nir: add direct lowered frag_coord building to replace lowering passes

Instead of lowering frag_coord 4 times during compilation,
just use this.

Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41227>

src/compiler/nir/nir_builder.c

@@ -790,3 +790,40 @@ nir_call_serialized(nir_builder *b, const uint32_t *serialized,
    ralloc_free(memctx);
    return ret;
 }
+
+/* Build frag_coord according to NIR options. This should generate the final
+ * lowered form expected by drivers.
+ *
+ * The purpose of "num_components" is to generate less dead code for the split
+ * form if some components are not needed.
+ */
+nir_def *
+nir_build_frag_coord(nir_builder *b, unsigned num_components)
+{
+   assert(b->shader->info.stage == MESA_SHADER_FRAGMENT);
+   assert(num_components && num_components <= 4);
+
+   if (b->shader->options->frag_coord_form >= nir_frag_coord_xy_z_w_separate) {
+      nir_def *xy = nir_load_frag_coord_xy(b);
+
+      if (num_components <= 2)
+         return nir_trim_vector(b, xy, num_components);
+
+      nir_def *z = nir_load_frag_coord_z(b);
+
+      if (num_components == 3)
+         return nir_vec3(b, nir_channel(b, xy, 0), nir_channel(b, xy, 1), z);
+
+      nir_def *w;
+
+      if (b->shader->options->frag_coord_form ==
+          nir_frag_coord_xy_z_w_rcp_separate)
+         w = nir_frcp(b, nir_load_frag_coord_w_rcp(b));
+      else
+         w = nir_load_frag_coord_w(b);
+
+      return nir_vec4(b, nir_channel(b, xy, 0), nir_channel(b, xy, 1), z, w);
+   } else {
+      return nir_trim_vector(b, nir_load_frag_coord(b), num_components);
+   }
+}

src/compiler/nir/nir_builder.h

@@ -2347,6 +2347,9 @@ nir_inverse_ballot_imm(nir_builder *build, uint64_t imm, unsigned bit_size)
    return nir_inverse_ballot(build, nir_imm_intN_t(build, imm, bit_size));
 }
 
+nir_def *
+nir_build_frag_coord(nir_builder *b, unsigned num_components);
+
 nir_def *
 nir_build_string(nir_builder *build, const char *value);
 

src/compiler/nir/nir_lower_system_values.c

@@ -299,6 +299,9 @@ lower_system_value_instr(nir_builder *b, nir_instr *instr, void *_state)
                                            bit_size, column, .base = 0,
                                            .range = intrin->def.num_components * bit_size / 8);
 
+      case SYSTEM_VALUE_FRAG_COORD:
+         return nir_build_frag_coord(b, 4);
+
       default:
          break;
       }

src/compiler/nir/nir_shader_compiler_options.h

@@ -243,6 +243,15 @@ typedef enum {
    nir_lower_packing_num_ops,
 } nir_lower_packing_op;
 
+typedef enum {
+   /* Build: frag_coord */
+   nir_frag_coord_regular,
+   /* Build: frag_coord_xy, frag_coord_z, frag_coord_w */
+   nir_frag_coord_xy_z_w_separate,
+   /* Build: frag_coord_xy, frag_coord_z, frcp(frag_coord_w_rcp) */
+   nir_frag_coord_xy_z_w_rcp_separate,
+} nir_frag_coord_form;
+
 typedef struct nir_shader_compiler_options {
    bool lower_fdiv;
    bool lower_ffma16;
@@ -803,6 +812,9 @@ typedef struct nir_shader_compiler_options {
    /** Lower VARYING_SLOT_LAYER in FS to SYSTEM_VALUE_LAYER_ID. */
    bool lower_layer_fs_input_to_sysval;
 
+   /** How nir_build_frag_coord generates frag_coord. */
+   nir_frag_coord_form frag_coord_form;
+
    /** clip/cull distance and tess level arrays use compact semantics */
    bool compact_arrays;
 

src/gallium/auxiliary/nir/tgsi_to_nir.c

@@ -604,7 +604,7 @@ ttn_src_for_file_and_index(struct ttn_compile *c, unsigned file, unsigned index,
          break;
       case TGSI_SEMANTIC_POSITION:
          assert(c->cap_position_is_sysval);
-         load = nir_load_frag_coord(b);
+         load = nir_build_frag_coord(b, 4);
          break;
       case TGSI_SEMANTIC_PCOORD:
          assert(c->cap_point_is_sysval);