glsl: Add an optimization pass to flatten simple nested if blocks.

GLBenchmark 2.7's shaders contain conditional blocks like:

if (x) {
    if (y) {
        ...
    }
}

where the outer conditional's then clause contains exactly one statement
(the nested if) and there are no else clauses.  This can easily be
optimized into:

if (x && y) {
    ...
}

This saves a few instructions in GLBenchmark 2.7:

    total instructions in shared programs: 11833 -> 11649 (-1.55%)
    instructions in affected programs:     8234 -> 8050 (-2.23%)

It also helps CS:GO slightly (-0.05%/-0.22%).  More importantly,
however, it simplifies the control flow graph, which could enable other
optimizations.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/glsl/glsl_parser_extras.cpp b/src/glsl/glsl_parser_extras.cpp
index 974090361b7..099229410c9 100644
--- a/src/glsl/glsl_parser_extras.cpp
+++ b/src/glsl/glsl_parser_extras.cpp
@@ -1218,6 +1218,7 @@ do_common_optimization(exec_list *ir, bool linked,
       progress = do_structure_splitting(ir) || progress;
    }
    progress = do_if_simplification(ir) || progress;
+   progress = opt_flatten_nested_if_blocks(ir) || progress;
    progress = do_copy_propagation(ir) || progress;
    progress = do_copy_propagation_elements(ir) || progress;
    if (linked)