SPIRV-Cross/spirv_cfg.cpp

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/*
2017-01-28 08:00:40 +00:00
* Copyright 2016-2017 ARM Limited
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "spirv_cfg.hpp"
#include "spirv_cross.hpp"
#include <algorithm>
#include <assert.h>
using namespace std;
namespace spirv_cross
{
CFG::CFG(Compiler &compiler_, const SPIRFunction &func_)
: compiler(compiler_)
, func(func_)
{
preceding_edges.resize(compiler.get_current_id_bound());
succeeding_edges.resize(compiler.get_current_id_bound());
visit_order.resize(compiler.get_current_id_bound());
immediate_dominators.resize(compiler.get_current_id_bound());
build_post_order_visit_order();
build_immediate_dominators();
}
uint32_t CFG::find_common_dominator(uint32_t a, uint32_t b) const
{
while (a != b)
{
if (visit_order[a] < visit_order[b])
a = immediate_dominators[a];
else
b = immediate_dominators[b];
}
return a;
}
uint32_t CFG::update_common_dominator(uint32_t a, uint32_t b)
{
auto dominator = find_common_dominator(immediate_dominators[a], immediate_dominators[b]);
immediate_dominators[a] = dominator;
immediate_dominators[b] = dominator;
return dominator;
}
void CFG::build_immediate_dominators()
{
// Traverse the post-order in reverse and build up the immediate dominator tree.
fill(begin(immediate_dominators), end(immediate_dominators), 0);
immediate_dominators[func.entry_block] = func.entry_block;
for (auto i = post_order.size(); i; i--)
{
uint32_t block = post_order[i - 1];
auto &pred = preceding_edges[block];
if (pred.empty()) // This is for the entry block, but we've already set up the dominators.
continue;
for (auto &edge : pred)
{
if (immediate_dominators[block])
{
assert(immediate_dominators[edge]);
immediate_dominators[block] = update_common_dominator(block, edge);
}
else
immediate_dominators[block] = edge;
}
}
}
bool CFG::is_back_edge(uint32_t to) const
{
// We have a back edge if the visit order is set with the temporary magic value 0.
// Crossing edges will have already been recorded with a visit order.
return visit_order[to] == 0;
}
bool CFG::post_order_visit(uint32_t block_id)
{
// If we have already branched to this block (back edge), stop recursion.
// If our branches are back-edges, we do not record them.
// We have to record crossing edges however.
if (visit_order[block_id] >= 0)
return !is_back_edge(block_id);
// Block back-edges from recursively revisiting ourselves.
visit_order[block_id] = 0;
// First visit our branch targets.
auto &block = compiler.get<SPIRBlock>(block_id);
switch (block.terminator)
{
case SPIRBlock::Direct:
if (post_order_visit(block.next_block))
add_branch(block_id, block.next_block);
break;
case SPIRBlock::Select:
if (post_order_visit(block.true_block))
add_branch(block_id, block.true_block);
if (post_order_visit(block.false_block))
add_branch(block_id, block.false_block);
break;
case SPIRBlock::MultiSelect:
for (auto &target : block.cases)
{
if (post_order_visit(target.block))
add_branch(block_id, target.block);
}
if (block.default_block && post_order_visit(block.default_block))
add_branch(block_id, block.default_block);
break;
default:
break;
}
// If this is a loop header, add an implied branch to the merge target.
// This is needed to avoid annoying cases with do { ... } while(false) loops often generated by inliners.
// To the CFG, this is linear control flow, but we risk picking the do/while scope as our dominating block.
// This makes sure that if we are accessing a variable outside the do/while, we choose the loop header as dominator.
if (block.merge == SPIRBlock::MergeLoop)
add_branch(block_id, block.merge_block);
// Then visit ourselves. Start counting at one, to let 0 be a magic value for testing back vs. crossing edges.
visit_order[block_id] = ++visit_count;
post_order.push_back(block_id);
return true;
}
void CFG::build_post_order_visit_order()
{
uint32_t block = func.entry_block;
visit_count = 0;
fill(begin(visit_order), end(visit_order), -1);
post_order.clear();
post_order_visit(block);
}
void CFG::add_branch(uint32_t from, uint32_t to)
{
const auto add_unique = [](vector<uint32_t> &l, uint32_t value) {
auto itr = find(begin(l), end(l), value);
if (itr == end(l))
l.push_back(value);
};
add_unique(preceding_edges[to], from);
add_unique(succeeding_edges[from], to);
}
DominatorBuilder::DominatorBuilder(const CFG &cfg_)
: cfg(cfg_)
{
}
void DominatorBuilder::add_block(uint32_t block)
{
if (!cfg.get_immediate_dominator(block))
{
// Unreachable block via the CFG, we will never emit this code anyways.
return;
}
if (!dominator)
{
dominator = block;
return;
}
if (block != dominator)
dominator = cfg.find_common_dominator(block, dominator);
}
void DominatorBuilder::lift_continue_block_dominator()
{
// It is possible for a continue block to be the dominator of a variable is only accessed inside the while block of a do-while loop.
// We cannot safely declare variables inside a continue block, so move any variable declared
// in a continue block to the entry block to simplify.
// It makes very little sense for a continue block to ever be a dominator, so fall back to the simplest
// solution.
if (!dominator)
return;
auto &block = cfg.get_compiler().get<SPIRBlock>(dominator);
auto post_order = cfg.get_visit_order(dominator);
// If we are branching to a block with a higher post-order traversal index (continue blocks), we have a problem
// since we cannot create sensible GLSL code for this, fallback to entry block.
bool back_edge_dominator = false;
switch (block.terminator)
{
case SPIRBlock::Direct:
if (cfg.get_visit_order(block.next_block) > post_order)
back_edge_dominator = true;
break;
case SPIRBlock::Select:
if (cfg.get_visit_order(block.true_block) > post_order)
back_edge_dominator = true;
if (cfg.get_visit_order(block.false_block) > post_order)
back_edge_dominator = true;
break;
case SPIRBlock::MultiSelect:
for (auto &target : block.cases)
{
if (cfg.get_visit_order(target.block) > post_order)
back_edge_dominator = true;
}
if (block.default_block && cfg.get_visit_order(block.default_block) > post_order)
back_edge_dominator = true;
break;
default:
break;
}
if (back_edge_dominator)
dominator = cfg.get_function().entry_block;
}
}