Pass rgba_color by const reference or value?

[hatcat]

gradient_stop::gradient_stop and gradient_stop::color take their color parameter by const reference, although in the paper it is taken by value. Which should it be?

[hatcat]

Also brush::brush.

[mikebmcl]

It's an optimization question here. Can the four floats that are the minimum state data for rgba_color be optimized into registers for the x64 architecture? For the x86 architecture? For ARM? And if not for one or more of those platforms (or other major platforms), is the by value cost (especially if it applies to ARM) significant enough that const ref is the right way to go?

This "best practice" is specifically stated as being only useful for cheap to copy situations and strongly states that the developers need to make this decision.

The only downside for const ref that I know of is that it limits optimization abilities. So if doing a pass-by-value with a four float type would cause non-trivial performance degradation on a major architecture, then pass-by-const-ref it is.

[mikebmcl]

We'll leave this as it is for now but keep the issue open pending additional feedback.