[Mesa-dev] Seeking advice speeding up glsl optimisation passes for AoA

[Timothy Arceri]

Hi guys,

As I've mentioned a couple of times in previous patches some of the cts AoA
tests are taking very long time to compile. This is due to excessive
optimisation passes mainly in the glsl optimisations (there are some slowdowns
in the intel backend too but these seemed to go away when I tried the new nir
vec4 backend).

I fixed part of the problem with this patch to do the dead code elimination in
a single pass [1]. 
These excessive passes exist in normal shaders but its generally not an issue
as the number of passes is generally quite low, and inexpensive. However when
you have an 8 dimensional array constantly walking this becomes quite
expensive.

The remaining issue I'm seeking some advice for is with constant
propagation/folding.

It seems for interators used in loops you can get into a situation where an
optimisation pass is needed for each loop iteration in order to make all
values of the iterator constant.

I didn't have look too find some real world examples of this in the public
shader-db. For example here is it happening for a Unity shader:

---------------------- Linker Pass 1 ----------------------
opt_function_inlining progress
opt_dead_functions progress
opt_copy_propagation_elements progress
opt_tree_grafting progress
opt_constant_folding progress
opt_algebraic progress
---------------------- Linker Pass 1 ----------------------
constant prop : l_3, Value : 0
constant prop : l_3, Value : 0
constant prop : l_3, Value : 0
constant prop : l_3, Value : 0
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 2 ----------------------
constant prop : l_3, Value : 1
constant prop : l_3, Value : 1
constant prop : l_3, Value : 1
constant prop : l_3, Value : 1
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 3 ----------------------
constant prop : l_3, Value : 2
constant prop : l_3, Value : 2
constant prop : l_3, Value : 2
constant prop : l_3, Value : 2
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 4 ----------------------
constant prop : l_3, Value : 3
constant prop : l_3, Value : 3
constant prop : l_3, Value : 3
constant prop : l_3, Value : 3
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 5 ----------------------
constant prop : l_3, Value : 4
constant prop : l_3, Value : 4
constant prop : l_3, Value : 4
constant prop : l_3, Value : 4
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 6 ----------------------
constant prop : l_3, Value : 5
constant prop : l_3, Value : 5
constant prop : l_3, Value : 5
constant prop : l_3, Value : 5
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 7 ----------------------
constant prop : l_3, Value : 6
constant prop : l_3, Value : 6
constant prop : l_3, Value : 6
constant prop : l_3, Value : 6
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 8 ----------------------
constant prop : l_3, Value : 7
constant prop : l_3, Value : 7
constant prop : l_3, Value : 7
constant prop : l_3, Value : 7
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 9 ----------------------
constant prop : l_3, Value : 8
constant prop : l_3, Value : 8
constant prop : l_3, Value : 8
constant prop : l_3, Value : 8
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 10 ----------------------
constant prop : l_3, Value : 9
constant prop : l_3, Value : 9
constant prop : l_3, Value : 9
constant prop : l_3, Value : 9
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 11 ----------------------
constant prop : l_3, Value : 10
constant prop : l_3, Value : 10
constant prop : l_3, Value : 10
constant prop : l_3, Value : 10
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 12 ----------------------
constant prop : l_3, Value : 11
constant prop : l_3, Value : 11
constant prop : l_3, Value : 11
constant prop : l_3, Value : 11
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 13 ----------------------
constant prop : l_3, Value : 12
constant prop : l_3, Value : 12
constant prop : l_3, Value : 12
constant prop : l_3, Value : 12
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 14 ----------------------
constant prop : l_3, Value : 13
constant prop : l_3, Value : 13
constant prop : l_3, Value : 13
constant prop : l_3, Value : 13
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 15 ----------------------
constant prop : l_3, Value : 14
constant prop : l_3, Value : 14
constant prop : l_3, Value : 14
constant prop : l_3, Value : 14
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 16 ----------------------
constant prop : l_3, Value : 15
constant prop : l_3, Value : 15
constant prop : l_3, Value : 15
constant prop : l_3, Value : 15
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 17 ----------------------
constant prop : l_3, Value : 16
constant prop : l_3, Value : 16
constant prop : l_3, Value : 16
constant prop : l_3, Value : 16
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 18 ----------------------
constant prop : l_3, Value : 17
constant prop : l_3, Value : 17
constant prop : l_3, Value : 17
constant prop : l_3, Value : 17
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 19 ----------------------
constant prop : l_3, Value : 18
constant prop : l_3, Value : 18
constant prop : l_3, Value : 18
constant prop : l_3, Value : 18
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 20 ----------------------
constant prop : l_3, Value : 19
constant prop : l_3, Value : 19
constant prop : l_3, Value : 19
constant prop : l_3, Value : 19
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 21 ----------------------
constant prop : l_3, Value : 20
constant prop : l_3, Value : 20
constant prop : l_3, Value : 20
constant prop : l_3, Value : 20
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 22 ----------------------
constant prop : l_3, Value : 21
constant prop : l_3, Value : 21
constant prop : l_3, Value : 21
constant prop : l_3, Value : 21
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 23 ----------------------
constant prop : l_3, Value : 22
constant prop : l_3, Value : 22
constant prop : l_3, Value : 22
constant prop : l_3, Value : 22
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 24 ----------------------
constant prop : l_3, Value : 23
constant prop : l_3, Value : 23
constant prop : l_3, Value : 23
constant prop : l_3, Value : 23
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 25 ----------------------
constant prop : l_3, Value : 24
constant prop : l_3, Value : 24
constant prop : l_3, Value : 24
constant prop : l_3, Value : 24
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 26 ----------------------
constant prop : l_3, Value : 25
constant prop : l_3, Value : 25
constant prop : l_3, Value : 25
constant prop : l_3, Value : 25
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 27 ----------------------
constant prop : l_3, Value : 26
constant prop : l_3, Value : 26
constant prop : l_3, Value : 26
constant prop : l_3, Value : 26
opt_constant_propagation progress
opt_constant_folding progress
---------------------- Linker Pass 28 ----------------------
constant prop : l_3, Value : 27
constant prop : l_3, Value : 27
constant prop : l_3, Value : 27
constant prop : l_3, Value : 27
opt_constant_propagation progress
opt_constant_folding progress


<snip>

shaders/unity/68.shader_test - FS SIMD8 shader: 1058 inst, 0 loops, 0:0
spills:fills, Promoted 144 constants, compacted 16928 to 13008 bytes.
shaders/unity/68.shader_test - FS SIMD16 shader: 1338 inst, 0 loops, 0:0
spills:fills, Promoted 144 constants, compacted 21408 to 17488 bytes.
shaders/unity/68.shader_test - VS vec4 shader: 10 inst, 0 loops, compacted 160
to 144 bytes.


The problem I'm having is that its expensive to walk over these large arrays
and in the tests I'm working with the interator goes through 256 optimisation
passes (and then is later dead code eliminated anyway).

So question for those who have more knowledge about these optimisations then
me is what would you recommend I do as a next step? Does it make sense to try
to do more of this work in a single pass or should I just try to avoid
constantly walking the arrays somehow.

Thanks for your time,
Tim

[1] http://lists.freedesktop.org/archives/mesa-dev/2015-July/088651.html