[Mesa-dev] [PATCH 0/4] i965: skip control-flow aware liveness analysis if we only have 1 block

[Iago Toral Quiroga]

This fixes the following test:

[require]
GL >= 3.3
GLSL >= 3.30
GL_ARB_shader_storage_buffer_object

[fragment shader]
#version 330
#extension GL_ARB_shader_storage_buffer_object: require

buffer SSBO {
    mat4 sm4;
};


mat4 um4;

void main() {
    sm4 *= um4;
    sm4[0][0] = 0.0;
}

[test]
link success

where our liveness analysis works really bad because the control-flow part
will expand register liveness to the end of only block we have (so every
register will be marked alive until the end of the program). This artificially
increases register pressure to a point in which we run out of registers.
Of course, this is only a simple optimization for a trivial case, the
underlying problem still exists and would manifest when we have more than
one block, but it helps simple shaders such as the one in the example above
without any effort. I guess the real fix would involve re-thinking parts of the
liveness analysis algorithm...

Jordan, there is another thing that we can improve for this shader that is
specific to SSBOs: we emit the same ssbo load multiple times because we are
playing it safe just in case there are writes in between. I think we can try to
do better and not re-issue the same load if we don't have ssbo stores to
the same address in between. I'll try to come up with a patch for this
(hopefully we can do this inside lower_ubo_reference).

The actual fix goes into patches 2 (FS) and 4 (VS). Patches 1,3 are just
indentation fixes in the code around these.

Iago Toral Quiroga (4):
  i965/fs: Fix indentation in fs_live_variables::compute_start_end
  i965/fs: skip control-flow aware liveness analysis if we only have 1
    block
  i965/vec4: fix indentation in vec4_visitor::calculate_live_intervals
  i965/vec4: skip control-flow aware liveness analysis if we only have 1
    block

 .../drivers/dri/i965/brw_fs_live_variables.cpp     | 24 ++++++++++++++--------
 .../drivers/dri/i965/brw_vec4_live_variables.cpp   | 23 +++++++++++++--------
 2 files changed, 30 insertions(+), 17 deletions(-)

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1.9.1