Memory Alignment

Why to align

• The scenario below causes unnecessary wastage of CPU cycles.

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• If we use padding (empty space) we can improve the CPU cycles, at the cost of the struct storing more memory.

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• The order of the elements matters, as it can introduce more padding than necessary:

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Memory Access

• Processors don't read 1 byte at a time from memory.

• They read 1 word  at a time.

• 32-bit Processor :

  • Word size is 4 bytes.

• 64-bit Processor :

  • Word size is 8 bytes.

Size

• The total number of bytes that a single element actually occupies in memory, including  any internal padding required by alignment.

Offset

• Defines where a field resides relative to a structure’s base address.

• Example :

  • In struct { int a; char b; } , if a  starts at offset 0 , b  might be at offset 4  due to alignment padding.

Stride

• Byte distance between consecutive elements in an array or buffer.

• It’s not “after the element finishes”, it’s the total distance between consecutive starts .

• That’s why stride includes all bytes (data + padding) in a single element.

• Example :

  • In a vertex buffer with position (12 bytes) + color (4 bytes), stride = 16 bytes. The next vertex starts 16 bytes after the previous one.

    • Vertex 0 starts at byte 0 .

    • Vertex 0 occupies bytes 0–15 .

    • Vertex 1 starts at byte 16 .