Fix output GLSL for primitive ID in a geometry shader (#1214)

We had been translating an `SV_PrimitiveID` input in a shader over to `gl_PrimitiveID` in GLSL.
That translation seemed to work just fine for users, so we thought it was correct.

It turns out that `gl_PrimitiveID` is the correct GLSL for a primitive ID input in every stage *except* a geometry shader.
In a geometry shader, `gl_PrimitiveID` is a primitive ID *output*, and if you want the input case you have to write `gl_PrimitiveIDIn` (note the `In` suffix).

This change sets aside my bewilderment at the above long enough to implement a workaround in the GLSL legalization step.
I also modified our current geometry shader cross compilation test to make use of an input primitive ID.

3 files changed, 41 insertions, 11 deletions

diff --git a/source/slang/slang-ir-glsl-legalize.cpp b/source/slang/slang-ir-glsl-legalize.cpp
index 263e56f55..2c889f341 100644
--- a/source/slang/slang-ir-glsl-legalize.cpp
+++ b/source/slang/slang-ir-glsl-legalize.cpp
@@ -414,9 +414,36 @@ GLSLSystemValueInfo* getGLSLSystemValueInfo(
     {
         // uint in hlsl, int in glsl
         // https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/gl_PrimitiveID.xhtml
-        name = "gl_PrimitiveID";
-
         requiredType = builder->getBasicType(BaseType::Int);
+
+        switch( context->getStage() )
+        {
+        default:
+            name = "gl_PrimitiveID";
+            break;
+
+        case Stage::Geometry:
+            // GLSL makes a confusing design choice here.
+            //
+            // All the non-GS stages use `gl_PrimitiveID` to access
+            // the *input* primitive ID, but a GS uses `gl_PrimitiveID`
+            // to acces an *output* primitive ID (that will be passed
+            // along to the fragment shader).
+            //
+            // For a GS to get an input primitive ID (the thing that
+            // other stages access with `gl_PrimitiveID`), the
+            // programmer must write `gl_PrimitiveIDIn`.
+            //
+            if( kind == LayoutResourceKind::VaryingInput )
+            {
+                name = "gl_PrimitiveIDIn";
+            }
+            else
+            {
+                name = "gl_PrimitiveID";
+            }
+            break;
+        }
     }
     else if (semanticName == "sv_rendertargetarrayindex")
     {
diff --git a/tests/cross-compile/geometry-shader.slang b/tests/cross-compile/geometry-shader.slang
index 614d71db9..8666b739a 100644
--- a/tests/cross-compile/geometry-shader.slang
+++ b/tests/cross-compile/geometry-shader.slang
@@ -19,7 +19,8 @@ struct RasterVertex
 [maxvertexcount(3)]
 void main(
     triangle CoarseVertex coarseVertices[3],
-    inout TriangleStream<RasterVertex> outputStream)
+    inout TriangleStream<RasterVertex> outputStream,
+    uint primitiveID : SV_PrimitiveID)
 {
     for(int ii = 0; ii < 3; ++ii)
     {
@@ -27,7 +28,7 @@ void main(
         RasterVertex rasterVertex;
         rasterVertex.position   = coarseVertex.position;
         rasterVertex.color      = coarseVertex.color;
-        rasterVertex.id         = coarseVertex.id;
+        rasterVertex.id         = coarseVertex.id + primitiveID;
         outputStream.Append(rasterVertex);
     }
 }
diff --git a/tests/cross-compile/geometry-shader.slang.glsl b/tests/cross-compile/geometry-shader.slang.glsl
index cdd0f94de..2dd9d35bc 100644
--- a/tests/cross-compile/geometry-shader.slang.glsl
+++ b/tests/cross-compile/geometry-shader.slang.glsl
@@ -52,13 +52,15 @@ void main()
 {
     int ii_0;
 
+    uint _S6 = uint(gl_PrimitiveIDIn);
+
     // TODO: Having to make this copy to transpose things is unfortunate.
     //
     // The front-end should be able to generate code using aggregate
     // types for the input, and/or eliminate the redundant temporary
     // by indexing directly into the sub-arrays.
     //
-    CoarseVertex_0  _S6[3] = {
+    CoarseVertex_0  _S7[3] = {
         CoarseVertex_0(input_position[0], input_color[0], input_id[0]),
         CoarseVertex_0(input_position[1], input_color[1], input_id[1]),
         CoarseVertex_0(input_position[2], input_color[2], input_id[2])
@@ -74,18 +76,18 @@ void main()
             break;
         }
 
-        CoarseVertex_0 coarseVertex_0 = _S6[ii_0];
+        CoarseVertex_0 coarseVertex_0 = _S7[ii_0];
 
         RasterVertex_0 rasterVertex_0;
         rasterVertex_0.position_0 = coarseVertex_0.position_1;
         rasterVertex_0.color_0 = coarseVertex_0.color_1;
-        rasterVertex_0.id_0 = coarseVertex_0.id_1;
+        rasterVertex_0.id_0 = coarseVertex_0.id_1 + _S6;
 
-        RasterVertex_0 _S7 = rasterVertex_0;
+        RasterVertex_0 _S8 = rasterVertex_0;
 
-        output_position = _S7.position_0;
-        output_color = _S7.color_0;
-        gl_Layer = int(_S7.id_0);
+        output_position = _S8.position_0;
+        output_color = _S8.color_0;
+        gl_Layer = int(_S8.id_0);
 
         EmitVertex();