Code folding (#1076)

Adds a pass that folds code, i.e. merges it when possible. See details in comment in the pass implementation cpp.

This is enabled by default in -Os and -Oz. Seems risky to enable anywhere else, as it does add branches - likely predictable ones so maybe no slowdown, but still some risk.

Code size numbers:

wasm-backend: 196331
+ binaryen -Os (before): 182598
+ binaryen -Os (with folding): 181943

asm2wasm -Os (before): 172463
asm2wasm -Os (with folding): 168774

So this reduces wasm-backend output by an additional 0.5% than it could before. Mainly this is because the wasm backend already has code folding, whereas on asm2wasm output, where we didn't have folding before, this saves over 2%. The 0.5% improvement on the wasm backend's output might be because this can fold more types of code than LLVM can (it can fold nested control flow, in particular).

Author: kripken

src/ast/branch-utils.h

@@ -18,6 +18,7 @@
 #define wasm_ast_branch_h
 
 #include "wasm.h"
+#include "wasm-traversal.h"
 
 namespace wasm {
 
@@ -36,6 +37,60 @@ inline bool isBranchTaken(Switch* sw) {
                         sw->condition->type != unreachable;
 }
 
+// returns the set of targets to which we branch that are
+// outside of a node
+inline std::set<Name> getExitingBranches(Expression* ast) {
+  struct Scanner : public PostWalker<Scanner> {
+    std::set<Name> targets;
+
+    void visitBreak(Break* curr) {
+      targets.insert(curr->name);
+    }
+    void visitSwitch(Switch* curr) {
+      for (auto target : targets) {
+        targets.insert(target);
+      }
+      targets.insert(curr->default_);
+    }
+    void visitBlock(Block* curr) {
+      if (curr->name.is()) {
+        targets.erase(curr->name);
+      }
+    }
+    void visitLoop(Loop* curr) {
+      if (curr->name.is()) {
+        targets.erase(curr->name);
+      }
+    }
+  };
+  Scanner scanner;
+  scanner.walk(ast);
+  // anything not erased is a branch out
+  return scanner.targets;
+}
+
+// returns the list of all branch targets in a node
+
+inline std::set<Name> getBranchTargets(Expression* ast) {
+  struct Scanner : public PostWalker<Scanner> {
+    std::set<Name> targets;
+
+    void visitBlock(Block* curr) {
+      if (curr->name.is()) {
+        targets.insert(curr->name);
+      }
+    }
+    void visitLoop(Loop* curr) {
+      if (curr->name.is()) {
+        targets.insert(curr->name);
+      }
+    }
+  };
+  Scanner scanner;
+  scanner.walk(ast);
+  return scanner.targets;
+}
+
 } // namespace BranchUtils
 
 } // namespace wasm

src/ast/label-utils.h

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2017 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_ast_label_h
+#define wasm_ast_label_h
+
+#include "wasm.h"
+#include "wasm-traversal.h"
+
+namespace wasm {
+
+namespace LabelUtils {
+
+// Handles branch/loop labels in a function; makes it easy to add new
+// ones without duplicates
+class LabelManager : public PostWalker<LabelManager> {
+public:
+  LabelManager(Function* func) {
+    walkFunction(func);
+  }
+
+  Name getUnique(std::string prefix) {
+    while (1) {
+      auto curr = Name(prefix + std::to_string(counter++));
+      if (labels.find(curr) == labels.end()) {
+        labels.insert(curr);
+        return curr;
+      }
+    }
+  }
+
+  void visitBlock(Block* curr) {
+    labels.insert(curr->name);
+  }
+  void visitLoop(Loop* curr) {
+    labels.insert(curr->name);
+  }
+
+private:
+  std::set<Name> labels;
+  size_t counter = 0;
+};
+
+} // namespace LabelUtils
+
+} // namespace wasm
+
+#endif // wasm_ast_label_h
+

src/mixed_arena.h

@@ -172,6 +172,10 @@ class ArenaVectorBase {
     return usedElements;
   }
 
+  bool empty() const {
+    return size() == 0;
+  }
+
   void resize(size_t size) {
     if (size > allocatedElements) {
       reallocate(size);

src/pass.h

@@ -237,29 +237,6 @@ class WalkerPass : public Pass, public WalkerType {
 // but registering them here in addition allows them to communicate
 // e.g. through PassRunner::getLast
 
-// Handles names in a module, in particular adding names without duplicates
-class NameManager : public WalkerPass<PostWalker<NameManager>> {
- public:
-  Name getUnique(std::string prefix);
-  // TODO: getUniqueInFunction
-
-  // visitors
-  void visitBlock(Block* curr);
-  void visitLoop(Loop* curr);
-  void visitBreak(Break* curr);
-  void visitSwitch(Switch* curr);
-  void visitCall(Call* curr);
-  void visitCallImport(CallImport* curr);
-  void visitFunctionType(FunctionType* curr);
-  void visitFunction(Function* curr);
-  void visitImport(Import* curr);
-  void visitExport(Export* curr);
-
-private:
-  std::set<Name> names;
-  size_t counter = 0;
-};
-
 // Prints out a module
 class Printer : public Pass {
 protected:

src/passes/CMakeLists.txt

@@ -2,6 +2,7 @@ SET(passes_SOURCES
   pass.cpp
   CoalesceLocals.cpp
   CodePushing.cpp
+  CodeFolding.cpp
   DeadCodeElimination.cpp
   DuplicateFunctionElimination.cpp
   ExtractFunction.cpp
@@ -15,7 +16,6 @@ SET(passes_SOURCES
   MemoryPacking.cpp
   MergeBlocks.cpp
   Metrics.cpp
-  NameManager.cpp
   NameList.cpp
   OptimizeInstructions.cpp
   PickLoadSigns.cpp