glsl: Properly throw compile-time errors for conflicting FS output writes.

We were checking for these at link time previously, which is not as
early as mandated, and would actually fail to detect conflicting
writes if dead code removal removed some writes.

Fixes failures in piglit
glsl-*/compiler/fragment-outputs/write-gl_Frag*

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ast_to_hir.cpp

@@ -57,6 +57,10 @@
 #include "program/hash_table.h"
 #include "ir.h"
 
+static void
+detect_conflicting_assignments(struct _mesa_glsl_parse_state *state,
+			       exec_list *instructions);
+
 void
 _mesa_ast_to_hir(exec_list *instructions, struct _mesa_glsl_parse_state *state)
 {
@@ -87,6 +91,7 @@ _mesa_ast_to_hir(exec_list *instructions, struct _mesa_glsl_parse_state *state)
       ast->hir(instructions, state);
 
    detect_recursion_unlinked(state, instructions);
+   detect_conflicting_assignments(state, instructions);
 
    state->toplevel_ir = NULL;
 }
@@ -4017,3 +4022,64 @@ ast_struct_specifier::hir(exec_list *instructions,
     */
    return NULL;
 }
+
+static void
+detect_conflicting_assignments(struct _mesa_glsl_parse_state *state,
+			       exec_list *instructions)
+{
+   bool gl_FragColor_assigned = false;
+   bool gl_FragData_assigned = false;
+   bool user_defined_fs_output_assigned = false;
+   ir_variable *user_defined_fs_output = NULL;
+
+   /* It would be nice to have proper location information. */
+   YYLTYPE loc;
+   memset(&loc, 0, sizeof(loc));
+
+   foreach_list(node, instructions) {
+      ir_variable *var = ((ir_instruction *)node)->as_variable();
+
+      if (!var)
+	 continue;
+
+      if (strcmp(var->name, "gl_FragColor") == 0)
+	 gl_FragColor_assigned = var->assigned;
+      else if (strcmp(var->name, "gl_FragData") == 0)
+	 gl_FragData_assigned = var->assigned;
+      else if (strncmp(var->name, "gl_", 3) != 0) {
+	 if (state->target == fragment_shader &&
+	     (var->mode == ir_var_out || var->mode == ir_var_inout)) {
+	    user_defined_fs_output_assigned = true;
+	    user_defined_fs_output = var;
+	 }
+      }
+   }
+
+   /* From the GLSL 1.30 spec:
+    *
+    *     "If a shader statically assigns a value to gl_FragColor, it
+    *      may not assign a value to any element of gl_FragData. If a
+    *      shader statically writes a value to any element of
+    *      gl_FragData, it may not assign a value to
+    *      gl_FragColor. That is, a shader may assign values to either
+    *      gl_FragColor or gl_FragData, but not both. Multiple shaders
+    *      linked together must also consistently write just one of
+    *      these variables.  Similarly, if user declared output
+    *      variables are in use (statically assigned to), then the
+    *      built-in variables gl_FragColor and gl_FragData may not be
+    *      assigned to. These incorrect usages all generate compile
+    *      time errors."
+    */
+   if (gl_FragColor_assigned && gl_FragData_assigned) {
+      _mesa_glsl_error(&loc, state, "fragment shader writes to both "
+		       "`gl_FragColor' and `gl_FragData'\n");
+   } else if (gl_FragColor_assigned && user_defined_fs_output_assigned) {
+      _mesa_glsl_error(&loc, state, "fragment shader writes to both "
+		       "`gl_FragColor' and `%s'\n",
+		       user_defined_fs_output->name);
+   } else if (gl_FragData_assigned && user_defined_fs_output_assigned) {
+      _mesa_glsl_error(&loc, state, "fragment shader writes to both "
+		       "`gl_FragData' and `%s'\n",
+		       user_defined_fs_output->name);
+   }
+}