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/ir_optimization.h b/src/glsl/ir_optimization.h
index 2454bbe6f66..a8885d7225b 100644
--- a/src/glsl/ir_optimization.h
+++ b/src/glsl/ir_optimization.h
@@ -82,6 +82,7 @@ bool do_function_inlining(exec_list *instructions);
 bool do_lower_jumps(exec_list *instructions, bool pull_out_jumps = true, bool lower_sub_return = true, bool lower_main_return = false, bool lower_continue = false, bool lower_break = false);
 bool do_lower_texture_projection(exec_list *instructions);
 bool do_if_simplification(exec_list *instructions);
+bool opt_flatten_nested_if_blocks(exec_list *instructions);
 bool do_discard_simplification(exec_list *instructions);
 bool lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth = 0);
 bool do_mat_op_to_vec(exec_list *instructions);