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JSSpecCompiler: Add bare-bones DCE pass
Right now the only dead code it eliminates is the unused phi nodes.
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/*
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* Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include "Compiler/Passes/DeadCodeEliminationPass.h"
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#include "AST/AST.h"
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#include "Compiler/ControlFlowGraph.h"
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#include "Compiler/StronglyConnectedComponents.h"
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#include "Function.h"
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namespace JSSpecCompiler {
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void DeadCodeEliminationPass::process_function()
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{
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with_graph(m_function->m_local_ssa_variables.size(), [&] {
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remove_unused_phi_nodes();
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});
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m_function->reindex_ssa_variables();
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}
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DeadCodeEliminationPass::Vertex DeadCodeEliminationPass::as_vertex(Variable* variable)
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{
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return Vertex(variable->m_ssa);
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}
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RecursionDecision DeadCodeEliminationPass::on_entry(Tree tree)
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{
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if (tree->is_statement())
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TODO();
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return RecursionDecision::Recurse;
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}
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void DeadCodeEliminationPass::on_leave(Tree tree)
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{
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if (auto variable = as<Variable>(tree); variable)
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as_vertex(variable)->is_referenced = true;
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}
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void DeadCodeEliminationPass::remove_unused_phi_nodes()
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{
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for (auto const& block : m_function->m_cfg->blocks) {
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for (auto const& phi_node : block->m_phi_nodes) {
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auto to = as_vertex(phi_node.var);
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for (auto const& branch : phi_node.branches) {
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auto from = as_vertex(branch.value);
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from->outgoing_edges.append(to);
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to->incoming_edges.append(from);
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}
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}
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for (auto& expr : block->m_expressions)
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run_in_subtree(expr);
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run_in_const_subtree(block->m_continuation);
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}
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// FIXME?: There surely must be a way to do this in a linear time without finding strongly
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// connected components.
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for (auto const& component : find_strongly_connected_components(m_nodes)) {
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bool is_referenced = false;
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for (Vertex u : component)
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for (Vertex v : u->outgoing_edges)
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is_referenced |= v->is_referenced;
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if (is_referenced)
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for (Vertex u : component)
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u->is_referenced = true;
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}
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for (auto const& block : m_function->m_cfg->blocks) {
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block->m_phi_nodes.remove_all_matching([&](auto const& node) {
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return !as_vertex(node.var)->is_referenced;
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});
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}
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m_function->m_local_ssa_variables.remove_all_matching([&](auto const& variable) {
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return !Vertex(variable)->is_referenced;
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});
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}
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}
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