feat(furc): start working on ssa optimizations

Compute dominance frontiers.

Refs: #2

furc/include/furc/front/ssa.hpp

@@ -0,0 +1,32 @@
+#ifndef FURC_FRONT_SSA_HPP
+#define FURC_FRONT_SSA_HPP
+
+#include "furlang/ir/function.hpp"
+#include "furlang/ir/instruction.hpp"
+#include "furlang/ir/module.hpp"
+
+#include <memory>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+namespace furc {
+namespace front {
+
+class ssa {
+    using block_map_t = std::unordered_map<furlang::ir::block_index, std::vector<furlang::ir::block_index>>;
+public:
+    static void optimize(furlang::ir::module& mod);
+private:
+    static void optimize(const std::unique_ptr<furlang::ir::function>& func);
+
+    static void dfs_rpo(furlang::ir::block_index      block,
+        const block_map_t&                            successors,
+        std::unordered_set<furlang::ir::block_index>& visited,
+        std::vector<furlang::ir::block_index>&        rpo);
+};
+
+} // namespace front
+} // namespace furc
+
+#endif // FURC_FRONT_SSA_HPP

furc/src/front/ssa.cpp

@@ -0,0 +1,129 @@
+#include "furc/front/ssa.hpp"
+
+#include "furlang/ir/instruction.hpp"
+
+#include <algorithm>
+#include <cstddef>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+namespace furc::front {
+
+void ssa::optimize(furlang::ir::module& mod) {
+    for (const auto& func : mod.functions()) {
+        ssa::optimize(func);
+    }
+}
+
+void ssa::optimize(const std::unique_ptr<furlang::ir::function>& func) {
+    block_map_t predecessors;
+    block_map_t successors;
+
+    std::unordered_map<furlang::ir::block_index, furlang::ir::block_index> idoms;
+
+    for (furlang::ir::block_index i = 0; i < func->blocks().size(); ++i) {
+        const auto& block = func->blocks()[i];
+        const auto& exit  = block->exit();
+        switch (exit->type()) {
+        case furlang::ir::instruction_t::Branch: {
+            const auto& br = reinterpret_cast<const furlang::ir::branch_instruction&>(*exit);
+            predecessors[br.block()].push_back(i);
+            successors[i].push_back(br.block());
+        } break;
+        case furlang::ir::instruction_t::BranchCond: {
+            const auto& br = reinterpret_cast<const furlang::ir::branch_cond_instruction&>(*exit);
+            predecessors[br.if_block()].push_back(i);
+            predecessors[br.else_block()].push_back(i);
+            successors[i].push_back(br.if_block());
+            successors[i].push_back(br.else_block());
+        } break;
+        default: break;
+        }
+    }
+
+    std::unordered_set<furlang::ir::block_index> visited;
+    std::vector<furlang::ir::block_index>        rpoOrder;
+    dfs_rpo(0, successors, visited, rpoOrder);
+    std::reverse(rpoOrder.begin(), rpoOrder.end());
+
+    std::unordered_map<furlang::ir::block_index, std::size_t> rpoIndex;
+    for (std::size_t i = 0; i < rpoOrder.size(); ++i) {
+        rpoIndex[rpoOrder[i]] = i;
+    }
+
+    auto intersect = [&](furlang::ir::block_index block1, furlang::ir::block_index block2) {
+        while (block1 != block2) {
+            while (rpoIndex[block1] > rpoIndex[block2]) {
+                block1 = idoms[block1];
+            }
+            while (rpoIndex[block2] > rpoIndex[block1]) {
+                block2 = idoms[block2];
+            }
+        }
+        return block1;
+    };
+
+    auto entry   = rpoOrder.front();
+    idoms[entry] = entry;
+
+    bool changed = true;
+    while (changed) {
+        changed = false;
+
+        for (std::size_t i = 1; i < rpoOrder.size(); ++i) {
+            auto block = rpoOrder[i];
+
+            furlang::ir::block_index newIdom = 0;
+            bool                     found   = false;
+            for (auto pred : predecessors[block]) {
+                if (idoms.find(pred) == idoms.end()) continue;
+                if (!found) {
+                    newIdom = pred;
+                    found   = true;
+                } else {
+                    newIdom = intersect(pred, newIdom);
+                }
+            }
+
+            if (idoms.find(block) == idoms.end() || idoms[block] != newIdom) {
+                idoms[block] = newIdom;
+                changed      = true;
+            }
+        }
+    }
+
+    std::unordered_map<furlang::ir::block_index, std::unordered_set<furlang::ir::block_index>> df;
+    for (auto block : rpoOrder) {
+        df[block] = std::unordered_set<furlang::ir::block_index>{};
+    }
+
+    for (auto block : rpoOrder) {
+        if (predecessors[block].size() < 2) continue;
+
+        for (auto pred : predecessors[block]) {
+            auto runner = pred;
+            while (runner != idoms[block]) {
+                df[runner].insert(block);
+                runner = idoms[runner];
+            }
+        }
+    }
+}
+
+void ssa::dfs_rpo(furlang::ir::block_index        block,
+    const block_map_t&                            successors,
+    std::unordered_set<furlang::ir::block_index>& visited,
+    std::vector<furlang::ir::block_index>&        rpo) {
+    visited.insert(block);
+    if (auto it = successors.find(block); it != successors.end()) {
+        for (auto successor : it->second) {
+            if (visited.find(successor) == visited.end()) {
+                dfs_rpo(successor, successors, visited, rpo);
+            }
+        }
+    }
+    rpo.push_back(block);
+}
+
+} // namespace furc::front

furc/src/main.cpp

@@ -3,6 +3,7 @@
 #include "furc/ast/program.hpp"
 #include "furc/front/ir_generator.hpp"
 #include "furc/front/parser.hpp"
+#include "furc/front/ssa.hpp"
 
 #include <iostream>
 
@@ -34,6 +35,8 @@ int main(void) {
         program->accept(generator);
 
         auto module = std::move(generator.move_module());
+        furc::front::ssa::optimize(module);
+
         std::cout << "Generated IR:\n";
         for (const auto& function : module.functions()) {
             std::cout << function->name() << ":\n";