[Mesa-dev] [PATCH 6/6] glsl: Don't constant propagate arrays.

[Kenneth Graunke]

Constant propagation on arrays doesn't make a lot of sense.  If the
array is only accessed with constant indexes, then opt_array_splitting
would split it up.  Otherwise, we have variable indexing.  If there's
multiple accesses, then constant propagation would end up replicating
the data.

The lower_const_arrays_to_uniforms pass creates uniforms for each
ir_constant with array type that it encounters.  This means that it
creates redundant uniforms for each copy of the constant, which means
uploading too much data.  It can even mean exceeding the maximum number
of uniform components, causing link failures.

We could try and teach the pass to de-duplicate the data by hashing
constants, but it makes more sense to avoid duplicating it in the first
place.  We should promote constant arrays to uniforms, then propagate
the uniform access.

Fixes the TressFX shaders from Tomb Raider, which exceeded the maximum
number of uniform components by a huge margin and failed to link.

On Broadwell:

total instructions in shared programs: 9067702 -> 9068202 (0.01%)
instructions in affected programs: 10335 -> 10835 (4.84%)
helped: 10 (Hoard, Shadow of Mordor, Amnesia: The Dark Descent)
HURT: 20 (Natural Selection 2)

loops in affected programs: 4 -> 0

The hurt programs appear to no longer have a constarray uniform, as
all constants were successfully propagated.  Apparently before this
patch, we successfully unrolled a loop containing array access, but
only after promoting constant arrays to uniforms.  With this patch,
we unroll it first, so all array access is direct, and the array
is split up, and individual constants are propagated.  This seems
better.

Cc: mesa-stable at lists.freedesktop.org
Reported-by: Karol Herbst <nouveau at karolherbst.de>
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/compiler/glsl/opt_constant_propagation.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/glsl/opt_constant_propagation.cpp b/src/compiler/glsl/opt_constant_propagation.cpp
index 6ec4ab4..69bca74 100644
--- a/src/compiler/glsl/opt_constant_propagation.cpp
+++ b/src/compiler/glsl/opt_constant_propagation.cpp
@@ -145,7 +145,7 @@ ir_constant_propagation_visitor::constant_folding(ir_rvalue **rvalue)
       this->progress = true;
 
    ir_dereference_variable *var_ref = (*rvalue)->as_dereference_variable();
-   if (var_ref) {
+   if (var_ref && !var_ref->type->is_array()) {
       ir_constant *constant = var_ref->constant_expression_value();
       if (constant) {
          *rvalue = constant;
-- 
2.9.0