Spill the stack pointer across calls in the interpreter.

[markshannon]

For performance we keep the stack pointer in a register, however this means that the stack pointer is invisible to debuggers and other tools. If we spill the stack pointer across "escaping" calls, then most of the time we want to inspect the evaluation the stack pointer should be in memory.
Spilling the stack pointer is also a necessary precursor to deferred reference counting: faster-cpython/ideas#677, #117139, as it means there is no need to redundantly set the unused part of the stack to NULL.

Since the C compiler needs to spill values around calls, manually spilling the stack pointer has quite a low cost. It limits the freedom of the C compiler to allocate registers, but not that much.

Linked PRs

• GH-119866: Spill stack pointer when making "escaping" calls. #119875
• GH-119866: Spill before escaping calls in all cases. #124392
• GH-119866: Don't clear frame->stackpointer on release builds #131750

[markshannon]

Benchmarking shows negligible slowdown even when spilling the stack pointer on all escaping calls.

[colesbury]

@markshannon, are you still planning to work on this? If not, I can take a look at it.