalone variable has an alignment of 1.
• But inside S, x is followed by y (which needs to be 8-byte aligned), so the compiler adds padding.

Total size = 1 + 7 (padding) + 8 = 16 bytes