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.

1 files changed, 18 insertions, 0 deletions

diff --git a/reference/opt/shaders-msl/vert/unused-position.vert b/reference/opt/shaders-msl/vert/unused-position.vert
new file mode 100644
index 00000000..7dc46721
--- /dev/null
+++ b/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;
+}
+