Properly flatten MRT outputs in MSL.

reference/opt/shaders-msl/frag/mrt-array.frag

@@ -13,7 +13,10 @@ struct main0_in
 
 struct main0_out
 {
-    float4 FragColor [[color(0)]][4];
+    float4 FragColor_0 [[color(0)]];
+    float4 FragColor_1 [[color(1)]];
+    float4 FragColor_2 [[color(2)]];
+    float4 FragColor_3 [[color(3)]];
 };
 
 // Implementation of the GLSL mod() function, which is slightly different than Metal fmod()
@@ -26,10 +29,15 @@ Tx mod(Tx x, Ty y)
 fragment main0_out main0(main0_in in [[stage_in]])
 {
     main0_out out = {};
-    out.FragColor[0] = mod(in.vA, in.vB);
-    out.FragColor[1] = in.vA + in.vB;
-    out.FragColor[2] = in.vA - in.vB;
-    out.FragColor[3] = in.vA * in.vB;
+    float4 FragColor[4];
+    FragColor[0] = mod(in.vA, in.vB);
+    FragColor[1] = in.vA + in.vB;
+    FragColor[2] = in.vA - in.vB;
+    FragColor[3] = in.vA * in.vB;
+    out.FragColor_0 = FragColor[0];
+    out.FragColor_1 = FragColor[1];
+    out.FragColor_2 = FragColor[2];
+    out.FragColor_3 = FragColor[3];
     return out;
 }
 

reference/shaders-msl/frag/mrt-array.frag

@@ -13,7 +13,10 @@ struct main0_in
 
 struct main0_out
 {
-    float4 FragColor [[color(0)]][4];
+    float4 FragColor_0 [[color(0)]];
+    float4 FragColor_1 [[color(1)]];
+    float4 FragColor_2 [[color(2)]];
+    float4 FragColor_3 [[color(3)]];
 };
 
 // Implementation of the GLSL mod() function, which is slightly different than Metal fmod()
@@ -23,13 +26,28 @@ Tx mod(Tx x, Ty y)
     return x - y * floor(x / y);
 }
 
+void write_deeper_in_function(thread float4 (&FragColor)[4], thread float4& vA, thread float4& vB)
+{
+    FragColor[3] = vA * vB;
+}
+
+void write_in_function(thread float4 (&FragColor)[4], thread float4& vA, thread float4& vB)
+{
+    FragColor[2] = vA - vB;
+    write_deeper_in_function(FragColor, vA, vB);
+}
+
 fragment main0_out main0(main0_in in [[stage_in]])
 {
     main0_out out = {};
-    out.FragColor[0] = mod(in.vA, in.vB);
-    out.FragColor[1] = in.vA + in.vB;
-    out.FragColor[2] = in.vA - in.vB;
-    out.FragColor[3] = in.vA * in.vB;
+    float4 FragColor[4];
+    FragColor[0] = mod(in.vA, in.vB);
+    FragColor[1] = in.vA + in.vB;
+    write_in_function(FragColor, in.vA, in.vB);
+    out.FragColor_0 = FragColor[0];
+    out.FragColor_1 = FragColor[1];
+    out.FragColor_2 = FragColor[2];
+    out.FragColor_3 = FragColor[3];
     return out;
 }
 

shaders-msl/frag/mrt-array.frag

@@ -5,10 +5,20 @@ layout(location = 0) out vec4 FragColor[4];
 layout(location = 0) in vec4 vA;
 layout(location = 1) in vec4 vB;
 
+void write_deeper_in_function()
+{
+	FragColor[3] = vA * vB;
+}
+
+void write_in_function()
+{
+	FragColor[2] = vA - vB;
+	write_deeper_in_function();
+}
+
 void main()
 {
 	FragColor[0] = mod(vA, vB);
 	FragColor[1] = vA + vB;
-	FragColor[2] = vA - vB;
-	FragColor[3] = vA * vB;
+	write_in_function();
 }

spirv_common.hpp

@@ -739,6 +739,10 @@ struct SPIRFunction : IVariant
 		arguments.push_back({ parameter_type, id, 0u, 0u, alias_global_variable });
 	}
 
+	// Statements to be emitted when the function returns.
+	// Mostly used for lowering internal data structures onto flattened structures.
+	std::vector<std::string> fixup_statements;
+
 	bool active = false;
 	bool flush_undeclared = true;
 	bool do_combined_parameters = true;

spirv_glsl.cpp

@@ -9063,6 +9063,8 @@ void CompilerGLSL::emit_block_chain(SPIRBlock &block)
 	}
 
 	case SPIRBlock::Return:
+		for (auto &line : current_function->fixup_statements)
+			statement(line);
 		if (processing_entry_point)
 			emit_fixup();
 

spirv_msl.cpp

@@ -550,8 +550,9 @@ uint32_t CompilerMSL::add_interface_block(StorageClass storage)
 				bool is_builtin = is_member_builtin(type, mbr_idx, &builtin);
 
 				if (should_move_to_input_buffer(mbr_type_id, is_builtin, storage))
+				{
 					move_member_to_input_buffer(type, mbr_idx);
-
+				}
 				else if (!is_builtin || has_active_builtin(builtin, storage))
 				{
 					// Add a reference to the member to the interface struct.
@@ -609,34 +610,78 @@ uint32_t CompilerMSL::add_interface_block(StorageClass storage)
 			}
 			else if (!is_builtin || has_active_builtin(builtin, storage))
 			{
-				// Add a reference to the variable type to the interface struct.
-				uint32_t ib_mbr_idx = uint32_t(ib_type.member_types.size());
-				type_id = ensure_correct_builtin_type(type_id, builtin);
-				p_var->basetype = type_id;
-				ib_type.member_types.push_back(type_id);
-
-				// Give the member a name
-				string mbr_name = ensure_valid_name(to_expression(p_var->self), "m");
-				set_member_name(ib_type_id, ib_mbr_idx, mbr_name);
-
-				// Update the original variable reference to include the structure reference
-				string qual_var_name = ib_var_ref + "." + mbr_name;
-				meta[p_var->self].decoration.qualified_alias = qual_var_name;
-
-				// Copy the variable location from the original variable to the member
-				if (get_decoration_bitset(p_var->self).get(DecorationLocation))
+				// Arrays of MRT output is not allowed in MSL, so need to handle it specially.
+				if (!is_builtin &&
+				    storage == StorageClassOutput &&
+				    get_entry_point().model == ExecutionModelFragment &&
+				    !type.array.empty())
 				{
-					uint32_t locn = get_decoration(p_var->self, DecorationLocation);
-					set_member_decoration(ib_type_id, ib_mbr_idx, DecorationLocation, locn);
-					mark_location_as_used_by_shader(locn, storage);
+					if (type.array.size() != 1)
+						SPIRV_CROSS_THROW("Cannot emit arrays-of-arrays with MRT.");
+
+					uint32_t num_mrts =
+							type.array_size_literal.back() ? type.array.back() : get<SPIRConstant>(type.array.back()).scalar();
+
+					auto *no_array_type = &type;
+					while (!no_array_type->array.empty())
+						no_array_type = &get<SPIRType>(no_array_type->parent_type);
+
+					auto &entry_func = get<SPIRFunction>(entry_point);
+					entry_func.add_local_variable(p_var->self);
+
+					for (uint32_t i = 0; i < num_mrts; i++)
+					{
+						// Add a reference to the variable type to the interface struct.
+						uint32_t ib_mbr_idx = uint32_t(ib_type.member_types.size());
+						ib_type.member_types.push_back(no_array_type->self);
+
+						// Give the member a name
+						string mbr_name = ensure_valid_name(join(to_expression(p_var->self), "_", i), "m");
+						set_member_name(ib_type_id, ib_mbr_idx, mbr_name);
+
+						// There is no qualified alias since we need to flatten the internal array on return.
+						if (get_decoration_bitset(p_var->self).get(DecorationLocation))
+						{
+							uint32_t locn = get_decoration(p_var->self, DecorationLocation) + i;
+							set_member_decoration(ib_type_id, ib_mbr_idx, DecorationLocation, locn);
+							mark_location_as_used_by_shader(locn, storage);
+						}
+
+						// Lower the internal array to flattened output when entry point returns.
+						entry_func.fixup_statements.push_back(join(ib_var_ref, ".", mbr_name, " = ", to_name(p_var->self), "[", i, "];"));
+					}
 				}
-
-				// Mark the member as builtin if needed
-				if (is_builtin)
+				else
 				{
-					set_member_decoration(ib_type_id, ib_mbr_idx, DecorationBuiltIn, builtin);
-					if (builtin == BuiltInPosition)
-						qual_pos_var_name = qual_var_name;
+					// Add a reference to the variable type to the interface struct.
+					uint32_t ib_mbr_idx = uint32_t(ib_type.member_types.size());
+					type_id = ensure_correct_builtin_type(type_id, builtin);
+					p_var->basetype = type_id;
+					ib_type.member_types.push_back(type_id);
+
+					// Give the member a name
+					string mbr_name = ensure_valid_name(to_expression(p_var->self), "m");
+					set_member_name(ib_type_id, ib_mbr_idx, mbr_name);
+
+					// Update the original variable reference to include the structure reference
+					string qual_var_name = ib_var_ref + "." + mbr_name;
+					meta[p_var->self].decoration.qualified_alias = qual_var_name;
+
+					// Copy the variable location from the original variable to the member
+					if (get_decoration_bitset(p_var->self).get(DecorationLocation))
+					{
+						uint32_t locn = get_decoration(p_var->self, DecorationLocation);
+						set_member_decoration(ib_type_id, ib_mbr_idx, DecorationLocation, locn);
+						mark_location_as_used_by_shader(locn, storage);
+					}
+
+					// Mark the member as builtin if needed
+					if (is_builtin)
+					{
+						set_member_decoration(ib_type_id, ib_mbr_idx, DecorationBuiltIn, builtin);
+						if (builtin == BuiltInPosition)
+							qual_pos_var_name = qual_var_name;
+					}
 				}
 			}
 		}