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.
---
src/glsl/glcpp/glcpp-lex.l | 45 +++++++++++++++++++++++++++++++++++++++------
1 file changed, 39 insertions(+), 6 deletions(-)
diff --git a/src/glsl/glcpp/glcpp-lex.l b/src/glsl/glcpp/glcpp-lex.l
index a029f62..5543edc 100644
--- a/src/glsl/glcpp/glcpp-lex.l
+++ b/src/glsl/glcpp/glcpp-lex.l
@@ -92,14 +92,47 @@ 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;
- } else {
+ 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;
}
}
--
1.8.5.2
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