GeezerVison is showing github.com — Optimize reference tracking and eliminate branching during returns and yields · Issue #144540 · python/cpython
Optimize reference tracking and eliminate branching during returns and yields #144540DescriptionThere 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 a return, 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 a yield, 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 split RETURN_VALUE and YIELD_VALUE into micro-ops:
macro(RETURN_VALUE) = _MAKE_HEAP_SAFE + _RETURN_VALUE
macro(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 that a and d are immortal, or borrowed, then instead of looping over all the variables, we could emit code just to decref b and c.
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
Activityadded a commit that references this issue on Mar 11, 2026gh-144540: Add _MAKE_HEAP_SAFE uop to eliminate unnecessary refcount … Fidget-Spinner commented on Mar 15, 2026 @cocolato I suspect we can do more with _MAKE_HEAP_SAFE, e.g. if we have the following (return of a return)
_MAKE_HEAP_SAFE
_RETURN_VALUE
_MAKE_HEAP_SAFE <----- this can be eliminated!
_RETURN_VALUE
This shows up commonly in the bm_generators benchmark in pyperformance. Fidget-Spinner commented on Mar 22, 2026
This shows up commonly in the bm_generators benchmark in pyperformance.
@Sacul0457 would you like to work on this? It's just adding this conditional in _MAKE_HEAP_SAFE:
+ if (sym_is_immortal(PyJitRef_Unwrap(value)) || !PyJitRef_IsBorrowed(value)) {
And adding a test. Fidget-Spinner commented on Mar 22, 2026 @cocolato if you don't mind, I'm sensing you're a little busier now, so I will let Sacul take over for this one. I have some very interesting problems for you in the future :). Thanks a lot for all the work you've done so far! cocolato commented on Mar 22, 2026
@cocolato if you don't mind, I'm sensing you're a little busier now, so I will let Sacul take over for this one. I have some very interesting problems for you in the future :). Thanks a lot for all the work you've done so far!
Never mind. I’ve been little busy because I’m in the middle of a job handover, but I should have plenty of time starting in early April. I’ll also look into some of the existing issues then. If there's anything I can do to help, please let me know! Reacted by Ken Jin and Sacul Sacul0457 commented on Mar 22, 2026 Thank you both! I'll look into this in a few hours!
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