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gh-160788: Fix exponential time in code object comparison - #160849
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iliev-blnk wants to merge 1 commit into
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code_richcompare() compared the constant key of the whole co_consts tuple. The key of a tuple is a pair of the item keys and the tuple itself, so each nested code object was compared twice per level of nesting. Compare the keys of the constants one by one instead.
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code_richcompare()compared_PyCode_ConstantKey(co_consts). For a tuple, that key is the pair(item_keys, tuple), so comparing two keys compares each nested code object twice: once through its item key, which is the code object itself, and once through the original tuple. That doubles the work at every level of nesting.This compares the keys of the constants one by one instead. The result is the same: equal item keys already imply equal items, so comparing the original tuple again never changed the result.
With the reproducer from the issue on a debug build, depth 20 took 3.0 s on main and depth 30 didn't finish. With this change, depth 30 takes under 1 ms.
Tests:
test_code_equality_nested: the 30-deep reproducer. It hangs on main.test_code_equality_uses_consts: checks that the comparison still tells apart0.0,-0.0,0,False,(0.0,)and an extra constant. It passes on main and with this change.