Some (mostly) easy ways to reduce the size of JIT generated code

Reducing the size of JIT generated code makes it faster, often super-linearly as there is less code to execute and less code to fill caches.

Looking at the top instructions from old but mostly still valid stats there are a few opportunities to reduce code size:

_SET_IP

Most uses of _SET_IP are setting the ip just in case something wants to inspect it. For these cases, we could store a 16 bit offset instead of the full 64 bit pointer. Code that reads the instruction pointer would need updating, but this should reduce the overhead of _SET_IP by more than half.

_LOAD_CONST_INLINE_BORROW and similar

We can do the tagging when generating the code, instead of while executing. This only saves a single instruction, but it helps.

_LOAD_FAST_BORROW

We already replicate _LOAD_FAST_BORROW, but that still leaves most _LOAD_FAST_BORROW using the slower version, and bloating the stencils.
We could manually generate the code, it is only a couple of instructions, reducing stencil bloat and covering all operands

Linked PRs

• gh-148081
• gh-148217
• gh-148436