What's the advantage of having nonvolatile registers in a calling convention?

Question

I\'m programming a JIT compiler and I\'ve been surprised to discover that so many of the x86-64 registers are nonvolatile (callee-preserved) in the Win64 calling convention.

Answer 1

If registers are caller-saves, then the caller always has to save or reload those registers around a function call. But if registers are callee-saves, then the callee only has to save the registers that it uses, and only when it knows they're going to be used (i.e. maybe not at all in an early-exit scenario). The disadvantage of this convention is that the callee doesn't have knowledge of the caller, so it might be saving registers that are dead anyway, but I guess that's seen as a smaller concern.