tgsi_to_nir: Support FACE and POSITION properly.

Previously, FACE was hard-coded as a sysval, but TTN emulated
it incorrectly. Also, POSITION was not supported when it was
a sysval. This patch fixes these by allowing both of them to
be sysvals or inputs, based on driver capabilities. It also
fixes the TGSI FACE emulation based on the TGSI spec.

Signed-Off-By: Timur Kristóf <timur.kristof@gmail.com>
Tested-by: Andre Heider <a.heider@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/gallium/auxiliary/nir/tgsi_to_nir.c

@@ -71,6 +71,9 @@ struct ttn_compile {
    nir_variable **inputs;
    nir_variable **outputs;
 
+   nir_variable *input_var_face;
+   nir_variable *input_var_position;
+
    /**
     * Stack of nir_cursors where instructions should be pushed as we pop
     * back out of the control flow stack.
@@ -284,8 +287,22 @@ ttn_emit_declaration(struct ttn_compile *c)
 
             if (c->scan->processor == PIPE_SHADER_FRAGMENT) {
                if (decl->Semantic.Name == TGSI_SEMANTIC_FACE) {
-                  var->data.location = SYSTEM_VALUE_FRONT_FACE;
-                  var->data.mode = nir_var_system_value;
+                  var->type = glsl_bool_type();
+                  if (c->cap_face_is_sysval) {
+                     var->data.mode = nir_var_system_value;
+                     var->data.location = SYSTEM_VALUE_FRONT_FACE;
+                  } else {
+                     var->data.location = VARYING_SLOT_FACE;
+                  }
+                  c->input_var_face = var;
+               } else if (decl->Semantic.Name == TGSI_SEMANTIC_POSITION) {
+                  if (c->cap_position_is_sysval) {
+                     var->data.mode = nir_var_system_value;
+                     var->data.location = SYSTEM_VALUE_FRAG_COORD;
+                  } else {
+                     var->data.location = VARYING_SLOT_POS;
+                  }
+                  c->input_var_position = var;
                } else {
                   var->data.location =
                      tgsi_varying_semantic_to_slot(decl->Semantic.Name,
@@ -441,20 +458,43 @@ ttn_array_deref(struct ttn_compile *c, nir_variable *var, unsigned offset,
 }
 
 /* Special case: Turn the frontface varying into a load of the
- * frontface intrinsic plus math, and appending the silly floats.
+ * frontface variable, and create the vector as required by TGSI.
  */
 static nir_ssa_def *
 ttn_emulate_tgsi_front_face(struct ttn_compile *c)
 {
-   nir_ssa_def *tgsi_frontface[4] = {
-      nir_bcsel(&c->build,
-                nir_load_front_face(&c->build, 1),
-                nir_imm_float(&c->build, 1.0),
-                nir_imm_float(&c->build, -1.0)),
-      nir_imm_float(&c->build, 0.0),
-      nir_imm_float(&c->build, 0.0),
-      nir_imm_float(&c->build, 1.0),
-   };
+   nir_ssa_def *tgsi_frontface[4];
+
+   if (c->cap_face_is_sysval) {
+      /* When it's a system value, it should be an integer vector: (F, 0, 0, 1)
+       * F is 0xffffffff if front-facing, 0 if not.
+       */
+
+      nir_ssa_def *frontface = nir_load_front_face(&c->build, 1);
+
+      tgsi_frontface[0] = nir_bcsel(&c->build,
+                             frontface,
+                             nir_imm_int(&c->build, 0xffffffff),
+                             nir_imm_int(&c->build, 0));
+      tgsi_frontface[1] = nir_imm_int(&c->build, 0);
+      tgsi_frontface[2] = nir_imm_int(&c->build, 0);
+      tgsi_frontface[3] = nir_imm_int(&c->build, 1);
+   } else {
+      /* When it's an input, it should be a float vector: (F, 0.0, 0.0, 1.0)
+       * F is positive if front-facing, negative if not.
+       */
+
+      assert(c->input_var_face);
+      nir_ssa_def *frontface = nir_load_var(&c->build, c->input_var_face);
+
+      tgsi_frontface[0] = nir_bcsel(&c->build,
+                             frontface,
+                             nir_imm_float(&c->build, 1.0),
+                             nir_imm_float(&c->build, -1.0));
+      tgsi_frontface[1] = nir_imm_float(&c->build, 0.0);
+      tgsi_frontface[2] = nir_imm_float(&c->build, 0.0);
+      tgsi_frontface[3] = nir_imm_float(&c->build, 1.0);
+   }
 
    return nir_vec(&c->build, tgsi_frontface, 4);
 }
@@ -521,6 +561,16 @@ ttn_src_for_file_and_index(struct ttn_compile *c, unsigned file, unsigned index,
          op = nir_intrinsic_load_instance_id;
          load = nir_load_instance_id(b);
          break;
+      case TGSI_SEMANTIC_FACE:
+         assert(c->cap_face_is_sysval);
+         op = nir_intrinsic_load_front_face;
+         load = ttn_emulate_tgsi_front_face(c);
+         break;
+      case TGSI_SEMANTIC_POSITION:
+         assert(c->cap_position_is_sysval);
+         op = nir_intrinsic_load_frag_coord;
+         load = nir_load_frag_coord(b);
+         break;
       default:
          unreachable("bad system value");
       }
@@ -535,7 +585,12 @@ ttn_src_for_file_and_index(struct ttn_compile *c, unsigned file, unsigned index,
    case TGSI_FILE_INPUT:
       if (c->scan->processor == PIPE_SHADER_FRAGMENT &&
           c->scan->input_semantic_name[index] == TGSI_SEMANTIC_FACE) {
+         assert(!c->cap_face_is_sysval && c->input_var_face);
          return nir_src_for_ssa(ttn_emulate_tgsi_front_face(c));
+      } else if (c->scan->processor == PIPE_SHADER_FRAGMENT &&
+          c->scan->input_semantic_name[index] == TGSI_SEMANTIC_POSITION) {
+         assert(!c->cap_position_is_sysval && c->input_var_position);
+         return nir_src_for_ssa(nir_load_var(&c->build, c->input_var_position));
       } else {
          /* Indirection on input arrays isn't supported by TTN. */
          assert(!dim);
@@ -1971,6 +2026,7 @@ ttn_finalize_nir(struct ttn_compile *c)
    NIR_PASS_V(nir, nir_lower_global_vars_to_local);
    NIR_PASS_V(nir, nir_split_var_copies);
    NIR_PASS_V(nir, nir_lower_var_copies);
+   NIR_PASS_V(nir, nir_lower_system_values);
 
    if (c->cap_packed_uniforms)
       NIR_PASS_V(nir, nir_lower_uniforms_to_ubo, 16);