Branched Path Tracing Sample all Direct not working with Volumetrics on GPU?

[GottfriedHofmann]

Hello Cycles devs,

I recently noticed that “Sample all Direct” in branched path tracing seems not to work with volumetrics, neither homogeneous nor heterogeneous.
In scenes with 0 volume bounces contributions from lights sources visible by shadow rays are the norm so sampling several light sources at once is attractive.
Here is an (extreme) example scene, try rendering with CPU and GPU: https://www.dropbox.com/s/8qrjcn25db3bvqs/branched_path_tracing_example_01.blend?dl=1

Left: GPU Right: CPU

Has this simply not been implemented yet on GPU or are there technical reasons it’s not supported?

[brecht]

Volume rendering on the CPU has much better sampling, this is a known limitation currently that we would like to fix. The algorithm we use currently can not work well on the GPU.