glsl/linker: Check the subroutine associated functions names

>From Section 6.1.2 (Subroutines) of the GLSL 4.00 specification

    "A program will fail to compile or link if any shader
     or stage contains two or more functions with the same
     name if the name is associated with a subroutine type."

v2:
  - error out earlier (Tapani)
  - style fixes (Iago)

Fixes:
    * no-overloads.vert

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108109
Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/compiler/glsl/linker.cpp

@@ -4639,6 +4639,45 @@ link_assign_subroutine_types(struct gl_shader_program *prog)
    }
 }
 
+static void
+verify_subroutine_associated_funcs(struct gl_shader_program *prog)
+{
+   unsigned mask = prog->data->linked_stages;
+   while (mask) {
+      const int i = u_bit_scan(&mask);
+      gl_program *p = prog->_LinkedShaders[i]->Program;
+      glsl_symbol_table *symbols = prog->_LinkedShaders[i]->symbols;
+
+      /*
+       * From OpenGL ES Shading Language 4.00 specification
+       * (6.1.2 Subroutines):
+       *     "A program will fail to compile or link if any shader
+       *     or stage contains two or more functions with the same
+       *     name if the name is associated with a subroutine type."
+       */
+      for (unsigned j = 0; j < p->sh.NumSubroutineFunctions; j++) {
+         unsigned definitions = 0;
+         char *name = p->sh.SubroutineFunctions[j].name;
+         ir_function *fn = symbols->get_function(name);
+
+         /* Calculate number of function definitions with the same name */
+         foreach_in_list(ir_function_signature, sig, &fn->signatures) {
+            if (sig->is_defined) {
+               if (++definitions > 1) {
+                  linker_error(prog, "%s shader contains two or more function "
+                               "definitions with name `%s', which is "
+                               "associated with a subroutine type.\n",
+                               _mesa_shader_stage_to_string(i),
+                               fn->name);
+                  return;
+               }
+            }
+         }
+      }
+   }
+}
+
+
 static void
 set_always_active_io(exec_list *ir, ir_variable_mode io_mode)
 {
@@ -5024,6 +5063,7 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
 
    check_explicit_uniform_locations(ctx, prog);
    link_assign_subroutine_types(prog);
+   verify_subroutine_associated_funcs(prog);
 
    if (!prog->data->LinkStatus)
       goto done;