spirv: Emit SYSTEM_VALUE_LAYER_ID for fragment shaders

We have nir_lower_sysvals_to_varyings() so we can just have that lower
it for the drivers who don't want a sysval.  Most have to support the
sysval version anyway for various lowering so making them all have to
support both is pretty annoying.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38562>

src/broadcom/vulkan/v3dv_pipeline.c

@@ -294,6 +294,7 @@ preprocess_nir(nir_shader *nir)
    const struct nir_lower_sysvals_to_varyings_options sysvals_to_varyings = {
       .frag_coord = true,
       .point_coord = true,
+      .layer_id = true,
    };
    NIR_PASS(_, nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);
 

src/compiler/spirv/vtn_variables.c

@@ -941,16 +941,17 @@ vtn_get_builtin_location(struct vtn_builder *b,
       break;
    case SpvBuiltInLayer:
    case SpvBuiltInLayerPerViewNV:
-      *location = VARYING_SLOT_LAYER;
       if (b->shader->info.stage == MESA_SHADER_FRAGMENT) {
-         *mode = nir_var_shader_in;
-         *interp_mode = INTERP_MODE_FLAT;
+         *location = SYSTEM_VALUE_LAYER_ID;
+         set_mode_system_value(b, mode);
       } else if (b->shader->info.stage == MESA_SHADER_GEOMETRY) {
+         *location = VARYING_SLOT_LAYER;
          *mode = nir_var_shader_out;
       } else if (b->supported_capabilities.ShaderViewportIndexLayerEXT &&
                (b->shader->info.stage == MESA_SHADER_VERTEX ||
                 b->shader->info.stage == MESA_SHADER_TESS_EVAL ||
                 b->shader->info.stage == MESA_SHADER_MESH)) {
+         *location = VARYING_SLOT_LAYER;
          *mode = nir_var_shader_out;
       } else {
          vtn_fail("invalid stage for SpvBuiltInLayer");

src/freedreno/vulkan/tu_shader.cc

@@ -232,6 +232,7 @@ tu_spirv_to_nir(struct tu_device *dev,
 
    const struct nir_lower_sysvals_to_varyings_options sysvals_to_varyings = {
       .point_coord = true,
+      .layer_id = true,
    };
    NIR_PASS(_, nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);
 

src/gallium/frontends/lavapipe/lvp_pipeline.c

@@ -333,6 +333,7 @@ lvp_shader_lower(struct lvp_device *pdevice, nir_shader *nir, struct lvp_pipelin
    const struct nir_lower_sysvals_to_varyings_options sysvals_to_varyings = {
       .frag_coord = true,
       .point_coord = true,
+      .layer_id = true,
    };
    NIR_PASS(_, nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);
 

src/microsoft/spirv_to_dxil/dxil_spirv_nir.c

@@ -905,6 +905,7 @@ dxil_spirv_nir_passes(nir_shader *nir,
       .frag_coord = true,
       .point_coord = true,
       .front_face = true,
+      .layer_id = true,
    };
    NIR_PASS(_, nir, nir_lower_sysvals_to_varyings, &sysvals_to_varyings);