glsl: Store a predicate for whether a built-in signature is available.

For the upcoming built-in function rewrite, we'll need to be able to
answer "Is this built-in function signature available?".

This is actually a somewhat complex question, since it depends on the
language version, GLSL vs. GLSL ES, enabled extensions, and the current
shader stage.

Storing such a set of constraints in a structure would be painful, so
instead we store a function pointer.  When creating a signature, we
simply point to a predicate that inspects _mesa_glsl_parse_state and
answers whether the signature is available in the current shader.

Unfortunately, IR reader doesn't actually know when built-in functions
are available, so this patch makes it lie and say that they're always
present.  This allows us to hook up the new functionality; it just won't
be useful until real data is populated.  In the meantime, the existing
profile mechanism ensures built-ins are available in the right places.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/ir_reader.cpp

@@ -211,6 +211,12 @@ ir_reader::read_function(s_expression *expr, bool skip_body)
    return added ? f : NULL;
 }
 
+static bool
+always_available(const _mesa_glsl_parse_state *)
+{
+   return true;
+}
+
 void
 ir_reader::read_function_sig(ir_function *f, s_expression *expr, bool skip_body)
 {
@@ -251,8 +257,11 @@ ir_reader::read_function_sig(ir_function *f, s_expression *expr, bool skip_body)
    ir_function_signature *sig = f->exact_matching_signature(&hir_parameters);
    if (sig == NULL && skip_body) {
       /* If scanning for prototypes, generate a new signature. */
-      sig = new(mem_ctx) ir_function_signature(return_type);
-      sig->is_builtin = true;
+      /* ir_reader doesn't know what languages support a given built-in, so
+       * just say that they're always available.  For now, other mechanisms
+       * guarantee the right built-ins are available.
+       */
+      sig = new(mem_ctx) ir_function_signature(return_type, always_available);
       f->add_signature(sig);
    } else if (sig != NULL) {
       const char *badvar = sig->qualifiers_match(&hir_parameters);