dead code elimination

.vscode/launch.json

@@ -10,7 +10,7 @@
             "name": "Debug",
             "program": "${workspaceFolder}/build/sysy",
             // "args": ["../sysytests/functional_2021/069_greatest_common_divisor.sy", "-S", "-o", "build/my.s", "-O1", "-emit-llvm"],
-            "args" : ["-S", "../sysytests/functional_2022/21_if_test2.sy", "-o", "build/my.s", "-emit-llvm", "-O1",],
+            "args" : ["-S", "../sysytests/functional_2022/38_op_priority4.sy", "-o", "build/dbg.s", "-emit-llvm", "-O1",],
             "cwd": "${workspaceFolder}"
         },
     ]

include/algos.h

@@ -8,4 +8,88 @@ void gen_dominance(FunctionPtr_t func);
 void gen_dominance_frontier(FunctionPtr_t func);
 void update_dfs_numbers(BasicBlockPtr_t bb, bool rst);
 
+struct CFGNode {
+  BasicBlock *bb                   = nullptr;
+  int special_node                 = 0;
+  std::vector<CFGNode *> pred_list = {};
+  std::vector<CFGNode *> succ_list = {};
+  std::unordered_set<CFGNode *> DF = {};
+  CFGNode *idom;                           // immediate dominator of this node
+  std::unordered_set<CFGNode *> dom;       // dominators of this node
+  std::unordered_set<CFGNode *> dominated; // nodes immediately dominated by this node
+  static sptr(CFGNode) New(BasicBlock *bb = nullptr) {
+    auto newnode = std::make_shared<CFGNode>();
+    newnode->bb  = bb;
+    return newnode;
+  }
+  static sptr(CFGNode) New(int special_node) {
+    auto newnode          = std::make_shared<CFGNode>();
+    newnode->bb           = nullptr;
+    newnode->special_node = special_node;
+    return newnode;
+  }
+  std::string to_string() const {
+    if (is_entry()) return "ENTRY";
+    if (is_exit()) return "EXIT";
+    return bb->name;
+  }
+  bool is_entry() const {
+    return special_node == 1;
+  }
+  bool is_exit() const {
+    return special_node == 2;
+  }
+  void print_cfg(std::ostream &ostr) const {
+    ostr << "---" << to_string() << "---\n";
+    ostr << "  pred: [";
+    for (auto pred : pred_list) {
+      ostr << pred->to_string() << ", ";
+    }
+    ostr << "]\n";
+    ostr << "  succ: [";
+    for (auto succ : succ_list) {
+      ostr << succ->to_string() << ", ";
+    }
+    ostr << "]\n";
+    ostr.flush();
+  }
+  void print_dom(std::ostream &ostr) const {
+    print_cfg(ostr);
+    ostr << "  idom: " << (!idom ? "NULL" : idom->to_string()) << "\n";
+    ostr << "  domer: [";
+    for (auto d : dom) {
+      ostr << d->to_string() << ", ";
+    }
+    ostr << "]\n";
+    ostr << "  domee: [";
+    for (auto domee : dominated) {
+      ostr << domee->to_string() << ", ";
+    }
+    ostr << "]\n";
+    ostr << "  DF: [";
+    for (auto df : DF) {
+      ostr << df->to_string() << ", ";
+    }
+    ostr << "]\n";
+    ostr.flush();
+  }
+};
+
+class CFG {
+public:
+  std::list<sptr(CFGNode)> node_pool;
+  CFGNode *entry_node;
+  CFGNode *exit_node;
+  std::unordered_map<BasicBlock *, CFGNode *> bb_node;
+  void build_from_func(Function *func, bool reversed);
+  void print_cfg();
+  void print_dom();
+  void calculate();
+
+private:
+  std::unordered_set<CFGNode *> visit;
+  void _print_cfg(CFGNode *);
+  void _print_dom(CFGNode *);
+};
+
 } // namespace CompSysY

include/common.h

@@ -42,10 +42,7 @@ inline sptr(DST) strict_shared_cast(SRC src) {
 #define INF (0x3f3f3f3f)
 #define BTWN(v, l, r) (l <= v && v <= r)
 
-#define panic(message)                                                                                                 \
+#define panic(message) __assert_fail(message, __FILE__, __LINE__, __ASSERT_FUNCTION)
-  do {                                                                                                                 \
-    throw GrammarException(__FILE__, __LINE__, (message));                                                             \
-  } while (0)
 
 #define DEF_PTR_T(type)                                                                                                \
   class type;                                                                                                          \

include/mc_pass.h

@@ -76,4 +76,4 @@ private:
   std::set<MInstMove *, MvCmp> active_moves;
 };
 
-}
+} // namespace CompSysY

include/pass.h

@@ -4,7 +4,7 @@ namespace CompSysY {
 
 class PassSCCP final : public Pass {
 public:
-  PassSCCP() : Pass("Constant Propagation") {}
+  PassSCCP() : Pass("constant propagation") {}
   void run(const Module &module) override;
   enum class ConstLatTag { Top = 0, Const, Bottom };
   struct ConstLat {
@@ -58,13 +58,19 @@ public:
 
 class PassBuildCFG final : public Pass {
 public:
-  PassBuildCFG() : Pass("build_cfg") {}
+  PassBuildCFG() : Pass("build control flow graph") {}
   void run(const Module &module) override;
 };
 
 class PassConstFold final : public Pass {
 public:
-  PassConstFold() : Pass("const fold") {}
+  PassConstFold() : Pass("constant fold") {}
+  void run(const Module &module) override;
+};
+
+class PassDCE final : public Pass {
+public:
+  PassDCE() : Pass("dead code elimination") {}
   void run(const Module &module) override;
 };
 

src/algo_dominance.cpp

@@ -1,3 +1,4 @@
+#include "algos.h"
 #include "llir.h"
 #include "visitor.h"
 
@@ -132,4 +133,164 @@ void update_dfs_numbers(BasicBlockPtr_t bb, bool rst) {
   bb->dom_dfs_out = dfs_num++;
 }
 
+void CFG::build_from_func(Function *func, bool reversed) {
+  // special node entry&exit
+  auto _entry_node = CFGNode::New(1);
+  auto _exit_node  = CFGNode::New(2);
+  node_pool.push_back(_entry_node);
+  node_pool.push_back(_exit_node);
+  entry_node = _entry_node.get();
+  exit_node  = _exit_node.get();
+  // create a cfgnode for each bb
+  for (auto bb : func->bb_list) {
+    auto _new_node = CFGNode::New(bb.get());
+    node_pool.push_back(_new_node);
+    bb_node.insert({bb.get(), _new_node.get()});
+  }
+  auto as_bb   = [](sptr(Value) value) { return dynamic_cast<BasicBlock *>(value.get()); };
+  auto connect = [&](CFGNode *pred, CFGNode *succ) {
+    if (reversed) std::swap(pred, succ);
+    pred->succ_list.push_back(succ);
+    succ->pred_list.push_back(pred);
+  };
+
+  connect(entry_node, bb_node.at(func->bb_list.front().get()));
+  for (auto bb : func->bb_list) {
+    auto _inst_br = bb->inst_list.back();
+    if (auto inst_br = shared_cast<InstBranch>(_inst_br)) {
+      if (inst_br->operand_list.size() == 1) {
+        connect(bb_node.at(bb.get()), bb_node.at(as_bb(inst_br->operand_list[0])));
+      }
+      else if (inst_br->operand_list.size() == 3) {
+        connect(bb_node.at(bb.get()), bb_node.at(as_bb(inst_br->operand_list[1])));
+        connect(bb_node.at(bb.get()), bb_node.at(as_bb(inst_br->operand_list[2])));
+      }
+      else {
+        panic("br should have either 1 or 3 operands");
+      }
+    }
+    else if (auto inst_ret = shared_cast<InstReturn>(_inst_br)) {
+      connect(bb_node.at(bb.get()), exit_node);
+    }
+    else {
+      panic("The last instruction of a basic block should be control transfer");
+    }
+  }
+  if (reversed) {
+    exit_node->special_node  = 1;
+    entry_node->special_node = 2;
+    std::swap(exit_node, entry_node);
+  }
+}
+
+void CFG::print_cfg() {
+  visit.clear();
+  _print_cfg(entry_node);
+}
+
+void CFG::_print_cfg(CFGNode *rt) {
+  rt->print_cfg(std::cout);
+  for (auto succ : rt->succ_list) {
+    if (!visit.count(succ)) {
+      visit.insert(succ);
+      _print_cfg(succ);
+    }
+  }
+}
+
+void CFG::print_dom() {
+  visit.clear();
+  _print_dom(entry_node);
+}
+
+void CFG::_print_dom(CFGNode *rt) {
+  rt->print_dom(std::cout);
+  for (auto succ : rt->succ_list) {
+    if (!visit.count(succ)) {
+      visit.insert(succ);
+      _print_dom(succ);
+    }
+  }
+}
+/*
+DOM(entry) = entry
+DOM(everything else) = all nodes
+change = true
+while change, do
+  change = false
+  for each BB (except the entry BB)
+    TMP(BB) = BB + {intersect of DOM of all predecessor BB’s}
+    if (TMP(BB) != DOM(BB))
+      DOM(BB) = TMP(BB)
+      change = true
+*/
+
+void CFG::calculate() {
+  entry_node->dom.clear();
+  entry_node->dom.insert(entry_node);
+  std::unordered_set<CFGNode *> all_node_set;
+  for (auto cfgnode : node_pool) {
+    all_node_set.insert(cfgnode.get());
+  }
+  for (auto cfgnode : node_pool) {
+    if (cfgnode->is_entry()) continue;
+    cfgnode->dom = all_node_set;
+  }
+  all_node_set.erase(entry_node); // now, it is every node but entry
+  bool changed = true;
+  while (changed) {
+    changed = false;
+    for (auto node : all_node_set) {
+      // std::unordered_set<CFGNode *> tmp = {node};
+      // for (auto pred : node->pred_list) {
+      //   tmp.insert(BEGINEND(pred->dom));
+      // }
+      // if (tmp != node->dom) {
+      //   changed   = true;
+      //   node->dom = tmp;
+      // }
+      for (auto it = node->dom.begin(); it != node->dom.end();) {
+        auto *x = *it;
+        auto check = [x](CFGNode *p) { return !INSET(p->dom, x); };
+        if (x != node && std::any_of(BEGINEND(node->pred_list), check)) {
+          changed = true;
+          it      = node->dom.erase(it);
+        }
+        else ++it;
+      }
+    }
+  }
+
+  entry_node->idom = nullptr;
+  for (auto bb : all_node_set) {
+    for (auto domer : bb->dom) {
+      // given domer dom bb, apparently, idom(bb) != bb, i.e. domer sdom bb
+      if (domer == bb) continue;
+      // if domer not sdom x where x sdom bb
+      auto check = [domer, bb](CFGNode *x) { return x == bb || x == domer || !INSET(x->dom, domer); };
+      if (std::all_of(BEGINEND(bb->dom), check)) {
+        bb->idom = domer;
+        domer->dominated.insert(bb);
+        break;
+      }
+    }
+  }
+  // calculate DF
+  for (auto &bb : bb_node) {
+    bb.second->DF.clear();
+  }
+  for (auto &bb : bb_node) {
+    auto n = bb.second;
+    if (n->pred_list.size() >= 2) {
+      for (auto pred : n->pred_list) {
+        auto runner = pred;
+        while (runner && runner != n->idom) {
+          runner->DF.insert(n);
+          runner = runner->idom;
+        }
+      }
+    }
+  }
+}
+
 } // namespace CompSysY

src/main.cpp

@@ -116,6 +116,7 @@ int main(int argc, const char **argv) {
   std::vector<sptr(Pass)> optpasses = {
     std::make_shared<PassConstFold>(),
     std::make_shared<PassSCCP>(),
+    std::make_shared<PassDCE>(),
   };
 
   if (flg_O1) {

src/pass_const_fold.cpp

@@ -2,15 +2,12 @@
 
 namespace CompSysY {
 
-
 static bool foldable(Instruction *inst) {
   if (InstTag::Add <= inst->tag && inst->tag <= InstTag::Ne) {
-    if (shared_cast<ConstantInt>(inst->operand_list[0]) && shared_cast<ConstantInt>(inst->operand_list[1]))
+    if (shared_cast<ConstantInt>(inst->operand_list[0]) && shared_cast<ConstantInt>(inst->operand_list[1])) return true;
-      return true;
   }
   else if (inst->tag == InstTag::Zext) {
-    if (shared_cast<ConstantInt>(inst->operand_list[0])) 
+    if (shared_cast<ConstantInt>(inst->operand_list[0])) return true;
-      return true;
   }
   return false;
 }
@@ -23,6 +20,7 @@ static InstBranch *is_cond_br(Instruction *inst) {
   return nullptr;
 }
 
+// not removing the block, this only means that pred no longer branch to bb
 static void rewrite_bb(sptr(BasicBlock) bb, sptr(BasicBlock) pred) {
   pred->succ_list.remove(bb);
   auto pred_index = GETINDEX(bb->pred_list, pred);
@@ -59,12 +57,30 @@ static void compute(Function* func) {
           case InstTag::Mod: result = val(0) % val(1); break;
           case InstTag::Mul: result = val(0) * val(1); break;
           case InstTag::Div: result = val(0) / val(1); break;
-          case InstTag::Lt: result = val(0) < val(1); type = TypeHelper::TYPE_I1; break;
+          case InstTag::Lt:
-          case InstTag::Le: result = val(0) <= val(1); type = TypeHelper::TYPE_I1; break;
+            result = val(0) < val(1);
-          case InstTag::Ge: result = val(0) >= val(1); type = TypeHelper::TYPE_I1; break;
+            type   = TypeHelper::TYPE_I1;
-          case InstTag::Gt: result = val(0) > val(1); type = TypeHelper::TYPE_I1; break;
+            break;
-          case InstTag::Eq: result = val(0) == val(1); type = TypeHelper::TYPE_I1; break;
+          case InstTag::Le:
-          case InstTag::Ne: result = val(0) != val(1); type = TypeHelper::TYPE_I1; break;
+            result = val(0) <= val(1);
+            type   = TypeHelper::TYPE_I1;
+            break;
+          case InstTag::Ge:
+            result = val(0) >= val(1);
+            type   = TypeHelper::TYPE_I1;
+            break;
+          case InstTag::Gt:
+            result = val(0) > val(1);
+            type   = TypeHelper::TYPE_I1;
+            break;
+          case InstTag::Eq:
+            result = val(0) == val(1);
+            type   = TypeHelper::TYPE_I1;
+            break;
+          case InstTag::Ne:
+            result = val(0) != val(1);
+            type   = TypeHelper::TYPE_I1;
+            break;
           case InstTag::Zext: result = val(0); break;
           default: break;
         }
@@ -96,4 +112,4 @@ void PassConstFold::run(const Module &module) {
   }
 }
 
-}
+} // namespace CompSysY

src/pass_dce.cpp

@@ -0,0 +1,196 @@
+#include "algos.h"
+#include "pass.h"
+
+namespace CompSysY {
+
+void get_reachable(BasicBlock *bb, std::unordered_set<BasicBlock *> &reachable) {
+  std::unordered_set<BasicBlock *> visited;
+  std::vector<BasicBlock *> stk = {bb};
+  while (!stk.empty()) {
+    auto u = stk.back();
+    stk.pop_back();
+    if (INSET(visited, u)) continue;
+    reachable.insert(u);
+    visited.insert(u);
+    for (auto succ : u->succ_list) {
+      stk.push_back(succ.get());
+    }
+  }
+}
+
+/*
+Remove block from the function
+1. remove from pred's succ
+2. remove from succ's pred
+  2.1 remove from succ's phi-nodes
+3. remove from function
+*/
+void remove_block(sptr(BasicBlock) victim) {
+  for (auto pred : victim->pred_list) {
+    pred->succ_list.remove(victim);
+  }
+  for (auto succ : victim->succ_list) {
+    auto pred_index = GETINDEX(succ->pred_list, victim);
+    succ->pred_list.remove(victim);
+    for (auto itr = succ->inst_list.begin(); itr != succ->inst_list.end();) {
+      auto inst = *itr++;
+      if (!shared_cast<InstPhi>(inst)) break;
+      auto inst_phi = shared_cast<InstPhi>(inst);
+      assert(inst_phi->operand_list.size() > pred_index);
+      inst_phi->operand_list.erase(inst_phi->operand_list.begin() + pred_index);
+    }
+  }
+  victim->parent_func->bb_list.erase(victim->itr);
+}
+
+// 高级编译器18.1
+bool Elim_Unreach_Code(Function *func) {
+  bool again   = true;
+  bool changed = false;
+  std::unordered_set<BasicBlock *> reachable;
+  do {
+    again = false;
+    reachable.clear();
+    get_reachable(func->bb_list.front().get(), reachable);
+    for (auto itr = func->bb_list.begin(); itr != func->bb_list.end();) {
+      auto bb = *itr++;
+      if (!INSET(reachable, bb.get())) {
+        again   = true;
+        changed = true;
+        VLOG(6) << "[DCE] remove unreachable block " << bb->to_string();
+        remove_block(bb);
+      }
+    }
+  } while (again);
+  return changed;
+}
+
+// Operations defined as critical: I/O statements, linkage code (entry & exit blocks), return values, calls to other
+// procedures From Efficiently Computing SSA Form and the Control Dependence Graph, Zadeck 1991
+bool Elim_Dead_Code(Function *func) {
+  CFG RCFG;
+  RCFG.build_from_func(func, true);
+  RCFG.calculate();
+  // RCFG.print_dom();
+  // Mark
+  std::unordered_map<Instruction *, bool> mark;
+  std::unordered_set<Instruction *> worklist;
+  for (auto bb : func->bb_list) {
+    for (auto inst : bb->inst_list) {
+      switch (inst->tag) {
+        case InstTag::Call:  // though not all functions are critical, we dont impl anything to get that info
+        case InstTag::Ret:   // useful for its caller, though could be useless, but we can't either know anything about
+                             // this
+        case InstTag::Store: // though not every store is critical, mark the operands conservatively
+          // case InstTag::Br: // branch, well, controls the flow
+          break;
+        default: continue;
+      }
+      mark[inst.get()] = true;
+      worklist.insert(inst.get());
+    }
+  }
+  while (!worklist.empty()) {
+    auto inst = *worklist.begin();
+    worklist.erase(worklist.begin());
+    if (auto inst_phi = dynamic_cast<InstPhi*>(inst)) {
+      for (auto pred : inst->parent_bb->pred_list) {
+        if (!mark[pred->inst_list.back().get()]) {
+          mark[pred->inst_list.back().get()]= true;
+          worklist.insert(pred->inst_list.back().get());
+        }
+      }
+    }
+    for (auto op : inst->operand_list) {
+      // we ignore globals and constants
+      if (auto inst_op = shared_cast<Instruction>(op)) {
+        if (!mark[inst_op.get()]) {
+          mark[inst_op.get()] = true;
+          worklist.insert(inst_op.get());
+        }
+      }
+    }
+    auto cfgnode = RCFG.bb_node.at(inst->parent_bb.get());
+    for (auto rdf : cfgnode->DF) {
+      assert(rdf->bb);
+      auto block_end_branch = rdf->bb->inst_list.back().get();
+      assert(dynamic_cast<InstBranch *>(block_end_branch));
+      if (!mark[block_end_branch]) {
+        mark[block_end_branch] = true;
+        worklist.insert(block_end_branch);
+      }
+    }
+  }
+  auto bb_useful = [&](BasicBlock *bb) {
+    for (auto inst : bb->inst_list) {
+      if (mark[inst.get()]) return true;
+    }
+    return false;
+  };
+  // sweep
+  bool changed = false;
+  for (auto bb : func->bb_list) {
+    for (auto itr = bb->inst_list.begin(); itr != bb->inst_list.end();) {
+      auto inst = *itr++;
+      if (mark[inst.get()]) continue;
+      // deal with unmakred
+      if (auto inst_br = shared_cast<InstBranch>(inst)) {
+        // in this algo, jump and branch are different
+        if (inst_br->operand_list.size() == 1) continue;
+        // Still unknown what this would be like, the only case by now is
+        // true_branch = false_branch
+        changed = true;
+        LOG(WARNING) << "rewrite dead branch";
+        auto cfgnode = RCFG.bb_node.at(bb.get());
+        // walk up through the dom-tree(idom), and check if there is any useful insts in it
+        auto useful_pdom = cfgnode->idom;
+        while (!useful_pdom->is_entry()) {
+          if (bb_useful(useful_pdom->bb)) {
+            break;
+          }
+          useful_pdom = useful_pdom->idom;
+        }
+        // rewrite branch to jmp
+        inst_br->u_remove_from_usees();
+        inst_br->operand_list.clear();
+        sptr(BasicBlock) tmp = nullptr;
+        for (auto _bb : func->bb_list) {
+          if (_bb.get() == useful_pdom->bb) {
+            tmp = _bb;
+            break;
+          }
+        }
+        inst_br->add_operand(tmp);
+      }
+      else {
+        changed = true;
+        // inst->u_replace_users(nullptr);
+        inst->u_remove_from_usees();
+        bb->inst_list.erase(inst->inst_itr);
+        VLOG(6) << "remove dead instruction";
+      }
+    }
+  }
+  return changed;
+}
+
+// 高级编译器18.2
+bool Fuse_Block(Function *func) {
+  return false;
+}
+
+void PassDCE::run(const Module &module) {
+  LOG(INFO) << "Run pass " << pass_name;
+  for (auto func : module.function_list) {
+    if (func->is_libfunc()) continue;
+    auto again = true;
+    while (again) {
+      again = false;
+      again = Elim_Dead_Code(func.get()) || again;
+      again = Elim_Unreach_Code(func.get()) && again;
+      again = Fuse_Block(func.get()) || again;
+    }
+  }
+}
+
+} // namespace CompSysY