MSL: Fix duplicate gl_Position outputs when gl_Position defined but unused.

When gl_Position is defined by SPIR-V, but neither used nor initialized,
it appeared twice in the MSL output, as gl_Position and glPosition_1.

The existing tests for whether an output is active check only that it is
used by an op, or initialized. Adding the implicit gl_Position also marked
the existing gl_Position as active, duplicating the output variable.

Fix is that when checking for the need to add an implicit gl_Position
output, also check if the var is already defined in the shader,
and just needs to be marked as active.
Add test shader.

reference/opt/shaders-msl/vert/unused-position.vert

@@ -0,0 +1,18 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 gl_Position [[position]];
+    float gl_PointSize [[point_size]];
+};
+
+vertex main0_out main0()
+{
+    main0_out out = {};
+    out.gl_PointSize = 1.0;
+    return out;
+}
+

reference/shaders-msl/vert/unused-position.vert

@@ -0,0 +1,18 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 gl_Position [[position]];
+    float gl_PointSize [[point_size]];
+};
+
+vertex main0_out main0()
+{
+    main0_out out = {};
+    out.gl_PointSize = 1.0;
+    return out;
+}
+

shaders-msl/vert/unused-position.vert

@@ -0,0 +1,13 @@
+#version 450
+
+out gl_PerVertex
+{
+    vec4 gl_Position;
+    float gl_PointSize;
+    float gl_ClipDistance[1];
+};
+
+void main()
+{
+    gl_PointSize = 1.0;
+}

spirv_msl.cpp

@@ -877,12 +877,36 @@ void CompilerMSL::build_implicit_builtins()
 	if (need_position)
 	{
 		// If we can get away with returning void from entry point, we don't need to care.
-		// If there is at least one other stage output, we need to return [[position]].
-		need_position = false;
+		// If there is at least one other stage output, we need to return [[position]],
+		// so we need to create one if it doesn't appear in the SPIR-V. Before adding the
+		// implicit variable, check if it actually exists already, but just has not been used
+		// or initialized, and if so, mark it as active, and do not create the implicit variable.
+		bool has_output = false;
 		ir.for_each_typed_id<SPIRVariable>([&](uint32_t, SPIRVariable &var) {
 			if (var.storage == StorageClassOutput && interface_variable_exists_in_entry_point(var.self))
-				need_position = true;
+			{
+				has_output = true;
+
+				// Check if the var is the Position builtin
+				if (has_decoration(var.self, DecorationBuiltIn) && get_decoration(var.self, DecorationBuiltIn) == BuiltInPosition)
+					active_output_builtins.set(BuiltInPosition);
+
+				// If the var is a struct, check if any members is the Position builtin
+				auto &var_type = get_variable_element_type(var);
+				if (var_type.basetype == SPIRType::Struct)
+				{
+					auto mbr_cnt = var_type.member_types.size();
+					for (uint32_t mbr_idx = 0; mbr_idx < mbr_cnt; mbr_idx++)
+					{
+						auto builtin = BuiltInMax;
+						bool is_builtin = is_member_builtin(var_type, mbr_idx, &builtin);
+						if (is_builtin && builtin == BuiltInPosition)
+							active_output_builtins.set(BuiltInPosition);
+					}
+				}
+			}
 		});
+		need_position = has_output && !active_output_builtins.get(BuiltInPosition);
 	}
 
 	if (need_position)