[Mesa-dev] [PATCH] glsl: Add an optimization pass to flatten simple nested if blocks.

[Eric Anholt]

Kenneth Graunke <kenneth at whitecape.org> writes:

> On 04/04/2013 09:08 AM, Eric Anholt wrote:
>> Kenneth Graunke <kenneth at whitecape.org> writes:
>>> diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
>>> index c294aa4..b5282a6 100644
>>> --- a/src/glsl/Makefile.sources
>>> +++ b/src/glsl/Makefile.sources
>>> @@ -80,6 +80,7 @@ LIBGLSL_FILES = \
>>>   	$(GLSL_SRCDIR)/opt_dead_code.cpp \
>>>   	$(GLSL_SRCDIR)/opt_dead_code_local.cpp \
>>>   	$(GLSL_SRCDIR)/opt_dead_functions.cpp \
>>> +	$(GLSL_SRCDIR)/opt_flatten_nested_if_blocks.cpp \
>>>   	$(GLSL_SRCDIR)/opt_function_inlining.cpp \
>>>   	$(GLSL_SRCDIR)/opt_if_simplification.cpp \
>>>   	$(GLSL_SRCDIR)/opt_noop_swizzle.cpp \
>>> diff --git a/src/glsl/glsl_parser_extras.cpp b/src/glsl/glsl_parser_extras.cpp
>>> index 9740903..0992294 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)
>>> diff --git a/src/glsl/ir_optimization.h b/src/glsl/ir_optimization.h
>>> index 2454bbe..a8885d7 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);
>>> diff --git a/src/glsl/opt_flatten_nested_if_blocks.cpp b/src/glsl/opt_flatten_nested_if_blocks.cpp
>>> new file mode 100644
>>> index 0000000..c702102
>>> --- /dev/null
>>> +++ b/src/glsl/opt_flatten_nested_if_blocks.cpp
>>> @@ -0,0 +1,103 @@
>>> +/*
>>> + * Copyright © 2013 Intel Corporation
>>> + *
>>> + * Permission is hereby granted, free of charge, to any person obtaining a
>>> + * copy of this software and associated documentation files (the "Software"),
>>> + * to deal in the Software without restriction, including without limitation
>>> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
>>> + * and/or sell copies of the Software, and to permit persons to whom the
>>> + * Software is furnished to do so, subject to the following conditions:
>>> + *
>>> + * The above copyright notice and this permission notice (including the next
>>> + * paragraph) shall be included in all copies or substantial portions of the
>>> + * Software.
>>> + *
>>> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
>>> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
>>> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
>>> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
>>> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
>>> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
>>> + * DEALINGS IN THE SOFTWARE.
>>> + */
>>> +
>>> +/**
>>> + * \file opt_flatten_nested_if_blocks.cpp
>>> + *
>>> + * Flattens nested if blocks such as:
>>> + *
>>> + * if (x) {
>>> + *    if (y) {
>>> + *       ...
>>> + *    }
>>> + * }
>>> + *
>>> + * into a single if block with a combined condition:
>>> + *
>>> + * if (x && y) {
>>> + *    ...
>>> + * }
>>> + */
>>> +
>>> +#include "ir.h"
>>> +#include "ir_builder.h"
>>> +
>>> +using namespace ir_builder;
>>> +
>>> +namespace {
>>> +
>>> +class nested_if_flattener : public ir_hierarchical_visitor {
>>> +public:
>>> +   nested_if_flattener()
>>> +   {
>>> +      progress = false;
>>> +   }
>>> +
>>> +   ir_visitor_status visit_leave(ir_if *);
>>> +   ir_visitor_status visit_enter(ir_assignment *);
>>> +
>>> +   bool progress;
>>> +};
>>> +
>>> +} /* unnamed namespace */
>>> +
>>> +/* We only care about the top level "if" instructions, so don't
>>> + * descend into expressions.
>>> + */
>>> +ir_visitor_status
>>> +nested_if_flattener::visit_enter(ir_assignment *ir)
>>> +{
>>> +   (void) ir;
>>> +   return visit_continue_with_parent;
>>> +}
>>> +
>>> +bool
>>> +opt_flatten_nested_if_blocks(exec_list *instructions)
>>> +{
>>> +   nested_if_flattener v;
>>> +
>>> +   v.run(instructions);
>>> +   return v.progress;
>>> +}
>>> +
>>> +
>>> +ir_visitor_status
>>> +nested_if_flattener::visit_leave(ir_if *ir)
>>> +{
>>> +   /* Only handle a single ir_if within the then clause of an ir_if.  No extra
>>> +    * instructions, no else clauses, nothing.
>>> +    */
>>> +   if (ir->then_instructions.is_empty() || !ir->else_instructions.is_empty())
>>> +      return visit_continue;
>>> +
>>> +   ir_if *inner = ((ir_instruction *) ir->then_instructions.head)->as_if();
>>> +   if (!inner || !inner->next->is_tail_sentinel() ||
>>> +       !inner->else_instructions.is_empty())
>>> +      return visit_continue;
>>> +
>>> +   ir->condition = logic_and(ir->condition, inner->condition);
>>> +   inner->then_instructions.move_nodes_to(&ir->then_instructions);
>>> +
>>
>> Also inner->remove() right now instead of leaving it up to the
>> empty-if-statement-remover?  Equivalent functionality, but may take one
>> less turn through the giant loop.
>
> It's not actually necessary.  move_nodes_to throws away all nodes in the 
> target list, effectively doing the remove for us.

that makes inner->then_instructions empty, but it still leaves inner in
the outer's instruction list.
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