When should I pass or return a struct by value?

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On small embedded architectures (8/16-bitters) — always pass by pointer, as non-trivial structures don’t fit into such tiny registers, and those machines are generally register-starved as well.

On PC-like architectures (32 and 64 bit processors) — passing a structure by value is OK provided sizeof(mystruct_t) <= 2*sizeof(mystruct_t*) and the function does not have many (usually more than 3 machine words’ worth of) other arguments. Under these circumstances, a typical optimizing compiler will pass/return the structure in a register or register pair. However, on x86-32, this advice should be taken with a hefty grain of salt, due to the extraordinary register pressure a x86-32 compiler must deal with — passing a pointer may still be faster due to reduced register spilling and filling.

Returning a structure by value on PC-likes, on the other hand, follows the same rule, save for the fact that when a structure is returned by pointer, the structure to be filled out should be passed in by pointer as well — otherwise, the callee and the caller are stuck having to agree on how to manage the memory for that structure.

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