Convert `DELETE_FAST` to `LOAD_FAST_CHECK POP_TOP DELETE_FAST` as a size optimization

[markshannon]

Both LOAD_FAST_CHECK and DELETE_FAST check for NULL and raise.
Replacing DELETE_FAST with LOAD_FAST_CHECK; POP_TOP DELETE_FAST could save some space.

How does replacing one instruction with three save space?
Although DELETE_FAST is dynamically rare, DELETE_FAST is statically much more common as it is used in every except Exception as name: block.
Currently at the end of named exception block we emit

LOAD_CONST None
STORE_FAST n
DELETE_FAST n

In the vast majority of these cases we could safely emit just the DELETE_FAST, using the same data flow analysis we use to convert almost all LOAD_FAST_CHECKs into LOAD_FAST

We can implement this by adding DELETE_FAST_CHECKED (for del) and DELETE_FAST_MAYBE_NULL (for the end of named exception blocks) virtual instructions to the compiler.
The data flow analysis would convert them to DELETE_FAST if it can prove that the local is defined.
The backend would convert the remaining DELETE_FAST_CHECKED to LOAD_FAST_CHECK; POP_TOP DELETE_FAST
and the remaining DELETE_FAST_MAYBE_NULL to LOAD_CONST None STORE_FAST DELETE_FAST.

@sweeneyde interested?

[sweeneyde]

Sure, I can work on this.

Another thing to look at, as you had suggested at one point, is that there could be a NULL_CHECK instruction, and that would work for both LOAD_FAST and DELETE_FAST. That should become possible once python/cpython#99075 is merged.

[markshannon]

NULL_CHECK; DELETE_FAST(unchecked) and LOAD_FAST_CHECK; DELETE_FAST(unchecked) are equivalent.
The question is which is faster. Are there more loads that need checking, or more deletes that need checking? I suspect that there are very few of each.

[markshannon]

Over the 3 years since this issue was created, we've gained a few new instructions, so we can now implement this as follows:

• Implicit delete (named exceptions, etc)

  We can replace LOAD_CONST None; STORE_FAST n; DELETE_FAST n with PUSH_NULL; STORE_FAST n

• Explicit delete (delete x)

  We can replace DELETE_FAST n with LOAD_FAST n; POP_TOP n; PUSH_NULL; STORE_FAST n
  In most cases, the compiler will detect that variable n is defined and leave only PUSH_NULL; STORE_FAST n

With that done, we can remove the DELETE_FAST instruction, freeing up an opcode.