From 865691df264788cc6b725a078f806eb11c1d2b78 Mon Sep 17 00:00:00 2001
From: CHatingPython <chatingpython@kpgpmc.org>
Date: Fri, 12 Jun 2026 13:45:35 +0200
Subject: [PATCH] feat(furc, ssa): implement phi instruction placement

Refs: #2
---
 furc/src/front/ssa.cpp                     | 71 +++++++++++++++++++++-
 furlang/include/furlang/ir/instruction.hpp | 24 +++++++-
 2 files changed, 89 insertions(+), 6 deletions(-)

diff --git a/furc/src/front/ssa.cpp b/furc/src/front/ssa.cpp
index e5a92db..34603ff 100644
--- a/furc/src/front/ssa.cpp
+++ b/furc/src/front/ssa.cpp
@@ -1,9 +1,11 @@
 #include "furc/front/ssa.hpp"
 
 #include "furlang/ir/instruction.hpp"
+#include "furlang/ir/operand.hpp"
 
 #include <algorithm>
 #include <cstddef>
+#include <memory>
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
@@ -20,19 +22,46 @@ void ssa::optimize(const std::unique_ptr<furlang::ir::function>& func) {
     block_map_t predecessors;
     block_map_t successors;
 
+    std::unordered_map<furlang::ir::register_operand, std::unordered_set<furlang::ir::block_index>> regSites;
+    std::unordered_map<furlang::ir::block_index, std::unordered_set<furlang::ir::register_operand>> regUses;
+
     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();
+
+        for (const auto& instr : block->instructions()) {
+            switch (instr->type()) {
+            case furlang::ir::instruction_t::Assign: {
+                const auto& assign = dynamic_cast<const furlang::ir::assign_instruction&>(*instr);
+                regSites[assign.destination().reg()].insert(i);
+                if (assign.source().type() == furlang::ir::operand_t::Register) {
+                    regUses[i].insert(assign.source().reg());
+                }
+            } break;
+            case furlang::ir::instruction_t::BinaryOp: {
+                const auto& binOp = dynamic_cast<const furlang::ir::binary_op_instruction&>(*instr);
+                regSites[binOp.dst().reg()].insert(i);
+                if (binOp.lhs().type() == furlang::ir::operand_t::Register) {
+                    regUses[i].insert(binOp.lhs().reg());
+                }
+                if (binOp.rhs().type() == furlang::ir::operand_t::Register) {
+                    regUses[i].insert(binOp.rhs().reg());
+                }
+            } break;
+            default: break;
+            }
+        }
+
+        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);
+            const auto& br = dynamic_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);
+            const auto& br = dynamic_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());
@@ -109,6 +138,42 @@ void ssa::optimize(const std::unique_ptr<furlang::ir::function>& func) {
             }
         }
     }
+
+    std::unordered_map<furlang::ir::block_index, std::unordered_set<furlang::ir::register_operand>> phis;
+
+    for (const auto& [reg, blocks] : regSites) {
+        std::vector<furlang::ir::block_index> worklist(blocks.begin(), blocks.end());
+
+        std::unordered_set<furlang::ir::block_index> added;
+        while (!worklist.empty()) {
+            auto block = worklist.back();
+            worklist.pop_back();
+            for (auto frontier : df[block]) {
+                if (added.find(frontier) != added.end()) continue;
+                phis[frontier].insert(reg);
+                added.insert(frontier);
+                if (blocks.find(frontier) == blocks.end()) {
+                    worklist.push_back(frontier);
+                }
+            }
+        }
+    }
+
+    for (furlang::ir::block_index i = 0; i < func->blocks().size(); ++i) {
+        if (phis.find(i) == phis.end()) continue;
+        const auto& block = func->blocks()[i];
+        const auto& preds = predecessors[i];
+
+        for (auto reg : phis[i]) {
+            if (regUses[i].find(reg) == regUses[i].end()) continue;
+
+            auto phiInstr = std::make_unique<furlang::ir::phi_instruction>(reg);
+            for (auto pred : preds) {
+                phiInstr->labels().emplace_back(furlang::ir::operand::new_reg(reg), pred);
+            }
+            block->instructions().emplace(block->instructions().begin(), std::move(phiInstr));
+        }
+    }
 }
 
 void ssa::dfs_rpo(furlang::ir::block_index        block,
diff --git a/furlang/include/furlang/ir/instruction.hpp b/furlang/include/furlang/ir/instruction.hpp
index 94978b6..bc84926 100644
--- a/furlang/include/furlang/ir/instruction.hpp
+++ b/furlang/include/furlang/ir/instruction.hpp
@@ -468,8 +468,8 @@ protected:
  */
 class phi_instruction final : public instruction {
 public:
-    phi_instruction(std::vector<std::pair<operand, block_index>>&& labels)
-      : m_labels(std::move(labels)) {}
+    phi_instruction(register_operand dst)
+      : m_dst(dst) {}
 
     ~phi_instruction() override = default;
 
@@ -494,6 +494,20 @@ public:
      */
     instruction_t type() const override { return instruction_t::Phi; }
 
+    /**
+     * @brief Returns this instruction's destination register.
+     *
+     * @return The register.
+     */
+    register_operand destination() const { return m_dst; }
+
+    /**
+     * @brief Returns this instruction's labels.
+     *
+     * @return The labels.
+     */
+    std::vector<std::pair<operand, block_index>>& labels() { return m_labels; }
+
     /**
      * @brief Returns this instruction's labels.
      *
@@ -501,11 +515,15 @@ public:
      */
     const std::vector<std::pair<operand, block_index>>& labels() const { return m_labels; }
 private:
+    register_operand                             m_dst;
     std::vector<std::pair<operand, block_index>> m_labels;
 protected:
     std::ostream& print(std::ostream& os) const override {
-        os << "phi";
+        os << "phi " << m_dst << " =";
+        bool first = true;
         for (const auto& pair : m_labels) {
+            if (!first) os << ',';
+            first = false;
             os << ' ' << pair.second << ": " << pair.first;
         }
         return os;