+/**
+ * When the GS is not in use, we assign the entire URB space to the VS. When
+ * the GS is in use, we split the URB space evenly between the VS and the GS.
+ * This is not ideal, but it's simple.
+ *
+ * URB size / 2 URB size / 2
+ * _____________-______________ _____________-______________
+ * / \ / \
+ * +-------------------------------------------------------------+
+ * | Vertex Shader Entries | Geometry Shader Entries |
+ * +-------------------------------------------------------------+
+ *
+ * Sandybridge GT1 has 32kB of URB space, while GT2 has 64kB.
+ * (See the Sandybridge PRM, Volume 2, Part 1, Section 1.4.7: 3DSTATE_URB.)
+ */