wasm2js: remove unneeded break/continue labels (#2058)

Author: kripken

src/emscripten-optimizer/simple_ast.cpp

@@ -183,65 +183,4 @@ void dump(const char *str, Ref node, bool pretty) {
   std::cerr << std::endl;
 }
 
-// Traversals
-
-struct TraverseInfo {
-  TraverseInfo() = default;
-  TraverseInfo(Ref node) : node(node) {
-    assert(node.get());
-    if (node->isArray()) {
-      for (size_t i = 0; i < node->size(); i++) {
-        maybeAdd(node[i]);
-      }
-    } else if (node->isAssign()) {
-      auto assign = node->asAssign();
-      maybeAdd(assign->target());
-      maybeAdd(assign->value());
-    } else if (node->isAssignName()) {
-      auto assign = node->asAssignName();
-      maybeAdd(assign->value());
-    } else {
-      // no children
-    }
-  }
-  Ref node;
-  size_t index = -1;
-  std::vector<Ref> children;
-
-private:
-  void maybeAdd(Ref child) {
-    if (child.get()) {
-      children.push_back(child);
-    }
-  }
-};
-
-// Traverse, calling visit after the children
-void traversePost(Ref node, std::function<void (Ref)> visit) {
-  std::vector<TraverseInfo> stack;
-  stack.push_back(TraverseInfo(node));
-  while (!stack.empty()) {
-    TraverseInfo& back = stack.back();
-    if (back.index == size_t(-1)) {
-      // This is the first time we see this. Push its children.
-      back.index = 0;
-      for (auto child : back.children) {
-        stack.emplace_back(child);
-      }
-      continue;
-    }
-    if (back.index < back.children.size()) {
-      // Visit this child.
-      back.index++;
-      visit(back.children[back.index - 1]);
-      continue;
-    }
-    assert(back.index == back.children.size());
-    // Time to visit the node itself
-    auto node = back.node;
-    stack.pop_back();
-    visit(node);
-  }
-}
-
 } // namespace cashew

src/emscripten-optimizer/simple_ast.h

@@ -394,6 +394,10 @@ struct Value {
     return arr->size();
   }
 
+  bool empty() {
+    return size() == 0;
+  }
+
   void setSize(size_t size) {
     assert(isArray());
     auto old = arr->size();
@@ -525,11 +529,6 @@ struct AssignName : public Value {
   }
 };
 
-// AST traversals
-
-// Traverse, calling visit after the children
-void traversePost(Ref node, std::function<void (Ref)> visit);
-
 // JS printing support
 
 struct JSPrinter {

src/tools/wasm2js.cpp

@@ -67,19 +67,79 @@ static void printJS(Ref ast, T& output) {
   output << jser.buffer << std::endl;
 }
 
+
+// Traversals
+
+struct TraverseInfo {
+  TraverseInfo() = default;
+  TraverseInfo(Ref node) : node(node) {
+    assert(node.get());
+    if (node->isArray()) {
+      for (size_t i = 0; i < node->size(); i++) {
+        maybeAdd(node[i]);
+      }
+    } else if (node->isAssign()) {
+      auto assign = node->asAssign();
+      maybeAdd(assign->target());
+      maybeAdd(assign->value());
+    } else if (node->isAssignName()) {
+      auto assign = node->asAssignName();
+      maybeAdd(assign->value());
+    } else {
+      // no children
+    }
+  }
+  Ref node;
+  bool scanned = false;
+  std::vector<Ref> children;
+
+private:
+  void maybeAdd(Ref child) {
+    if (child.get()) {
+      children.push_back(child);
+    }
+  }
+};
+
+// Traverse, calling visit after the children
+static void traversePrePost(Ref node, std::function<void (Ref)> visitPre, std::function<void (Ref)> visitPost) {
+  std::vector<TraverseInfo> stack;
+  stack.push_back(TraverseInfo(node));
+  while (!stack.empty()) {
+    TraverseInfo& back = stack.back();
+    if (!back.scanned) {
+      back.scanned = true;
+      // This is the first time we see this.
+      visitPre(back.node);
+      for (auto child : back.children) {
+        stack.emplace_back(child);
+      }
+      continue;
+    }
+    // Time to post-visit the node itself
+    auto node = back.node;
+    stack.pop_back();
+    visitPost(node);
+  }
+}
+
+static void traversePost(Ref node, std::function<void (Ref)> visit) {
+  traversePrePost(node, [](Ref node) {}, visit);
+}
+
 static void optimizeJS(Ref ast) {
   // helpers
 
   auto isOrZero = [](Ref node) {
-    return node->isArray() && node->size() > 0 && node[0] == BINARY && node[1] == OR && node[3]->isNumber() && node[3]->getNumber() == 0;
+    return node->isArray() && !node->empty() && node[0] == BINARY && node[1] == OR && node[3]->isNumber() && node[3]->getNumber() == 0;
   };
 
   auto isPlus = [](Ref node) {
-    return node->isArray() && node->size() > 0 && node[0] == UNARY_PREFIX && node[1] == PLUS;
+    return node->isArray() && !node->empty() && node[0] == UNARY_PREFIX && node[1] == PLUS;
   };
 
   auto isBitwise = [](Ref node) {
-    if (node->isArray() && node->size() > 0 && node[0] == BINARY) {
+    if (node->isArray() && !node->empty() && node[0] == BINARY) {
       auto op = node[1];
       return op == OR || op == AND || op == XOR || op == RSHIFT || op == TRSHIFT || op == LSHIFT;
     }
@@ -88,7 +148,7 @@ static void optimizeJS(Ref ast) {
 
   // x >> 0  =>  x | 0
   traversePost(ast, [](Ref node) {
-    if (node->isArray() && node->size() > 0 && node[0] == BINARY && node[1] == RSHIFT && node[3]->isNumber()) {
+    if (node->isArray() && !node->empty() && node[0] == BINARY && node[1] == RSHIFT && node[3]->isNumber()) {
       if (node[3]->getNumber() == 0) {
         node[1]->setString(OR);
       }
@@ -124,6 +184,63 @@ static void optimizeJS(Ref ast) {
       }
     }
   });
+
+  // Remove unnecessary break/continue labels, when referring to the top level.
+
+  // XXX IString invalid("__wasm2js$INVALID_LABEL__");
+  std::vector<Ref> breakCapturers;
+  std::vector<Ref> continueCapturers;
+  std::unordered_map<IString, Ref> labelToValue; // maps the label to the loop/etc.
+  std::unordered_set<Value*> labelled; // all things with a label on them.
+  Value INVALID;
+  traversePrePost(ast, [&](Ref node) {
+    if (node->isArray() && !node->empty()) {
+      if (node[0] == LABEL) {
+        auto label = node[1]->getIString();
+        labelToValue[label] = node[2];
+        labelled.insert(node[2].get());
+      } else if (node[0] == WHILE || node[0] == DO || node[0] == FOR) {
+        breakCapturers.push_back(node);
+        continueCapturers.push_back(node);
+      } else if (node[0] == cashew::BLOCK) {
+        if (labelled.count(node.get())) {
+          // Cannot break to a block without the label.
+          breakCapturers.push_back(Ref(&INVALID));
+        }
+      } else if (node[0] == SWITCH) {
+        breakCapturers.push_back(node);
+      }
+    }
+  }, [&](Ref node) {
+    if (node->isArray() && !node->empty()) {
+      if (node[0] == LABEL) {
+        auto label = node[1]->getIString();
+        labelToValue.erase(label);
+        labelled.erase(node[2].get());
+      } else if (node[0] == WHILE || node[0] == DO || node[0] == FOR) {
+        breakCapturers.pop_back();
+        continueCapturers.pop_back();
+      } else if (node[0] == cashew::BLOCK) {
+        if (labelled.count(node.get())) {
+          breakCapturers.pop_back();
+        }
+      } else if (node[0] == SWITCH) {
+        breakCapturers.pop_back();
+      } else if (node[0] == BREAK || node[0] == CONTINUE) {
+        if (!node[1]->isNull()) {
+          auto label = node[1]->getIString();
+          assert(labelToValue.count(label));
+          auto& capturers = node[0] == BREAK ? breakCapturers : continueCapturers;
+          assert(!capturers.empty());
+          if (capturers.back() == labelToValue[label]) {
+            // Success, the break/continue goes exactly where we would if we
+            // didn't have the label!
+            node[1]->setNull();
+          }
+        }
+      }
+    }
+  });
 }
 
 static void emitWasm(Module& wasm, Output& output, Wasm2JSBuilder::Flags flags, PassOptions options, Name name) {

test/wasm2js/br_table_to_loop.2asm.js

@@ -28,7 +28,7 @@ function asmFunc(global, env, buffer) {
     case 0:
      break block;
     case 1:
-     continue loop;
+     continue;
     default:
      break block;
     };
@@ -41,11 +41,11 @@ function asmFunc(global, env, buffer) {
    loop : while (1) {
     switch (1 | 0) {
     case 0:
-     continue loop;
+     continue;
     case 1:
      break block;
     default:
-     continue loop;
+     continue;
     };
    };
   }

test/wasm2js/fac.2asm.js

@@ -132,7 +132,7 @@ function asmFunc(global, env, buffer) {
       $1$hi = i64toi32_i32$5;
      }
     }
-    continue loop_in;
+    continue;
    };
   }
   i64toi32_i32$5 = $2$hi;
@@ -179,7 +179,7 @@ function asmFunc(global, env, buffer) {
       i$hi = i64toi32_i32$5;
      }
     }
-    continue loop;
+    continue;
    };
   }
   i64toi32_i32$5 = res$hi;
@@ -265,9 +265,9 @@ function asmFunc(global, env, buffer) {
      }
     }
     if ($13) {
-     continue loop_in
+     continue
     }
-    break loop_in;
+    break;
    };
   }
   i64toi32_i32$2 = $1$hi;

test/wasm2js/i32.2asm.js

@@ -208,7 +208,7 @@ function asmFunc(global, env, buffer) {
     }
     var$0 = var$0 & var$0 - 1;
     var$1 = var$1 + 1 | 0;
-    continue label$2;
+    continue;
    };
   }
   return $5_1;

test/wasm2js/i64-ctz.2asm.js

@@ -236,7 +236,7 @@ function asmFunc(global, env, buffer) {
     }
     var$1 = i64toi32_i32$1;
     var$1$hi = i64toi32_i32$4;
-    continue label$2;
+    continue;
    };
   }
   i64toi32_i32$4 = $5$hi;

test/wasm2js/i64.2asm.js

@@ -3357,9 +3357,9 @@ function asmFunc(global, env, buffer) {
         var$7$hi = i64toi32_i32$3;
         var$2 = var$2 + -1 | 0;
         if (var$2) {
-         continue label$15
+         continue
         }
-        break label$15;
+        break;
        };
        break label$13;
       }
@@ -3556,7 +3556,7 @@ function asmFunc(global, env, buffer) {
     }
     var$1 = i64toi32_i32$1;
     var$1$hi = i64toi32_i32$4;
-    continue label$2;
+    continue;
    };
   }
   i64toi32_i32$4 = $5$hi;