Feature or enhancement

Proposal:

Motivation

We should implement unique reference tracking in Tier 2 to facilitate optimizations that reduce reference counting overhead. For example, when a tuple is known to be uniquely referenced, we can "steal" its element references during unpacking without performing any reference counting operations.

For reference: discussion in #142952

Technical Approach

1. Reference Tracking Infrastructure
   • Add an REF_IS_UNIQUE bit (bit 1) to the JitOptRef union in pycore_optimizer.h (code reference).
   • Implement PyJitRef_MakeUnique() and PyJitRef_IsUnique() helper functions.
   • Update helper utilities including PyJitRef_StripReferenceInfo and JIT_BITS_TO_PTR_MASKED to support this unique reference bit.
2. Apply unique reference tracking to UNPACK_SEQUENCE uops
3. Expand support to more uops
   • After verifying performance and correctness, extend the use of unique reference tracking to additional uops and optimizations as identified.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-144145: Track nullness of properties in the Tier 2 JIT optimizer #144122
• gh-143414: Implement unique reference tracking for JIT, optimize unpacking of such tuples #144300