[Bug 89597] Implement SSBOs in GLSL front-end and i965

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89597

--- Comment #60 from Samuel Iglesias <siglesias at igalia.com> ---
Francisco, I have a question for you related to untyped write messages source
code:

Today, I have been rebasing our SSBO patch series to master, finding that FS
builder was already pushed upstream and a lot of code was migrated to use it. I
found a crash that is related to one of the patches we picked from you some
time ago that emits untyped write messages. This is the problem:

When processing nir_intrinsic_store_ssbo in brw_fs_nir.cpp, there is a call to
surface_access::emit_untyped_write() which ends up internally calling to
emit_header().

The function emit_header() (in
src/mesa/drivers/dri/i965/brw_fs_surface_builder.cpp)
copies the sample mask to the header register:

       exec_all(ubld.MOV(component(dst, 7), sample_mask));

This MOV fails the following assert in function emit(instruction *inst) at
src/mesa/drivers/dri/i965/brw_fs_builder.h:

    assert(inst->exec_size == dispatch_width() || force_writemask_all);

According to GDB, those variables have the following values:

    * inst->exec_size = 1
    * dispatch_width() = 8
    * force_writemask_all = 0

Removing this assert makes the code to work properly but I wonder if we need to
update our code in some way or if you have already a fix for this.

If you want to test it, I have published a branch which includes the assert
deletion in the last commit:

$ git clone -b wip/siglesias/ARB_shader_storage_buffer_object-v2.3
https://github.com/Igalia/mesa.git

You can run the following piglit test to reproduce the crash, if you revert
last commit:

$ bin/arb_shader_storage_buffer_object-layout-std140-write-shader -fbo -auto

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