kconfig: fix ambiguous grammar in terms of new lines

This commit decreases 8 shift/reduce conflicts.

A certain amount of grammatical ambiguity comes from how to reduce
excessive T_EOL tokens.

Let's take a look at the example code below:

  1  config A
  2          bool "a"
  3
  4          depends on B
  5
  6  config B
  7          def_bool y

The line 3 is melt into "config_option_list", but the line 5 can be
either a part of "config_option_list" or "common_stmt" by itself.

Currently, the lexer converts '\n' to T_EOL verbatim. In Kconfig,
a new line works as a statement terminator, but new lines in empty
lines are not critical since empty lines (or lines that contain only
whitespaces/comments) are just no-op.

If the lexer simply discards no-op lines, the parser will not be
bothered by excessive T_EOL tokens.

Of course, this means we are shifting the complexity from the parser
to the lexer, but it is much easier than tackling on shift/reduce
conflicts.

I introduced the second stage lexer to tweak the behavior.

Discard T_EOL if the previous token is T_EOL or T_HELPTEXT.
Two T_EOL tokens in a row is meaningless. T_HELPTEXT is a special
token that is reduced without T_EOL.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/scripts/kconfig/zconf.l b/scripts/kconfig/zconf.l
index b7bc164fdf00..847ba4248092 100644
--- a/scripts/kconfig/zconf.l
+++ b/scripts/kconfig/zconf.l
@@ -16,6 +16,8 @@
 
 #include "lkc.h"
 
+#define YY_DECL		static int yylex1(void)
+
 #define START_STRSIZE	16
 
 static struct {
@@ -23,6 +25,7 @@ static struct {
 	int lineno;
 } current_pos;
 
+static int prev_token = T_EOL;
 static char *text;
 static int text_size, text_asize;
 
@@ -268,6 +271,24 @@ n	[A-Za-z0-9_-]
 }
 
 %%
+
+/* second stage lexer */
+int yylex(void)
+{
+	int token;
+
+repeat:
+	token = yylex1();
+
+	/* Do not pass unneeded T_EOL to the parser. */
+	if ((prev_token == T_EOL || prev_token == T_HELPTEXT) && token == T_EOL)
+		goto repeat;
+
+	prev_token = token;
+
+	return token;
+}
+
 static char *expand_token(const char *in, size_t n)
 {
 	char *out;