There are a few optimizations we can make to returns and yields, but they are somewhat related so I'm grouping them into a single issue.

During areturn, the VM needs to make the returned reference "heap safe" (convert any borrowed references to strong references) and then pop and clear the frame, which can involve quite a lot of decrefs.
During ayield, the VM needs to make the returned reference heap safe and then pop the frame, but not clear it.

We want to minimize the amount of refcounting operations that we do.

Before we can do much else, we should splitRETURN_VALUE andYIELD_VALUE into micro-ops:

macro(RETURN_VALUE) = _MAKE_HEAP_SAFE + _RETURN_VALUEmacro(YIELD_VALUE) = _MAKE_HEAP_SAFE + _YIELD_VALUE

so that we can optimize the uops independently

Eliminate making heap safe if we know that reference already is

If TOS is a strong reference_MAKE_HEAP_SAFE ->_NOP

Avoid the branch in_RETURN_VALUE by splitting into_RETURN_VALUE_GEN and_RETURN_VALUE_FUNC

To avoid using up opcodes, we'll need to do this in the JIT, not the interpreter.

Track borrows/immortals to optimize clearing frames

E.g. if a frame has four local variables,a,b,c,d but we can tell thata andd are immortal, or borrowed, then instead of looping over all the variables, we could emit code just to decrefb andc.
This might end up bloating the code, as we would need to inline the decrefs, but it could be quite a lot faster.

Linked PRs

• gh-144540: Add _MAKE_HEAP_SAFE uop to eliminate unnecessary refcount operations in RETURN_VALUE and YIELD_VALUE #144414
• gh-144540: Eliminate_MAKE_HEAP_FUNCTION for owned references #146320