ir3: Add layer_zero variant bit

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5732>

src/freedreno/ir3/ir3_nir.c

@@ -305,6 +305,61 @@ ir3_nir_post_finalize(struct ir3_compiler *compiler, nir_shader *s)
 	ir3_optimize_loop(s);
 }
 
+static bool
+ir3_nir_lower_layer_id(nir_shader *nir)
+{
+	unsigned layer_id_loc = ~0;
+	nir_foreach_variable(var, &nir->inputs) {
+		if (var->data.location == VARYING_SLOT_LAYER) {
+			layer_id_loc = var->data.driver_location;
+			break;
+		}
+	}
+
+	assert(layer_id_loc != ~0);
+
+	bool progress = false;
+	nir_builder b;
+
+	nir_foreach_function(func, nir) {
+		nir_builder_init(&b, func->impl);
+
+		nir_foreach_block(block, func->impl) {
+			nir_foreach_instr_safe(instr, block) {
+				if (instr->type != nir_instr_type_intrinsic)
+					continue;
+
+				nir_intrinsic_instr *intrin =
+					nir_instr_as_intrinsic(instr);
+
+				if (intrin->intrinsic != nir_intrinsic_load_input)
+					continue;
+
+				unsigned base = nir_intrinsic_base(intrin);
+				if (base != layer_id_loc)
+					continue;
+
+				b.cursor = nir_before_instr(&intrin->instr);
+				nir_ssa_def *zero = nir_imm_int(&b, 0);
+				nir_ssa_def_rewrite_uses(&intrin->dest.ssa,
+										 nir_src_for_ssa(zero));
+				nir_instr_remove(&intrin->instr);
+				progress = true;
+			}
+		}
+
+		if (progress) {
+			nir_metadata_preserve(func->impl,
+								  nir_metadata_block_index |
+								  nir_metadata_dominance);
+		} else {
+			nir_metadata_preserve(func->impl, nir_metadata_all);
+		}
+	}
+
+	return progress;
+}
+
 void
 ir3_nir_lower_variant(struct ir3_shader_variant *so, nir_shader *s)
 {
@@ -352,6 +407,8 @@ ir3_nir_lower_variant(struct ir3_shader_variant *so, nir_shader *s)
 			progress |= OPT(s, nir_lower_clip_fs, so->key.ucp_enables, false);
 		if (so->key.fclamp_color)
 			progress |= OPT(s, nir_lower_clamp_color_outputs);
+		if (so->key.layer_zero && (s->info.inputs_read & VARYING_BIT_LAYER))
+			progress |= OPT(s, ir3_nir_lower_layer_id);
 	}
 	if (so->key.color_two_side) {
 		OPT_V(s, nir_lower_two_sided_color);

src/freedreno/ir3/ir3_shader.c

@@ -354,6 +354,10 @@ ir3_setup_used_key(struct ir3_shader *shader)
 			key->color_two_side = true;
 		}
 
+		if (info->inputs_read & VARYING_BIT_LAYER) {
+			key->layer_zero = true;
+		}
+
 		if ((info->outputs_written & ~(FRAG_RESULT_DEPTH |
 								FRAG_RESULT_STENCIL |
 								FRAG_RESULT_SAMPLE_MASK)) != 0) {

src/freedreno/ir3/ir3_shader.h

@@ -318,6 +318,9 @@ struct ir3_shader_key {
 			 * the limit:
 			 */
 			unsigned safe_constlen : 1;
+
+			/* Whether gl_Layer must be forced to 0 because it isn't written. */
+			unsigned layer_zero : 1;
 		};
 		uint32_t global;
 	};
@@ -385,6 +388,9 @@ ir3_shader_key_changes_fs(struct ir3_shader_key *key, struct ir3_shader_key *las
 	if (last_key->rasterflat != key->rasterflat)
 		return true;
 
+	if (last_key->layer_zero != key->layer_zero)
+		return true;
+
 	if (last_key->ucp_enables != key->ucp_enables)
 		return true;