8000 gh-106149: Simplify stack depth calculation. Replace asserts by exceptions. by iritkatriel · Pull Request #107255 · python/cpython · GitHub
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gh-106149: Simplify stack depth calculation. Replace asserts by exceptions. #107255

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Merged
merged 6 commits into from
Jul 26, 2023
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gh-106149: Simplify stack depth calculation. Replace asserts by exceptions. #107255

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962e87c
gh-106149: Simplify stack depth calculation. Replace asserts by excep…
iritkatriel Jul 25, 2023
1cea1e4
increment g_maxdepth when adding RETURN_GENERATOR (matches previous s…
iritkatriel Jul 25, 2023
ead648f
Revert "increment g_maxdepth when adding RETURN_GENERATOR (matches pr…
iritkatriel Jul 25, 2023
a2fa3cb
move the call to _PyCfg_Stackdepth back to compile.c. return value only
iritkatriel Jul 25, 2023
e546717
assert that stackdepth is not 0 when it's a generator
iritkatriel Jul 25, 2023
fce0919
remove redundant assert. Explain the next one.
iritkatriel Jul 25, 2023
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remove redundant assert. Explain the next one.
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@iritkatriel
iritkatriel committed Jul 25, 2023
commit fce09194f14c71b3b2bf86ec468667cc434fa2b3
8 changes: 7 additions & 1 deletion Python/compile.c
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Expand Up @@ -7731,7 +7731,13 @@ optimize_and_assemble_code_unit(struct compiler_unit *u, PyObject *const_cache,
if (stackdepth < 0) {
goto error;
}
assert(stackdepth >= 0);

/* prepare_localsplus adds instructions for generators that push
* and pop an item on the stack. This assertion makes sure there
* is space on the stack for that.
* It should always be true, because at least one expression is
* required to turn a function into a generator.
Comment on lines +7738 to +7739
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I'm not sure this explanation is accurate? This function is a generator:

async def f():
    return

Here is the disassembly of it:

>>> dis.dis(f)
  1           0 RETURN_GENERATOR
              2 POP_TOP
              4 RESUME                   0

  2           6 RETURN_CONST             0 (None)
        >>    8 CALL_INTRINSIC_1         3 (INTRINSIC_STOPITERATION_ERROR)
             10 RERAISE                  1
ExceptionTable:
  4 to 6 -> 8 [0] lasti

I think if RETURN_CONST didn't exist, it would be enough to say "all functions must return a value, which always must go on the stack before it is returned." But with RETURN_CONST that's not true either. It really does seem like the StopIteration handler is what we are relying on here.

*/
assert(!(IS_GENERATOR(code_flags) && stackdepth == 0));

/** Assembly **/
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0