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glcpp: Add a more descriptive comment for the SKIP state manipulation

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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 <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
This commit is contained in:
Carl Worth 2013-12-19 15:14:19 -08:00
parent 5a51c1b01a
commit 61cea49014
1 changed files with 36 additions and 5 deletions
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41
src/glsl/glcpp/glcpp-lex.l
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@ -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;
}