My question is about passing variables to GLSL shader. I'm not sure how that works and what are the performance implications.
Say I got a function that accepts a "vec4" variable. The question is - is that variable copied at the entrance? I guess it makes impact on performance if so. And if it happens to be that way is there a way to pass only references like in C/C++?
GLSL always uses copying, but this doesn't have the same performance implications as C++. In particular, because there is no recursion, there isn't a stack, and typically either functions are inlined and optimized there by the GLSL compiler, or parameters are in fixed register locations and copying is unnecessary.
Note that passing by const& in C++ is completely different from "passing by reference." In passing by reference, the function is allowed to change the argument, and these changes are visible to the calling function. "inout" achieves a similar result, but is not quite the same as vec4& in C++ if you pass the same variable to different parameters of a function.
vec4 is a primitive type in GLSL, you can expect it to behave like a int or float for performance. Finally, vec4 is only 128 bits, which is one or two cycles to copy.
const& IS passing by reference. It's just a read-only reference.
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Commented
Sep 25, 2020 at 15:13
You can mark the variable in the signature as "input". This will work as a reference pass (at least I'm the way you have asked).
