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.

1 files changed, 29 insertions, 2 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")
     {