nir: add lowering from FS LAYER input to LAYER_ID sysval

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26274>

src/compiler/nir/nir.c

@@ -2195,6 +2195,8 @@ nir_intrinsic_from_system_value(gl_system_value val)
       return nir_intrinsic_load_sample_pos_or_center;
    case SYSTEM_VALUE_SAMPLE_MASK_IN:
       return nir_intrinsic_load_sample_mask_in;
+   case SYSTEM_VALUE_LAYER_ID:
+      return nir_intrinsic_load_layer_id;
    case SYSTEM_VALUE_LOCAL_INVOCATION_ID:
       return nir_intrinsic_load_local_invocation_id;
    case SYSTEM_VALUE_LOCAL_INVOCATION_INDEX:
@@ -2348,6 +2350,8 @@ nir_system_value_from_intrinsic(nir_intrinsic_op intrin)
       return SYSTEM_VALUE_SAMPLE_POS_OR_CENTER;
    case nir_intrinsic_load_sample_mask_in:
       return SYSTEM_VALUE_SAMPLE_MASK_IN;
+   case nir_intrinsic_load_layer_id:
+      return SYSTEM_VALUE_LAYER_ID;
    case nir_intrinsic_load_local_invocation_id:
       return SYSTEM_VALUE_LOCAL_INVOCATION_ID;
    case nir_intrinsic_load_local_invocation_index:

src/compiler/nir/nir.h

@@ -4019,6 +4019,9 @@ typedef struct nir_shader_compiler_options {
 
    /** Lower f2f16 to f2f16_rtz when execution mode is not rtne. */
    bool force_f2f16_rtz;
+
+   /** Lower VARYING_SLOT_LAYER in FS to SYSTEM_VALUE_LAYER_ID. */
+   bool lower_layer_fs_input_to_sysval;
 } nir_shader_compiler_options;
 
 typedef struct nir_shader {

src/compiler/nir/nir_lower_system_values.c

@@ -146,6 +146,14 @@ lower_system_value_instr(nir_builder *b, nir_instr *instr, void *_state)
       }
    }
 
+   case nir_intrinsic_load_input:
+      if (b->shader->options->lower_layer_fs_input_to_sysval &&
+          b->shader->info.stage == MESA_SHADER_FRAGMENT &&
+          nir_intrinsic_io_semantics(intrin).location == VARYING_SLOT_LAYER)
+         return nir_load_layer_id(b);
+      else
+         return NULL;
+
    case nir_intrinsic_load_deref: {
       nir_deref_instr *deref = nir_src_as_deref(intrin->src[0]);
       if (!nir_deref_mode_is(deref, nir_var_system_value))

src/compiler/shader_enums.c

@@ -333,6 +333,7 @@ gl_system_value_name(gl_system_value sysval)
      ENUM(SYSTEM_VALUE_SAMPLE_ID),
      ENUM(SYSTEM_VALUE_SAMPLE_POS),
      ENUM(SYSTEM_VALUE_SAMPLE_MASK_IN),
+     ENUM(SYSTEM_VALUE_LAYER_ID),
      ENUM(SYSTEM_VALUE_HELPER_INVOCATION),
      ENUM(SYSTEM_VALUE_COLOR0),
      ENUM(SYSTEM_VALUE_COLOR1),

src/compiler/shader_enums.h

@@ -788,6 +788,7 @@ typedef enum
    SYSTEM_VALUE_SAMPLE_POS,
    SYSTEM_VALUE_SAMPLE_POS_OR_CENTER,
    SYSTEM_VALUE_SAMPLE_MASK_IN,
+   SYSTEM_VALUE_LAYER_ID,
    SYSTEM_VALUE_HELPER_INVOCATION,
    SYSTEM_VALUE_COLOR0,
    SYSTEM_VALUE_COLOR1,