As written, PEP 659 says that individual specializations are restricted to a single instruction.
PEP 669 relies on this, as it also wants to replace instructions at runtime, and it would break if specialization occurs across multiple instructions.

Currently there are a two places where we break this design by specializing pairs of instructions together:

• COMPARE_OPPOP_JUMP_IF_ pairs are specialized together
• FOR_ITERSTORE_FAST are specialized together

The second will go away with the register VM, and doesn't seem to be an issue in practice.
It is theCOMPARE_OPPOP_JUMP_IF_ specialization that is problematic, as PEP 669 wants to instrument branches.
Instrumenting thePOP_JUMP_IF_ doesn't work if theCOMPARE_OP specialization jumps over it.

The solution is to replace theCOMPARE_OPPOP_JUMP_IF_ pair with a singleCOMPARE_AND_BRANCH instruction that can be specialized or instrumented atomically.

Linked PRs

• GH-100982: AddCOMPARE_AND_BRANCH instruction #100983
• GH-100982: RestrictFOR_ITER_RANGE to a single instruction to allow instrumentation. #101985
• GH-100982: Break upCOMPARE_AND_BRANCH #102801