JSSpecCompiler: Add bare-bones DCE pass

Right now the only dead code it eliminates is the unused phi nodes.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Compiler/Passes/DeadCodeEliminationPass.cpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Compiler/Passes/DeadCodeEliminationPass.h"
+#include "AST/AST.h"
+#include "Compiler/ControlFlowGraph.h"
+#include "Compiler/StronglyConnectedComponents.h"
+#include "Function.h"
+
+namespace JSSpecCompiler {
+
+void DeadCodeEliminationPass::process_function()
+{
+    with_graph(m_function->m_local_ssa_variables.size(), [&] {
+        remove_unused_phi_nodes();
+    });
+    m_function->reindex_ssa_variables();
+}
+
+DeadCodeEliminationPass::Vertex DeadCodeEliminationPass::as_vertex(Variable* variable)
+{
+    return Vertex(variable->m_ssa);
+}
+
+RecursionDecision DeadCodeEliminationPass::on_entry(Tree tree)
+{
+    if (tree->is_statement())
+        TODO();
+    return RecursionDecision::Recurse;
+}
+
+void DeadCodeEliminationPass::on_leave(Tree tree)
+{
+    if (auto variable = as<Variable>(tree); variable)
+        as_vertex(variable)->is_referenced = true;
+}
+
+void DeadCodeEliminationPass::remove_unused_phi_nodes()
+{
+    for (auto const& block : m_function->m_cfg->blocks) {
+        for (auto const& phi_node : block->m_phi_nodes) {
+            auto to = as_vertex(phi_node.var);
+            for (auto const& branch : phi_node.branches) {
+                auto from = as_vertex(branch.value);
+
+                from->outgoing_edges.append(to);
+                to->incoming_edges.append(from);
+            }
+        }
+
+        for (auto& expr : block->m_expressions)
+            run_in_subtree(expr);
+        run_in_const_subtree(block->m_continuation);
+    }
+
+    // FIXME?: There surely must be a way to do this in a linear time without finding strongly
+    //         connected components.
+    for (auto const& component : find_strongly_connected_components(m_nodes)) {
+        bool is_referenced = false;
+
+        for (Vertex u : component)
+            for (Vertex v : u->outgoing_edges)
+                is_referenced |= v->is_referenced;
+
+        if (is_referenced)
+            for (Vertex u : component)
+                u->is_referenced = true;
+    }
+
+    for (auto const& block : m_function->m_cfg->blocks) {
+        block->m_phi_nodes.remove_all_matching([&](auto const& node) {
+            return !as_vertex(node.var)->is_referenced;
+        });
+    }
+
+    m_function->m_local_ssa_variables.remove_all_matching([&](auto const& variable) {
+        return !Vertex(variable)->is_referenced;
+    });
+}
+
+}