Daniel Sun created GROOVY-12173:
-----------------------------------
Summary: Left-factor parser rules to reduce lookahead on hot parse
paths
Key: GROOVY-12173
URL: https://issues.apache.org/jira/browse/GROOVY-12173
Project: Groovy
Issue Type: Improvement
Reporter: Daniel Sun
h3. Motivation
Successful parse cost in the ANTLR4 (Parrot) parser is dominated by adaptive
prediction. Several grammar rules still force the simulator to explore *sibling
alternatives that share a long common prefix*, or to re-run an expensive
semantic predicate that was already evaluated. That inflates DFA growth and SLL
work even when the input is valid.
This improvement left-factors and simplifies a small set of non-left-recursive
rules so decisions commit earlier, *without* changing language semantics or the
AST shapes {{AstBuilder}} relies on.
h3. Changes
*1. {{importDeclaration}} — left-factor {{annotationsOpt IMPORT}}*
** Before: two alternatives both started with {{annotationsOpt IMPORT}}, so
prediction replayed the shared prefix to choose module vs ordinary import.
** After: one {{IMPORT}}, then {{MODULE qualifiedName}} vs {{STATIC?
qualifiedName ...}}. Decision is effectively one token after {{IMPORT}}.
*2. {{typeNamePairs}} — left-factor {{LPAREN}} / {{RPAREN}}*
** Before: two alternatives both wrapped the list in parentheses (positional
multi-assign vs GEP-20 keyed pairs).
** After: open paren once; choose {{typeNamePair}} vs {{keyedPair}} (keyed form
distinguished by {{COLON}}).
*3. {{pathElement}} — left-factor {{DOT}}*
** Before: {{DOT nls NEW creator}} and {{DOT nls namePart}} were separate
alternatives, both starting with {{DOT}}, so every property access paid for
exploring both.
** After: {{DOT nls}} then {{NEW creator}} vs name selection. After {{DOT}},
{{NEW}} is LL(1). Non-DOT selectors ({{?.}}, {{*.}}, {{??.}}, {{.&}}, {{::}})
stay separate.
*4. {{blockStatement}} — drop redundant local-variable alternative*
** Before:
{noformat}
blockStatement
: localVariableDeclaration
| statement
;
{noformat}
** {{statement}} already includes {{localVariableDeclaration}}
({{#localVariableDeclarationStmtAlt}}). Inside a block, command-style calls
such as {{foo bar}} therefore evaluated {{isInvalidLocalVariableDeclaration}}
*twice* (fail on the first alt, fail again via {{statement}}).
** After: {{blockStatement : statement}} only. Observationally equivalent;
halves predicate traffic on that path.
** {{AstBuilder#visitBlockStatement}} updated to visit {{statement}} only.
h3. Non-goals / deliberate non-changes
* Do *not* left-factor the three {{#relationalExprAlt}} alternatives under
left-recursive {{expression}} into one subrule group. ANTLR's left-recursion
rewriter only preserves operator precedence for direct {{expression OP
expression}} forms; factoring the operator into a parenthesised group lets
{{right=expression}} accept lower-precedence ops (e.g. {{&&}}), so {{x >= min
&& x <= max}} can parse as {{x >= (min && ...)}} and break type checking.
* Index vs named property args ({{indexPropertyArgs}} / {{namedPropertyArgs}})
still share {{SAFE_INDEX|LBRACK}}; merging them would reshape the CST and force
a larger {{AstBuilder}} change for limited gain — left for a follow-up if
measured.
h3. Compatibility
* Language surface and AST construction paths are unchanged for correct
programs.
* No public API additions.
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