[Mesa-dev] memoryBarrier + SSBO

[Ilia Mirkin]

Hi Ian (and other spec experts),

The ARB_ssbo spec mentions the following:

    OpenGL 4.0 (either core or compatibility profile) is required.

...

    Additionally, the shading language provides the memoryBarrier() function
    to control the relative order of memory accesses within individual shader
    invocations and provides various memory qualifiers controlling how the
    memory corresponding to individual variables is accessed.

However the memoryBarrier() function only becomes available in GLSL
4.20 [along with the glMemoryBarrier() function that the spec also
refers to] or with ARB_shader_image_load_store.

Is the implication that such functionality should be auto-exposed in
image-less drivers that support ssbo? Or that this functionality
should just not exist unless images are supported?

This is relevant to me as I plan on adding ssbo support to gallium
before images. Also, is there really anything in there that would
prevent this from being exposed on a GL 3.1 driver (I'm thinking of
freedreno which doesn't support GS yet and is my initial target for
this, since I can make atomics actually work on there, unlike NVIDIA
where the damn things just won't budge without some magic
yet-to-be-found make-it-work bit).

Cheers,

  -ilia