Spawned from this bug report. The availability of builtins to gets the index of the current instance and vertex in the vertex shader vary between APIs.

D3D12

• SV_InstanceID: starts from 0, irrespective of the firstInstance in Draw / DrawIndexed
• SV_VertexID: starts from 0, irrespective of the firstVertex in Draw and baseVertex in DrawIndexed

@RafaelCintron the docs aren't super clear, can you check for SV_VertexID? (edit: confirmed that it starts from 0)

Metal (section "5.2.3.1 Vertex Function Input Attributes" of the MSL 2.3 "spec")

• [[base_instance]]: the firstInstance in DrawIndexed and Draw
• [[base_vertex]]: the firstVertex in Draw and baseVertex in DrawIndexed
• [[instance_id]]: starts from [[base_instance]]
• [[vertex_id]]: starts from [[base_vertex]]

Vulkan

• BaseInstance: the firstInstance in DrawIndexed and Draw, requires the VK_KHR_shader_draw_parameters extension / feature
• BaseVertex: the firstVertex in Draw and baseVertex in DrawIndexed, requires the VK_KHR_shader_draw_parameters extension / feature
• InstanceIndex: starts from [[base_instance]]
• VertexIndex: starts from [[base_vertex]]

OpenGL

• gl_VertexID: starts from baseVertex/firstVertex.
• gl_InstanceID: starts from 0.
• gl_BaseVertex / gl_BaseInstance: require GL_ARB_shader_draw_parameters or OpenGL 4.6

Conclusion

It's a mess and we're heavily limited by what D3D12 does, so we don't really have a choice. We should name builtins clearly to indicate if they start at 0 or not, to avoid the gl_InstanceID vs. gl_InstanceIndex confusion.