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https://issues.apache.org/jira/browse/GROOVY-12398?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul King closed GROOVY-12398.
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> Improve lexer and parser performance
> ------------------------------------
>
> Key: GROOVY-12398
> URL: https://issues.apache.org/jira/browse/GROOVY-12398
> Project: Groovy
> Issue Type: Improvement
> Reporter: Daniel Sun
> Assignee: Daniel Sun
> Priority: Major
> Fix For: 7.0.0-alpha-1, 6.0.0
>
>
> The Groovy ANTLR4 grammar already uses left-recursive expressions, which is
> the main idea of the [optimized Java
> grammar|https://github.com/antlr/grammars-v4/tree/master/java/java]. Several
> remaining lexer and parser costs still match patterns that grammar was
> written to avoid.
> h3. Problem
> * Number literals are assembled from a chain of one-character fragments
> ({{Digit}}, {{Zero}}, {{NonZeroDigit}}, ...). The invalid-octal alternative
> also counts digits in a mid-rule action loop, which prevents DFA compilation
> of {{IntegerLiteral}}.
> * Every identifier, including ASCII names such as {{Foo}} and {{foo}},
> evaluates {{Character.isUpperCase}} to choose {{CapitalizedIdentifier}} vs
> {{Identifier}}.
> * GString and comment predicates ({{isFollowedByJavaLetterInGString}},
> {{isFollowedByWhiteSpaces}}) allocate a one-character {{String}} and run
> {{Pattern.matcher}} on each lookahead.
> * {{elementValues}} is ambiguous with assignment: {{@Foo(a = 1)}} can be
> named pairs or an assignment expression, so {{AdaptivePredict}} explores both
> (see [antlr/grammars-v4#215|https://github.com/antlr/grammars-v4/issues/215]).
> h3. Approach
> Keep token types and language semantics unchanged.
> * Flatten number and float fragments to character classes, as in the
> optimized Java lexer. Validate invalid octal once at the end of the
> alternative.
> * Split ASCII identifiers on a predicate-free DFA path: uppercase {{A-Z}}
> becomes {{CapitalizedIdentifier}}; {{a-z}}, dollar and underscore become
> {{Identifier}}. Unicode still uses predicates.
> * Replace the regex-based predicates with direct character tests.
> * Gate {{elementValue}} with {{!isIdentifierAssign(_input)}}, the same idea
> as Java's {{IsNotIdentifierAssign}}.
> h3. Result
> Local parse-only measurement (SLL then LL) of {{src/main/groovy}} +
> {{src/test-resources/core}} (245 files, approximately 570k characters):
> ||Run||Best||Mean||
> |before|0.231s|0.401s|
> |after|0.146s|0.169s|
> Best run is about 1.6x faster. Mean improves more because prediction is more
> stable after DFA warmup.
> Related: GROOVY-10454, GROOVY-9588
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