Passing small structs in C

Hi prog. Got a C89 problem.

I want to pass small (less than 64B) structs throught functions that I can't inline because they are used several times elsewhere and are quite big. But it still need it to be very fast.

Should I pass the structs directly (thus copying them) or use a pointer (and getting a dereference penalty) ?

Depends on the calling convention. If it fits in a machine word, pass by value, the compiler will optimize it as if you passed in a integer variable. Some calling conventions use between 3-4 CPU registers to pass values to functions. In that case, the struct can be up to 3-4 machine words in size, and the compiler will pack it into the registers for you.

So on 64-bit machines, anything larger than 32 bytes should always be passed by references.

>>4
In no calling convention I have ever heard of will arguments be "packed" in registers. It's always one register per argument, and some CPUs have really weird additional contraints regarding sizes and alignment. Unless the struct is 4 members or less, use a pointer, unless you know exactly how your target platform works.