Avoid quadratic value-expression check on and/or chains - #4221
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Earlopain merged 1 commit intoSep 5, 2026
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pm_check_value_expression descended the left operand of every and/or node it visited. Because pm_and_node_create and pm_or_node_create already assert the value of their left operand when the node is built, walking a left-associative chain such as `a && a && ... && a` re-checked the whole left spine once per operator, which is quadratic in the length of the chain: 16k operators took seconds. The left operand of an existing and/or node was therefore already checked, so stop at the node instead of descending. This makes the check linear. The only observable change is that a void value on the left spine of a chain is now reported once, at the innermost node where it is created, rather than once per enclosing operator; the errors fixture that pinned the duplicate is updated to match.
makenowjust
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Sep 5, 2026
| @@ -1,6 +1,5 @@ | |||
| t next&&do end&= | |||
| ^~ unexpected 'do'; expected an expression after the operator | |||
| ^~~~ unexpected void value expression | |||
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parse.y still reports this duplicated warning, but it is redundant; I'll open a pull request to fix it for parse.y too.
Earlopain
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Nice, thanks.
BTW, you can do Prism.profile to get a more accurate number for just the parse step.
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(ruby/prism#4221) Ref https://bugs.ruby-lang.org/issues/22294 pm_check_value_expression descended the left operand of every and/or node it visited. Because pm_and_node_create and pm_or_node_create already assert the value of their left operand when the node is built, walking a left-associative chain such as `a && a && ... && a` re-checked the whole left spine once per operator, which is quadratic in the length of the chain: 16k operators took seconds. The left operand of an existing and/or node was therefore already checked, so stop at the node instead of descending. This makes the check linear. The only observable change is that a void value on the left spine of a chain is now reported once, at the innermost node where it is created, rather than once per enclosing operator; the errors fixture that pinned the duplicate is updated to match. ruby/prism@b19a0a5bc0
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Ref https://bugs.ruby-lang.org/issues/22294
pm_check_value_expression()descends the left operand of every and/or node.Because
pm_and_node_createandpm_or_node_createalready assert the value of their left operand when the node is built, walking a left-associative chain such asa && a && ... && are-checks the whole left spine once per operator, which is quadratic in the length of the chain.The left operand of an existing and/or node was therefore already checked, so stop at the node instead of descending.
This makes the check linear.
The only observable change is that a void value on the left branch of a chain is now reported once, at the innermost node where it is created, rather than once per enclosing operator; the errors fixture that pinned the duplicate is updated to match.