I'm curious too. Even if it will be optimized isn't it still be better to pass-by-reference?
There may be cases where by-reference (yes, that's an absolutely acceptable thing to say, even in C) is more efficient: If the struct is a kind of "this pointer" (as in object-oriented programming) that is passed to a bunch of functions but relatively rarely used, then passing it by reference causes much less register pressure. Passing it by value would mean occupying a lot of registers with values that are probably not accessed.
On the other hand, if you expect the callee to do actual computation on all or most of the fields of the struct you pass in, passing it by value should probably be the way to go. This way, the values to be processed are already in registers and don't have to be stored to the stack by the caller and loaded back from the stack by the callee.
There is no general answer that is uniformly best, it depends on the characteristics of the actual program and their interactions with the compiler's optimizations. With inlining in particular, which should often allow the compiler to generate the same code for both versions.
I don't want to be nit-picking. But I would like to know why "by-reference" is an acceptable thing to say.
When I learnt C, C++, Java, etc. I was repeatedly told that pass by reference exists only in C++. In C, a pointer is passed by value. In Java, a reference is passed by value.
Doesn't calling "pass pointer by value" as "pass by reference" lead to confusion and distortion of the true meaning of "by reference"?
C++ and Java have their own specific language-lawyer definitions of the word reference, but outside of those languages, "reference" is a catch-all term that includes pointers, handles, and indexes.
You have no problem using the terms 'loop' or 'recurse' to describe C code, right? But these high-level ideas are implemented without using those terms in the code. Same with 'pass by reference'.