Reorder instructions in with/async-with exception handler to reduce the size of the exception table. #93621

iritkatriel · 2022-06-08T23:05:46Z

The dis for this function:

def f1(cm):
    with cm:
        x = 1
    x = 2

is this:

 11           0 RESUME                   0

 12           2 LOAD_FAST                0 (cm)
              4 BEFORE_WITH
              6 POP_TOP

 13           8 LOAD_CONST               1 (1)
             10 STORE_FAST               1 (x)

 12          12 LOAD_CONST               0 (None)
             14 LOAD_CONST               0 (None)
             16 LOAD_CONST               0 (None)
             18 CALL                     2
             28 POP_TOP

 14          30 LOAD_CONST               2 (2)
             32 STORE_FAST               1 (x)
             34 LOAD_CONST               0 (None)
             36 RETURN_VALUE

 12     >>   38 PUSH_EXC_INFO
             40 WITH_EXCEPT_START
             42 POP_JUMP_FORWARD_IF_TRUE     4 (to 52)
             44 RERAISE                  2
        >>   46 COPY                     3
             48 POP_EXCEPT
             50 RERAISE                  1
        >>   52 POP_TOP
             54 POP_EXCEPT
             56 POP_TOP
             58 POP_TOP

 14          60 LOAD_CONST               2 (2)
             62 STORE_FAST               1 (x)
             64 LOAD_CONST               0 (None)
             66 RETURN_VALUE
ExceptionTable:
  6 to 10 -> 38 [1] lasti
  38 to 44 -> 46 [3] lasti
  52 to 52 -> 46 [3] lasti

Note the last row of the exception table: inst 52 goes to 46, same as 38-44. We can reorder the code so that this block is consecutive, and we save one entry in the exception table for each with/async with:

 11           0 RESUME                   0

 12           2 LOAD_FAST                0 (cm)
              4 BEFORE_WITH
              6 POP_TOP

 13           8 LOAD_CONST               1 (1)
             10 STORE_FAST               1 (x)

 12          12 LOAD_CONST               0 (None)
             14 LOAD_CONST               0 (None)
             16 LOAD_CONST               0 (None)
             18 CALL                     2
             28 POP_TOP

 14          30 LOAD_CONST               2 (2)
             32 STORE_FAST               1 (x)
             34 LOAD_CONST               0 (None)
             36 RETURN_VALUE

 12     >>   38 PUSH_EXC_INFO
             40 WITH_EXCEPT_START
             42 POP_JUMP_FORWARD_IF_TRUE     1 (to 46)
             44 RERAISE                  2
        >>   46 POP_TOP
             48 POP_EXCEPT
             50 POP_TOP
             52 POP_TOP

 14          54 LOAD_CONST               2 (2)
             56 STORE_FAST               1 (x)
             58 LOAD_CONST               0 (None)
             60 RETURN_VALUE
        >>   62 COPY                     3
             64 POP_EXCEPT
             66 RERAISE                  1
ExceptionTable:
  6 to 10 -> 38 [1] lasti
  38 to 46 -> 62 [3] lasti

The text was updated successfully, but these errors were encountered:

…ce the size of the exception table (GH-93622)

iritkatriel added the type-feature label Jun 8, 2022

iritkatriel mentioned this issue Jun 8, 2022

GH-93621: reorder code in with/async-with exception exit path to reduce the size of the exception table #93622

Merged

iritkatriel self-assigned this Jun 9, 2022

iritkatriel closed this as completed in #93622 Jun 10, 2022

iritkatriel added a commit that referenced this issue Jun 10, 2022

GH-93621: reorder code in with/async-with exception exit path to redu…

cf730b5

…ce the size of the exception table (GH-93622)

Reorder instructions in with/async-with exception handler to reduce the size of the exception table. #93621

Reorder instructions in with/async-with exception handler to reduce the size of the exception table. #93621

Comments

iritkatriel commented Jun 8, 2022