[Mesa-dev] [PATCH] RFC: glsl/glcpp: Allow for '#' characters to appear in shader body

[Carl Worth]

Strictly speaking, the GLSL specification only allows for the '#' to appear in
a shader in two places:

	1. To introduce a pre-processing directive

	   In this case, the '#' must be the first character on a line after
	   ignoring any comments and horizontal whitespace.

	2. As part of the token-pasting operator

	   In this case, the '#' must appear as part of the sequence "##" and
	   can only appear in the replacement list of a macro definition.

Since commit f062f0506a5b827667b7eb52136d8420b7e8113b, glcpp has been strict
about '#' characters, such that any occurence outside of either of the above
two cases would result in a syntax error of "Unexpected HASH_TOKEN" from the
grammar.

(Prior to that commit, these illegal '#' would be silently ignored and printed
to stdout. And if it hadn't been for that commit, these illegal '#' would have
triggered internal errors with later commits.)

With this commit, glcpp relaxes its strictness and treats such appearances of
'#' as just another piece of punctuation. This would make it legal to do
something like:

	#define FOO # something here

and then use "#ifdef FOO" and similar, (though if FOO were ever directly
expanded, this should trigger a syntax error in the main GLSL compiler).

Relaxing strict conformance compared to GLSL seems somewhat dubious here, but
might be justified for a few reasons:

	1. GCC's preprocessor allows this, (though C is not GLSL, of course)

	2. glcpp is generally soft about illegal characters, (compare to
	   non-ASCII characters which glcpp happily passes through, trusting
	   the GLSL compiler to catch them).

	3. At least one game in the wild (Reactive Quake) has been observed
	   with a bogus '#' in it:

		#define USE_DISCARD#line 0

	   Note that the shader (that we've seen) with this line does not ever
	   *use* the USE_DISCARD macro. Note also that this shader is accepted
	   by both Nvidia's OpenGL implementation as well as Mesa prior to
	   the commit referenced above.
---
 src/glsl/glcpp/glcpp-lex.l   | 12 +++++++-----
 src/glsl/glcpp/glcpp-parse.y |  1 +
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/glsl/glcpp/glcpp-lex.l b/src/glsl/glcpp/glcpp-lex.l
index c126850..cf517ad 100644
--- a/src/glsl/glcpp/glcpp-lex.l
+++ b/src/glsl/glcpp/glcpp-lex.l
@@ -270,15 +270,17 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 
 	/* If the '#' is the first non-whitespace, non-comment token on this
 	 * line, then it introduces a directive, switch to the <HASH> start
-	 * condition.
+	 * condition and return a HASH_TOKEN token.
 	 *
-	 * Otherwise, this is just punctuation, so return the HASH_TOKEN
-         * token. */
+	 * Otherwise, this is just punctuation, so just return a literal '#'
+	 * token, (which will not be interpreted as a directive).
+	 */
 	if (parser->first_non_space_token_this_line) {
 		BEGIN HASH;
+		RETURN_TOKEN_NEVER_SKIP (HASH_TOKEN);
+	} else {
+		RETURN_TOKEN ('#');
 	}
-
-	RETURN_TOKEN_NEVER_SKIP (HASH_TOKEN);
 }
 
 <HASH>version{HSPACE}+ {
diff --git a/src/glsl/glcpp/glcpp-parse.y b/src/glsl/glcpp/glcpp-parse.y
index 4ee4110..b281b01 100644
--- a/src/glsl/glcpp/glcpp-parse.y
+++ b/src/glsl/glcpp/glcpp-parse.y
@@ -744,6 +744,7 @@ operator:
 |	PASTE			{ $$ = PASTE; }
 |	PLUS_PLUS		{ $$ = PLUS_PLUS; }
 |	MINUS_MINUS		{ $$ = MINUS_MINUS; }
+|	'#'			{ $$ = '#'; }
 ;
 
 %%
-- 
2.0.0