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Avoid lookahead when parsing a statement.

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If a statement started with an identifier, then we'd normally lookahead to see if the next token
was a colon.  Lookahead is expensive (as it has to rescan tokens).  Instead, we do the same thing
we do for arrow-functions.  We just parse out the identifier, and afterwards we check if there is
a colon as the current token.  Now, no lookahead is required.
This commit is contained in:
Cyrus Najmabadi 2014-12-12 11:38:05 -08:00
parent 90a9b3472a
commit 26badc686f
1 changed files with 19 additions and 23 deletions
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42
src/compiler/parser.ts
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@ -3123,27 +3123,25 @@ module ts {
return finishNode(node);
}
function isLabel(): boolean {
return isIdentifier() && lookAhead(nextTokenIsColonToken);
}
function parseExpressionOrLabeledStatement(): ExpressionStatement | LabeledStatement {
// Avoiding having to do the lookahead for a labeled statement by just trying to parse
// out an expression, seeing if it is identifier and then seeing if it is followed by
// a colon.
var fullStart = scanner.getStartPos();
var expression = allowInAnd(parseExpression);
function nextTokenIsColonToken() {
return nextToken() === SyntaxKind.ColonToken;
}
function parseLabeledStatement(): LabeledStatement {
var node = <LabeledStatement>createNode(SyntaxKind.LabeledStatement);
node.label = parseIdentifier();
parseExpected(SyntaxKind.ColonToken);
node.statement = parseStatement();
return finishNode(node);
}
function parseExpressionStatement(): ExpressionStatement {
var node = <ExpressionStatement>createNode(SyntaxKind.ExpressionStatement);
node.expression = allowInAnd(parseExpression);
parseSemicolon();
return finishNode(node);
if (expression.kind === SyntaxKind.Identifier && parseOptional(SyntaxKind.ColonToken)) {
var labeledStatement = <LabeledStatement>createNode(SyntaxKind.LabeledStatement, fullStart);
labeledStatement.label = <Identifier>expression;
labeledStatement.statement = parseStatement();
return finishNode(labeledStatement);
}
else {
var expressionStatement = <ExpressionStatement>createNode(SyntaxKind.ExpressionStatement, fullStart);
expressionStatement.expression = expression;
parseSemicolon();
return finishNode(expressionStatement);
}
}
function isStatement(inErrorRecovery: boolean): boolean {
@ -3264,9 +3262,7 @@ module ts {
}
// Else parse it like identifier - fall through
default:
return isLabel()
? parseLabeledStatement()
: parseExpressionStatement();
return parseExpressionOrLabeledStatement();
}
}