From 61cea49014df6dcf3aa41ad18b2e998e6921320f Mon Sep 17 00:00:00 2001 From: Carl Worth Date: Thu, 19 Dec 2013 15:14:19 -0800 Subject: [PATCH] glcpp: Add a more descriptive comment for the SKIP state manipulation Two things make this code confusing: 1. The uncharacteristic manipulation of lexer start state outside of flex rules. 2. The confusing semantics of the skip_stack (including the "lexing_if" override and the SKIP_NO_SKIP state). This new comment is intended to bring a bit more clarity for any readers. There is no intended beahvioral change to the code here. The actual code changes include better indentation to avoid an excessively-long line, and using the more descriptive INITIAL rather than 0. Reviewed-by: Kenneth Graunke Reviewed-by: Ian Romanick --- src/glsl/glcpp/glcpp-lex.l | 41 +++++++++++++++++++++++++++++++++----- 1 file changed, 36 insertions(+), 5 deletions(-) diff --git a/src/glsl/glcpp/glcpp-lex.l b/src/glsl/glcpp/glcpp-lex.l index a029f620367..f1db165d199 100644 --- a/src/glsl/glcpp/glcpp-lex.l +++ b/src/glsl/glcpp/glcpp-lex.l @@ -92,13 +92,44 @@ OCTAL_INTEGER 0[0-7]*[uU]? HEXADECIMAL_INTEGER 0[xX][0-9a-fA-F]+[uU]? %% - /* Implicitly switch between SKIP and INITIAL (non-skipping); - * don't switch if some other state was explicitly set. + + /* The handling of the SKIP vs INITIAL start states requires + * some special handling. Typically, a lexer would change + * start states with statements like "BEGIN SKIP" within the + * lexer rules. We can't get away with that here, since we + * need the parser to actually evaluate expressions for + * directives like "#if". + * + * So, here, in code that will be executed on every call to + * the lexer,and before any rules, we examine the skip_stack + * as set by the parser to know whether to change from INITIAL + * to SKIP or from SKIP back to INITIAL. + * + * Three cases cause us to switch out of the SKIP state and + * back to the INITIAL state: + * + * 1. The top of the skip_stack is of type SKIP_NO_SKIP + * This means we're still evaluating some #if + * hierarchy, but we're on a branch of it where + * content should not be skipped (such as "#if 1" or + * "#else" or so). + * + * 2. The skip_stack is NULL meaning that we've reached + * the last #endif. + * + * 3. The lexing_if bit is set. This indicates that we + * are lexing the expression following an "#if" of + * "#elif". Even inside an "#if 0" we need to lex this + * expression so the parser can correctly update the + * skip_stack state. */ glcpp_parser_t *parser = yyextra; - if (YY_START == 0 || YY_START == SKIP) { - if (parser->lexing_if || parser->skip_stack == NULL || parser->skip_stack->type == SKIP_NO_SKIP) { - BEGIN 0; + if (YY_START == INITIAL || YY_START == SKIP) { + if (parser->lexing_if || + parser->skip_stack == NULL || + parser->skip_stack->type == SKIP_NO_SKIP) + { + BEGIN INITIAL; } else { BEGIN SKIP; } -- 2.30.2