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 thetypes 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 inbold are probably a bit harder, tasks initalics are probably a bit easier.

Narrow types to constants in branches involving truthiness:

• _TO_BOOL + _GUARD_IS_*_POPgh-130659
• _TO_BOOL_BOOL + _GUARD_IS_*_POPgh-130659
• _TO_BOOL_INT + _GUARD_IS_*_POPgh-130772
• _TO_BOOL_LIST + _GUARD_IS_*_POP
• _TO_BOOL_STR + _GUARD_IS_*_POPgh-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 teachremove_unneeded_uops about this new instruction.

• _COMPARE_OP (==,!=,<,>,<=,>=)
• _COMPARE_OP_FLOAT (==,!=,<,>,<=,>=)gh-137062
• _COMPARE_OP_INT (==,!=,<,>,<=,>=)gh-131489
• _COMPARE_OP_STR (==,!=,<,>,<=,>=)gh-137062
• _CONTAINS_OP (in,not in)
• _IS_OP (is,is not)

Linked PRs

• GH-130415: Narrowstr 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: Narrowint to0 based on boolean tests #130772
• GH-130415: Optimize constant comparison in JIT builds #131489
• GH-130415: Remove redundantsym_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: UsePOP_TWO_LOAD_CONST_INLINE_BORROW #134241
• GH-132732: Use pure op machinery to optimizeCOMPARE_OP_INT/FLOAT/STR #137062