If statement without else part generates unnecessary JUMP_FORWARD insn with jumps right to the next insn:

>>> def foo(x):
	if x: x = 1

>>> dis(foo)
  2           0 LOAD_FAST                0 (x) 
              3 POP_JUMP_IF_FALSE       15 
              6 LOAD_CONST               1 (1) 
              9 STORE_FAST               0 (x) 
             12 JUMP_FORWARD             0 (to 15) 
        >>   15 LOAD_CONST               0 (None) 
             18 RETURN_VALUE         

This patch suppresses generation of this jump.

Testing revealed another issue: when AST is produced from string empty 'orelse' sequences are represented with NULLs. However when AST is converted from Python AST objects empty 'orelse' is a pointer to 0-length sequence. I've changed this to produce NULL pointers, like in the string case. This uses less memory and doesn't introduce different code path in compiler. Without this change test_compile failed with my first change.

make test passes.

Test case (needed some refactoring to avoid duplication).

Python-ast.c can't be changed; it is auto-generated.  But the whole thing can be handled in compile.c I think -- see attached patch.  Test suite passes (except for test_dis, which checks compilation result against a given list of bytecodes).

I'll make a complete patch with test suite additions (and fixing test_dis) if this is deemed to be a correct approach.

I think it's fine with the simplification I suggested.

Updated patch with test suite update.

lgtm

Just for the record, have you passed the whole test suite after cleaning up the pyc files,

Yep :)

LGTM.

Attila Fazekas just has provided almost the same patch in issue22358.

New changeset c0ca9d32aed4 by Antoine Pitrou in branch 'default':
Closes #11471: avoid generating a JUMP_FORWARD instruction at the end of an if-block if there is no else-clause.
https://hg.python.org/cpython/rev/c0ca9d32aed4

Pushed after applying Serhiy's suggestion. Thank you!