Please don't work on this. I'm planning on sprinting on this with new contributors at an event this weekend.

Currently, the JIT optimizer uses the _COMPARE_OP family, _CONTAINS_OP, _IS_OP, and the _TO_BOOL family to narrow the types of the input values and the type of the output value.

However, by "peeking ahead" and seeing how a value will be used, we can narrow these types to constants as well. As a simple example, consider _TO_BOOL_INT + _GUARD_IS_FALSE_CHECK on an unknown value. After the _TO_BOOL_INT, it can be narrowed to a known class, int (we do this today). However, after the _GUARD_IS_FALSE_CHECK, we can actually narrow it to a constant value, 0.

An example implementation of this idea for _TO_BOOL_BOOL is here: main...brandtbucher:cpython:hack-night-to-bool-bool
 
I've divided this work into 3 "waves" of increasing complexity. Tasks in bold are probably a bit harder, tasks in italics are probably a bit easier.

Narrow types to constants in branches involving truthiness:

• _TO_BOOL + _GUARD_IS_*_POP gh-130659
• _TO_BOOL_BOOL + _GUARD_IS_*_POP gh-130659
• _TO_BOOL_INT + _GUARD_IS_*_POP gh-130772
• _TO_BOOL_LIST + _GUARD_IS_*_POP
• _TO_BOOL_STR + _GUARD_IS_*_POP gh-130476

Narrow types to constants in branches involving comparisons with a constant:

• _COMPARE_OP + _GUARD_IS_*_POP (==, !=)
• _COMPARE_OP_FLOAT + _GUARD_IS_*_POP (==, !=)
• _COMPARE_OP_INT + _GUARD_IS_*_POP (==, !=)
• _COMPARE_OP_STR + _GUARD_IS_*_POP (==, !=)
• _CONTAINS_OP + _GUARD_IS_*_POP (in, not in)
• _IS_OP + _GUARD_IS_*_POP (is, is not)

Evaluate comparisons involving two constants:

This is related, but a bit more involved, since we need a way to pop two values from the stack and push a constant (_POP_TWO_LOAD_CONST_INLINE_BORROW). We should also teach remove_unneeded_uops about this new instruction.

• _COMPARE_OP (==, !=, <, >, <=, >=)
• _COMPARE_OP_FLOAT (==, !=, <, >, <=, >=)
• _COMPARE_OP_INT (==, !=, <, >, <=, >=) gh-131489
• _COMPARE_OP_STR (==, !=, <, >, <=, >=)
• _CONTAINS_OP (in, not in)
• _IS_OP (is, is not)

Linked PRs

• GH-130415: Narrow str to "" based on boolean tests #130476
• GH-130415: Optimize JIT path for _TO_BOOL_INT branching  #130477
• GH-130415: Use boolean guards to narrow types to values in the JIT #130659
• GH-130415: Narrow int to 0 based on boolean tests #130772
• GH-130415: Optimize constant comparison in JIT builds #131489
• GH-130415: Remove redundant sym_matches_type calls in the JIT optimizer #131778
• GH-130415: Improve the JIT's unneeded uop removal pass #132333
• gh-130415: Eliminate guards for constant CALL_BUILTIN_O/FAST #132708
• GH-130415: Use POP_TWO_LOAD_CONST_INLINE_BORROW #134241