[wasm] Refactor handling of operands to bytecodes.

src/wasm/ast-decoder.cc

Index: src/wasm/ast-decoder.cc
diff --git a/src/wasm/ast-decoder.cc b/src/wasm/ast-decoder.cc
index cfee4f3cf14171df7815cc7a9465bf66c8d53a25..6d847b8e3ff4fb266fae060aeadb44807c3518ea 100644
--- a/src/wasm/ast-decoder.cc
+++ b/src/wasm/ast-decoder.cc
@@ -41,7 +41,6 @@ struct Tree {
   WasmOpcode opcode() const { return static_cast<WasmOpcode>(*pc); }
 };
 
-
 // A production represents an incomplete decoded tree in the LR decoder.
 struct Production {
   Tree* tree;  // the root of the syntax tree.
@@ -103,8 +102,8 @@ struct IfEnv {
 class WasmDecoder : public Decoder {
  public:
   WasmDecoder() : Decoder(nullptr, nullptr), function_env_(nullptr) {}
-
- protected:
+  WasmDecoder(FunctionEnv* env, const byte* start, const byte* end)
+      : Decoder(start, end), function_env_(env) {}
   FunctionEnv* function_env_;
 
   void Reset(FunctionEnv* function_env, const byte* start, const byte* end) {
@@ -136,62 +135,209 @@ class WasmDecoder : public Decoder {
     return read_u64(pc + 1);
   }
 
-  LocalType LocalOperand(const byte* pc, uint32_t* index, int* length) {
-    *index = UnsignedLEB128Operand(pc, length);
-    if (function_env_->IsValidLocal(*index)) {
-      return function_env_->GetLocalType(*index);
+  inline bool Validate(const byte* pc, LocalIndexOperand& operand) {
+    if (operand.index < function_env_->total_locals) {
+      operand.type = function_env_->GetLocalType(operand.index);
+      return true;
     }
-    error(pc, "invalid local variable index");
-    return kAstStmt;
+    error(pc, pc + 1, "invalid local index");
+    return false;
   }
 
-  LocalType GlobalOperand(const byte* pc, uint32_t* index, int* length) {
-    *index = UnsignedLEB128Operand(pc, length);
-    if (function_env_->module->IsValidGlobal(*index)) {
-      return WasmOpcodes::LocalTypeFor(
-          function_env_->module->GetGlobalType(*index));
+  inline bool Validate(const byte* pc, GlobalIndexOperand& operand) {
+    ModuleEnv* m = function_env_->module;
+    if (m && m->module && operand.index < m->module->globals->size()) {
+      operand.machine_type = m->module->globals->at(operand.index).type;
+      operand.type = WasmOpcodes::LocalTypeFor(operand.machine_type);
+      return true;
     }
-    error(pc, "invalid global variable index");
-    return kAstStmt;
+    error(pc, pc + 1, "invalid global index");
+    return false;
   }
 
-  FunctionSig* FunctionSigOperand(const byte* pc, uint32_t* index,
-                                  int* length) {
-    *index = UnsignedLEB128Operand(pc, length);
-    if (function_env_->module->IsValidFunction(*index)) {
-      return function_env_->module->GetFunctionSignature(*index);
+  inline bool Validate(const byte* pc, FunctionIndexOperand& operand) {
+    ModuleEnv* m = function_env_->module;
+    if (m && m->module && operand.index < m->module->functions->size()) {
+      operand.sig = m->module->functions->at(operand.index).sig;
+      return true;
     }
-    error(pc, "invalid function index");
-    return nullptr;
+    error(pc, pc + 1, "invalid function index");
+    return false;
   }
 
-  FunctionSig* SigOperand(const byte* pc, uint32_t* index, int* length) {
-    *index = UnsignedLEB128Operand(pc, length);
-    if (function_env_->module->IsValidSignature(*index)) {
-      return function_env_->module->GetSignature(*index);
+  inline bool Validate(const byte* pc, SignatureIndexOperand& operand) {
+    ModuleEnv* m = function_env_->module;
+    if (m && m->module && operand.index < m->module->signatures->size()) {
+      operand.sig = m->module->signatures->at(operand.index);
+      return true;
     }
-    error(pc, "invalid signature index");
-    return nullptr;
+    error(pc, pc + 1, "invalid signature index");
+    return false;
   }
 
-  uint32_t UnsignedLEB128Operand(const byte* pc, int* length) {
-    uint32_t result = 0;
-    ReadUnsignedLEB128ErrorCode error_code =
-        ReadUnsignedLEB128Operand(pc + 1, limit_, length, &result);
-    if (error_code == kInvalidLEB128) error(pc, "invalid LEB128 varint");
-    if (error_code == kMissingLEB128) error(pc, "expected LEB128 varint");
-    (*length)++;
-    return result;
+  inline bool Validate(const byte* pc, BreakDepthOperand& operand,
+                       ZoneVector<Block>& blocks) {
+    if (operand.depth < blocks.size()) {
+      operand.target = &blocks[blocks.size() - operand.depth - 1];
+      return true;
+    }
+    error(pc, pc + 1, "invalid break depth");
+    return false;
   }
 
-  void MemoryAccessOperand(const byte* pc, int* length, uint32_t* offset) {
-    byte bitfield = ByteOperand(pc, "missing memory access operand");
-    if (MemoryAccess::OffsetField::decode(bitfield)) {
-      *offset = UnsignedLEB128Operand(pc + 1, length);
-      (*length)++;  // to account for the memory access byte
-    } else {
-      *offset = 0;
-      *length = 2;
+  bool Validate(const byte* pc, TableSwitchOperand& operand,
+                size_t block_depth) {
+    if (operand.table_count == 0) {
+      error(pc, "tableswitch with 0 entries");
+      return false;
+    }
+    // Verify table.
+    for (uint32_t i = 0; i < operand.table_count; i++) {
+      uint16_t target = operand.read_entry(this, i);
+      if (target >= 0x8000) {
+        size_t depth = target - 0x8000;
+        if (depth > block_depth) {
+          error(operand.table + i * 2, "improper branch in tableswitch");
+          return false;
+        }
+      } else {
+        if (target >= operand.case_count) {
+          error(operand.table + i * 2, "invalid case target in tableswitch");
+          return false;
+        }
+      }
+    }
+    return true;
+  }
+
+  int OpcodeArity(const byte* pc) {
+#define DECLARE_ARITY(name, ...)                          \
+  static const LocalType kTypes_##name[] = {__VA_ARGS__}; \
+  static const int kArity_##name =                        \
+      static_cast<int>(arraysize(kTypes_##name) - 1);
+
+    FOREACH_SIGNATURE(DECLARE_ARITY);
+#undef DECLARE_ARITY
+
+    switch (static_cast<WasmOpcode>(*pc)) {
+      case kExprI8Const:
+      case kExprI32Const:
+      case kExprI64Const:
+      case kExprF64Const:
+      case kExprF32Const:
+      case kExprGetLocal:
+      case kExprLoadGlobal:
+      case kExprNop:
+      case kExprUnreachable:
+        return 0;
+
+      case kExprBr:
+      case kExprStoreGlobal:
+      case kExprSetLocal:
+        return 1;
+
+      case kExprIf:
+      case kExprBrIf:
+        return 2;
+      case kExprIfElse:
+      case kExprSelect:
+        return 3;
+
+      case kExprBlock:
+      case kExprLoop: {
+        BlockCountOperand operand(this, pc);
+        return operand.count;
+      }
+
+      case kExprCallFunction: {
+        FunctionIndexOperand operand(this, pc);
+        return static_cast<int>(
+            function_env_->module->GetFunctionSignature(operand.index)
+                ->parameter_count());
+      }
+      case kExprCallIndirect: {
+        SignatureIndexOperand operand(this, pc);
+        return 1 + static_cast<int>(
+                       function_env_->module->GetSignature(operand.index)
+                           ->parameter_count());
+      }
+      case kExprReturn: {
+        return static_cast<int>(function_env_->sig->return_count());
+      }
+      case kExprTableSwitch: {
+        TableSwitchOperand operand(this, pc);
+        return 1 + operand.case_count;
+      }
+
+#define DECLARE_OPCODE_CASE(name, opcode, sig) \
+  case kExpr##name:                            \
+    return kArity_##sig;
+
+        FOREACH_LOAD_MEM_OPCODE(DECLARE_OPCODE_CASE)
+        FOREACH_STORE_MEM_OPCODE(DECLARE_OPCODE_CASE)
+        FOREACH_MISC_MEM_OPCODE(DECLARE_OPCODE_CASE)
+        FOREACH_SIMPLE_OPCODE(DECLARE_OPCODE_CASE)
+#undef DECLARE_OPCODE_CASE
+    }
+    UNREACHABLE();
+    return 0;
+  }
+
+  int OpcodeLength(const byte* pc) {
+    switch (static_cast<WasmOpcode>(*pc)) {
+#define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
+      FOREACH_LOAD_MEM_OPCODE(DECLARE_OPCODE_CASE)
+      FOREACH_STORE_MEM_OPCODE(DECLARE_OPCODE_CASE)
+#undef DECLARE_OPCODE_CASE
+      {
+        MemoryAccessOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprBlock:
+      case kExprLoop: {
+        BlockCountOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprBr:
+      case kExprBrIf: {
+        BreakDepthOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprStoreGlobal:
+      case kExprLoadGlobal: {
+        GlobalIndexOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+
+      case kExprCallFunction: {
+        FunctionIndexOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprCallIndirect: {
+        SignatureIndexOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+
+      case kExprSetLocal:
+      case kExprGetLocal: {
+        LocalIndexOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprTableSwitch: {
+        TableSwitchOperand operand(this, pc);
+        return 1 + operand.length;
+      }
+      case kExprI8Const:
+        return 2;
+      case kExprI32Const:
+      case kExprF32Const:
+        return 5;
+      case kExprI64Const:
+      case kExprF64Const:
+        return 9;
+
+      default:
+        return 1;
     }
   }
 };
@@ -431,25 +577,25 @@ class LR_WasmDecoder : public WasmDecoder {
           Leaf(kAstStmt);
           break;
         case kExprBlock: {
-          int length = ByteOperand(pc_);
-          if (length < 1) {
+          BlockCountOperand operand(this, pc_);
+          if (operand.count < 1) {
             Leaf(kAstStmt);
           } else {
-            Shift(kAstEnd, length);
+            Shift(kAstEnd, operand.count);
             // The break environment is the outer environment.
             SsaEnv* break_env = ssa_env_;
             PushBlock(break_env);
             SetEnv("block:start", Steal(break_env));
           }
-          len = 2;
+          len = 1 + operand.length;
           break;
         }
         case kExprLoop: {
-          int length = ByteOperand(pc_);
-          if (length < 1) {
+          BlockCountOperand operand(this, pc_);
+          if (operand.count < 1) {
             Leaf(kAstStmt);
           } else {
-            Shift(kAstEnd, length);
+            Shift(kAstEnd, operand.count);
             // The break environment is the outer environment.
             SsaEnv* break_env = ssa_env_;
             PushBlock(break_env);
@@ -461,7 +607,7 @@ class LR_WasmDecoder : public WasmDecoder {
             PushBlock(cont_env);
             blocks_.back().stack_depth = -1;  // no production for inner block.
           }
-          len = 2;
+          len = 1 + operand.length;
           break;
         }
         case kExprIf:
@@ -474,58 +620,27 @@ class LR_WasmDecoder : public WasmDecoder {
           Shift(kAstStmt, 3);  // Result type is typeof(x) in {c ? x : y}.
           break;
         case kExprBr: {
-          uint32_t depth = ByteOperand(pc_);
-          Shift(kAstEnd, 1);
-          if (depth >= blocks_.size()) {
-            error("improperly nested branch");
+          BreakDepthOperand operand(this, pc_);
+          if (Validate(pc_, operand, blocks_)) {
+            Shift(kAstEnd, 1);
           }
-          len = 2;
+          len = 1 + operand.length;
           break;
         }
         case kExprBrIf: {
-          uint32_t depth = ByteOperand(pc_);
-          Shift(kAstStmt, 2);
-          if (depth >= blocks_.size()) {
-            error("improperly nested conditional branch");
+          BreakDepthOperand operand(this, pc_);
+          if (Validate(pc_, operand, blocks_)) {
+            Shift(kAstStmt, 2);
           }
-          len = 2;
+          len = 1 + operand.length;
           break;
         }
         case kExprTableSwitch: {
-          if (!checkAvailable(5)) {
-            error("expected #tableswitch <cases> <table>, fell off end");
-            break;
-          }
-          uint16_t case_count = read_u16(pc_ + 1);
-          uint16_t table_count = read_u16(pc_ + 3);
-          len = 5 + table_count * 2;
-
-          if (table_count == 0) {
-            error("tableswitch with 0 entries");
-            break;
-          }
-
-          if (!checkAvailable(len)) {
-            error("expected #tableswitch <cases> <table>, fell off end");
-            break;
-          }
-
-          Shift(kAstEnd, 1 + case_count);
-
-          // Verify table.
-          for (int i = 0; i < table_count; i++) {
-            uint16_t target = read_u16(pc_ + 5 + i * 2);
-            if (target >= 0x8000) {
-              size_t depth = target - 0x8000;
-              if (depth > blocks_.size()) {
-                error(pc_ + 5 + i * 2, "improper branch in tableswitch");
-              }
-            } else {
-              if (target >= case_count) {
-                error(pc_ + 5 + i * 2, "invalid case target in tableswitch");
-              }
-            }
+          TableSwitchOperand operand(this, pc_);
+          if (Validate(pc_, operand, blocks_.size())) {
+            Shift(kAstEnd, 1 + operand.case_count);
           }
+          len = 1 + operand.length;
           break;
         }
         case kExprReturn: {
@@ -546,59 +661,66 @@ class LR_WasmDecoder : public WasmDecoder {
           break;
         }
         case kExprI8Const: {
-          int32_t value = bit_cast<int8_t>(ByteOperand(pc_));
-          Leaf(kAstI32, BUILD(Int32Constant, value));
-          len = 2;
+          ImmI8Operand operand(this, pc_);
+          Leaf(kAstI32, BUILD(Int32Constant, operand.value));
+          len = 1 + operand.length;
           break;
         }
         case kExprI32Const: {
-          uint32_t value = Uint32Operand(pc_);
-          Leaf(kAstI32, BUILD(Int32Constant, value));
-          len = 5;
+          ImmI32Operand operand(this, pc_);
+          Leaf(kAstI32, BUILD(Int32Constant, operand.value));
+          len = 1 + operand.length;
           break;
         }
         case kExprI64Const: {
-          uint64_t value = Uint64Operand(pc_);
-          Leaf(kAstI64, BUILD(Int64Constant, value));
-          len = 9;
+          ImmI64Operand operand(this, pc_);
+          Leaf(kAstI64, BUILD(Int64Constant, operand.value));
+          len = 1 + operand.length;
           break;
         }
         case kExprF32Const: {
-          float value = bit_cast<float>(Uint32Operand(pc_));
-          Leaf(kAstF32, BUILD(Float32Constant, value));
-          len = 5;
+          ImmF32Operand operand(this, pc_);
+          Leaf(kAstF32, BUILD(Float32Constant, operand.value));
+          len = 1 + operand.length;
           break;
         }
         case kExprF64Const: {
-          double value = bit_cast<double>(Uint64Operand(pc_));
-          Leaf(kAstF64, BUILD(Float64Constant, value));
-          len = 9;
+          ImmF64Operand operand(this, pc_);
+          Leaf(kAstF64, BUILD(Float64Constant, operand.value));
+          len = 1 + operand.length;
           break;
         }
         case kExprGetLocal: {
-          uint32_t index;
-          LocalType type = LocalOperand(pc_, &index, &len);
-          TFNode* val =
-              build() && type != kAstStmt ? ssa_env_->locals[index] : nullptr;
-          Leaf(type, val);
+          LocalIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            TFNode* val = build() ? ssa_env_->locals[operand.index] : nullptr;
+            Leaf(operand.type, val);
+          }
+          len = 1 + operand.length;
           break;
         }
         case kExprSetLocal: {
-          uint32_t index;
-          LocalType type = LocalOperand(pc_, &index, &len);
-          Shift(type, 1);
+          LocalIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            Shift(operand.type, 1);
+          }
+          len = 1 + operand.length;
           break;
         }
         case kExprLoadGlobal: {
-          uint32_t index;
-          LocalType type = GlobalOperand(pc_, &index, &len);
-          Leaf(type, BUILD(LoadGlobal, index));
+          GlobalIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            Leaf(operand.type, BUILD(LoadGlobal, operand.index));
+          }
+          len = 1 + operand.length;
           break;
         }
         case kExprStoreGlobal: {
-          uint32_t index;
-          LocalType type = GlobalOperand(pc_, &index, &len);
-          Shift(type, 1);
+          GlobalIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            Shift(operand.type, 1);
+          }
+          len = 1 + operand.length;
           break;
         }
         case kExprI32LoadMem8S:
@@ -647,27 +769,25 @@ class LR_WasmDecoder : public WasmDecoder {
           Shift(kAstI32, 1);
           break;
         case kExprCallFunction: {
-          uint32_t unused;
-          FunctionSig* sig = FunctionSigOperand(pc_, &unused, &len);
-          if (sig) {
-            LocalType type =
-                sig->return_count() == 0 ? kAstStmt : sig->GetReturn();
-            Shift(type, static_cast<int>(sig->parameter_count()));
-          } else {
-            Leaf(kAstI32);  // error
+          FunctionIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            LocalType type = operand.sig->return_count() == 0
+                                 ? kAstStmt
+                                 : operand.sig->GetReturn();
+            Shift(type, static_cast<int>(operand.sig->parameter_count()));
           }
+          len = 1 + operand.length;
           break;
         }
         case kExprCallIndirect: {
-          uint32_t unused;
-          FunctionSig* sig = SigOperand(pc_, &unused, &len);
-          if (sig) {
-            LocalType type =
-                sig->return_count() == 0 ? kAstStmt : sig->GetReturn();
-            Shift(type, static_cast<int>(1 + sig->parameter_count()));
-          } else {
-            Leaf(kAstI32);  // error
+          SignatureIndexOperand operand(this, pc_);
+          if (Validate(pc_, operand)) {
+            LocalType type = operand.sig->return_count() == 0
+                                 ? kAstStmt
+                                 : operand.sig->GetReturn();
+            Shift(type, static_cast<int>(1 + operand.sig->parameter_count()));
           }
+          len = 1 + operand.length;
           break;
         }
         default:
@@ -690,19 +810,15 @@ class LR_WasmDecoder : public WasmDecoder {
   }
 
   int DecodeLoadMem(const byte* pc, LocalType type) {
-    int length = 2;
-    uint32_t offset;
-    MemoryAccessOperand(pc, &length, &offset);
+    MemoryAccessOperand operand(this, pc);
     Shift(type, 1);
-    return length;
+    return 1 + operand.length;
   }
 
   int DecodeStoreMem(const byte* pc, LocalType type) {
-    int length = 2;
-    uint32_t offset;
-    MemoryAccessOperand(pc, &length, &offset);
+    MemoryAccessOperand operand(this, pc);
     Shift(type, 2);
-    return length;
+    return 1 + operand.length;
   }
 
   void AddImplicitReturnAtEnd() {
@@ -876,31 +992,23 @@ class LR_WasmDecoder : public WasmDecoder {
         break;
       }
       case kExprBr: {
-        uint32_t depth = ByteOperand(p->pc());
-        if (depth >= blocks_.size()) {
-          error("improperly nested branch");
-          break;
-        }
-        Block* block = &blocks_[blocks_.size() - depth - 1];
-        ReduceBreakToExprBlock(p, block);
+        BreakDepthOperand operand(this, p->pc());
+        CHECK(Validate(p->pc(), operand, blocks_));
+        ReduceBreakToExprBlock(p, operand.target);
         break;
       }
       case kExprBrIf: {
         if (p->index == 1) {
           TypeCheckLast(p, kAstI32);
         } else if (p->done()) {
-          uint32_t depth = ByteOperand(p->pc());
-          if (depth >= blocks_.size()) {
-            error("improperly nested branch");
-            break;
-          }
-          Block* block = &blocks_[blocks_.size() - depth - 1];
+          BreakDepthOperand operand(this, p->pc());
+          CHECK(Validate(p->pc(), operand, blocks_));
           SsaEnv* fenv = ssa_env_;
           SsaEnv* tenv = Split(fenv);
           BUILD(Branch, p->tree->children[0]->node, &tenv->control,
                 &fenv->control);
           ssa_env_ = tenv;
-          ReduceBreakToExprBlock(p, block);
+          ReduceBreakToExprBlock(p, operand.target);
           ssa_env_ = fenv;
         }
         break;
@@ -909,18 +1017,22 @@ class LR_WasmDecoder : public WasmDecoder {
         if (p->index == 1) {
           // Switch key finished.
           TypeCheckLast(p, kAstI32);
+          if (failed()) break;
 
-          uint16_t table_count = read_u16(p->pc() + 3);
+          TableSwitchOperand operand(this, p->pc());
+          DCHECK(Validate(p->pc(), operand, blocks_.size()));
 
           // Build the switch only if it has more than just a default target.
-          bool build_switch = table_count > 1;
+          bool build_switch = operand.table_count > 1;
           TFNode* sw = nullptr;
-          if (build_switch) sw = BUILD(Switch, table_count, p->last()->node);
+          if (build_switch)
+            sw = BUILD(Switch, operand.table_count, p->last()->node);
 
           // Allocate environments for each case.
-          uint16_t case_count = read_u16(p->pc() + 1);
-          SsaEnv** case_envs = zone_->NewArray<SsaEnv*>(case_count);
-          for (int i = 0; i < case_count; i++) case_envs[i] = UnreachableEnv();
+          SsaEnv** case_envs = zone_->NewArray<SsaEnv*>(operand.case_count);
+          for (uint32_t i = 0; i < operand.case_count; i++) {
+            case_envs[i] = UnreachableEnv();
+          }
 
           ifs_.push_back({nullptr, nullptr, case_envs});
           SsaEnv* break_env = ssa_env_;
@@ -929,13 +1041,14 @@ class LR_WasmDecoder : public WasmDecoder {
           ssa_env_ = copy;
 
           // Build the environments for each case based on the table.
-          for (int i = 0; i < table_count; i++) {
-            uint16_t target = read_u16(p->pc() + 5 + i * 2);
+          for (uint32_t i = 0; i < operand.table_count; i++) {
+            uint16_t target = operand.read_entry(this, i);
             SsaEnv* env = copy;
             if (build_switch) {
               env = Split(env);
-              env->control = (i == table_count - 1) ? BUILD(IfDefault, sw)
-                                                    : BUILD(IfValue, i, sw);
+              env->control = (i == operand.table_count - 1)
+                                 ? BUILD(IfDefault, sw)
+                                 : BUILD(IfValue, i, sw);
             }
             if (target >= 0x8000) {
               // Targets an outer block.
@@ -981,12 +1094,11 @@ class LR_WasmDecoder : public WasmDecoder {
         break;
       }
       case kExprSetLocal: {
-        int unused = 0;
-        uint32_t index;
-        LocalType type = LocalOperand(p->pc(), &index, &unused);
+        LocalIndexOperand operand(this, p->pc());
+        CHECK(Validate(p->pc(), operand));
         Tree* val = p->last();
-        if (type == val->type) {
-          if (build()) ssa_env_->locals[index] = val->node;
+        if (operand.type == val->type) {
+          if (build()) ssa_env_->locals[operand.index] = val->node;
           p->tree->node = val->node;
         } else {
           error(p->pc(), val->pc, "Typecheck failed in SetLocal");
@@ -994,12 +1106,11 @@ class LR_WasmDecoder : public WasmDecoder {
         break;
       }
       case kExprStoreGlobal: {
-        int unused = 0;
-        uint32_t index;
-        LocalType type = GlobalOperand(p->pc(), &index, &unused);
+        GlobalIndexOperand operand(this, p->pc());
+        CHECK(Validate(p->pc(), operand));
         Tree* val = p->last();
-        if (type == val->type) {
-          BUILD(StoreGlobal, index, val->node);
+        if (operand.type == val->type) {
+          BUILD(StoreGlobal, operand.index, val->node);
           p->tree->node = val->node;
         } else {
           error(p->pc(), val->pc, "Typecheck failed in StoreGlobal");
@@ -1068,34 +1179,29 @@ class LR_WasmDecoder : public WasmDecoder {
         return;
 
       case kExprCallFunction: {
-        int len;
-        uint32_t index;
-        FunctionSig* sig = FunctionSigOperand(p->pc(), &index, &len);
-        if (!sig) break;
+        FunctionIndexOperand operand(this, p->pc());
+        CHECK(Validate(p->pc(), operand));
         if (p->index > 0) {
-          TypeCheckLast(p, sig->GetParam(p->index - 1));
+          TypeCheckLast(p, operand.sig->GetParam(p->index - 1));
         }
         if (p->done() && build()) {
           uint32_t count = p->tree->count + 1;
           TFNode** buffer = builder_->Buffer(count);
-          FunctionSig* sig = FunctionSigOperand(p->pc(), &index, &len);
-          USE(sig);
           buffer[0] = nullptr;  // reserved for code object.
           for (uint32_t i = 1; i < count; i++) {
             buffer[i] = p->tree->children[i - 1]->node;
           }
-          p->tree->node = builder_->CallDirect(index, buffer);
+          p->tree->node = builder_->CallDirect(operand.index, buffer);
         }
         break;
       }
       case kExprCallIndirect: {
-        int len;
-        uint32_t index;
-        FunctionSig* sig = SigOperand(p->pc(), &index, &len);
+        SignatureIndexOperand operand(this, p->pc());
+        CHECK(Validate(p->pc(), operand));
         if (p->index == 1) {
           TypeCheckLast(p, kAstI32);
         } else {
-          TypeCheckLast(p, sig->GetParam(p->index - 2));
+          TypeCheckLast(p, operand.sig->GetParam(p->index - 2));
         }
         if (p->done() && build()) {
           uint32_t count = p->tree->count;
@@ -1103,7 +1209,7 @@ class LR_WasmDecoder : public WasmDecoder {
           for (uint32_t i = 0; i < count; i++) {
             buffer[i] = p->tree->children[i]->node;
           }
-          p->tree->node = builder_->CallIndirect(index, buffer);
+          p->tree->node = builder_->CallIndirect(operand.index, buffer);
         }
         break;
       }
@@ -1152,11 +1258,9 @@ class LR_WasmDecoder : public WasmDecoder {
     DCHECK_EQ(1, p->index);
     TypeCheckLast(p, kAstI32);  // index
     if (build()) {
-      int length = 0;
-      uint32_t offset = 0;
-      MemoryAccessOperand(p->pc(), &length, &offset);
+      MemoryAccessOperand operand(this, p->pc());
       p->tree->node =
-          builder_->LoadMem(type, mem_type, p->last()->node, offset);
+          builder_->LoadMem(type, mem_type, p->last()->node, operand.offset);
     }
   }
 
@@ -1167,11 +1271,10 @@ class LR_WasmDecoder : public WasmDecoder {
       DCHECK_EQ(2, p->index);
       TypeCheckLast(p, type);
       if (build()) {
-        int length = 0;
-        uint32_t offset = 0;
-        MemoryAccessOperand(p->pc(), &length, &offset);
+        MemoryAccessOperand operand(this, p->pc());
         TFNode* val = p->tree->children[1]->node;
-        builder_->StoreMem(mem_type, p->tree->children[0]->node, offset, val);
+        builder_->StoreMem(mem_type, p->tree->children[0]->node, operand.offset,
+                           val);
         p->tree->node = val;
       }
     }
@@ -1194,7 +1297,7 @@ class LR_WasmDecoder : public WasmDecoder {
   void SetEnv(const char* reason, SsaEnv* env) {
     TRACE("  env = %p, block depth = %d, reason = %s", static_cast<void*>(env),
           static_cast<int>(blocks_.size()), reason);
-    if (env->control != nullptr && FLAG_trace_wasm_decoder) {
+    if (FLAG_trace_wasm_decoder && env && env->control) {
       TRACE(", control = ");
       compiler::WasmGraphBuilder::PrintDebugName(env->control);
     }
@@ -1447,158 +1550,29 @@ ReadUnsignedLEB128ErrorCode ReadUnsignedLEB128Operand(const byte* pc,
                                                       const byte* limit,
                                                       int* length,
                                                       uint32_t* result) {
-  *result = 0;
-  const byte* ptr = pc;
-  const byte* end = pc + 5;  // maximum 5 bytes.
-  if (end > limit) end = limit;
-  int shift = 0;
-  byte b = 0;
-  while (ptr < end) {
-    b = *ptr++;
-    *result = *result | ((b & 0x7F) << shift);
-    if ((b & 0x80) == 0) break;
-    shift += 7;
-  }
-  DCHECK_LE(ptr - pc, 5);
-  *length = static_cast<int>(ptr - pc);
-  if (ptr == end && (b & 0x80)) {
-    return kInvalidLEB128;
-  } else if (*length == 0) {
-    return kMissingLEB128;
-  } else {
-    return kNoError;
-  }
+  Decoder decoder(pc, limit);
+  *result = decoder.checked_read_u32v(pc, 0, length);
+  if (decoder.ok()) return kNoError;
+  return (limit - pc) > 1 ? kInvalidLEB128 : kMissingLEB128;
 }
 
-
-// TODO(titzer): move this into WasmDecoder and bounds check accesses.
-int OpcodeLength(const byte* pc) {
-  switch (static_cast<WasmOpcode>(*pc)) {
-#define DECLARE_OPCODE_CASE(name, opcode, sig) case kExpr##name:
-    FOREACH_LOAD_MEM_OPCODE(DECLARE_OPCODE_CASE)
-    FOREACH_STORE_MEM_OPCODE(DECLARE_OPCODE_CASE)
-#undef DECLARE_OPCODE_CASE
-    {
-      // Loads and stores have an optional offset.
-      byte bitfield = pc[1];
-      if (MemoryAccess::OffsetField::decode(bitfield)) {
-        int length;
-        uint32_t result = 0;
-        ReadUnsignedLEB128Operand(pc + 2, pc + 7, &length, &result);
-        return 2 + length;
-      }
-      return 2;
-    }
-    case kExprI8Const:
-    case kExprBlock:
-    case kExprLoop:
-    case kExprBr:
-    case kExprBrIf:
-      return 2;
-    case kExprI32Const:
-    case kExprF32Const:
-      return 5;
-    case kExprI64Const:
-    case kExprF64Const:
-      return 9;
-    case kExprStoreGlobal:
-    case kExprSetLocal:
-    case kExprLoadGlobal:
-    case kExprCallFunction:
-    case kExprCallIndirect:
-    case kExprGetLocal: {
-      int length;
-      uint32_t result = 0;
-      ReadUnsignedLEB128Operand(pc + 1, pc + 6, &length, &result);
-      return 1 + length;
-    }
-    case kExprTableSwitch: {
-      uint16_t table_count = *reinterpret_cast<const uint16_t*>(pc + 3);
-      return 5 + table_count * 2;
-    }
-
-    default:
-      return 1;
-  }
+int OpcodeLength(const byte* pc, const byte* end) {
+  WasmDecoder decoder(nullptr, pc, end);
+  return decoder.OpcodeLength(pc);
 }
 
-
-// TODO(titzer): move this into WasmDecoder and bounds check accesses.
-int OpcodeArity(FunctionEnv* env, const byte* pc) {
-#define DECLARE_ARITY(name, ...)                          \
-  static const LocalType kTypes_##name[] = {__VA_ARGS__}; \
-  static const int kArity_##name =                        \
-      static_cast<int>(arraysize(kTypes_##name) - 1);
-
-  FOREACH_SIGNATURE(DECLARE_ARITY);
-#undef DECLARE_ARITY
-
-  switch (static_cast<WasmOpcode>(*pc)) {
-    case kExprI8Const:
-    case kExprI32Const:
-    case kExprI64Const:
-    case kExprF64Const:
-    case kExprF32Const:
-    case kExprGetLocal:
-    case kExprLoadGlobal:
-    case kExprNop:
-    case kExprUnreachable:
-      return 0;
-
-    case kExprBr:
-    case kExprStoreGlobal:
-    case kExprSetLocal:
-      return 1;
-
-    case kExprIf:
-    case kExprBrIf:
-      return 2;
-    case kExprIfElse:
-    case kExprSelect:
-      return 3;
-    case kExprBlock:
-    case kExprLoop:
-      return *(pc + 1);
-
-    case kExprCallFunction: {
-      int index = *(pc + 1);
-      return static_cast<int>(
-          env->module->GetFunctionSignature(index)->parameter_count());
-    }
-    case kExprCallIndirect: {
-      int index = *(pc + 1);
-      return 1 + static_cast<int>(
-                     env->module->GetSignature(index)->parameter_count());
-    }
-    case kExprReturn:
-      return static_cast<int>(env->sig->return_count());
-    case kExprTableSwitch: {
-      uint16_t case_count = *reinterpret_cast<const uint16_t*>(pc + 1);
-      return 1 + case_count;
-    }
-
-#define DECLARE_OPCODE_CASE(name, opcode, sig) \
-  case kExpr##name:                            \
-    return kArity_##sig;
-
-      FOREACH_LOAD_MEM_OPCODE(DECLARE_OPCODE_CASE)
-      FOREACH_STORE_MEM_OPCODE(DECLARE_OPCODE_CASE)
-      FOREACH_MISC_MEM_OPCODE(DECLARE_OPCODE_CASE)
-      FOREACH_SIMPLE_OPCODE(DECLARE_OPCODE_CASE)
-#undef DECLARE_OPCODE_CASE
-  }
-  UNREACHABLE();
-  return 0;
+int OpcodeArity(FunctionEnv* env, const byte* pc, const byte* end) {
+  WasmDecoder decoder(env, pc, end);
+  return decoder.OpcodeArity(pc);
 }
 
-
-
 void PrintAst(FunctionEnv* env, const byte* start, const byte* end) {
+  WasmDecoder decoder(env, start, end);
   const byte* pc = start;
   std::vector<int> arity_stack;
   while (pc < end) {
-    int arity = OpcodeArity(env, pc);
-    size_t length = OpcodeLength(pc);
+    int arity = decoder.OpcodeArity(pc);
+    size_t length = decoder.OpcodeLength(pc);
 
     for (auto arity : arity_stack) {
       printf("  ");
@@ -1623,7 +1597,6 @@ void PrintAst(FunctionEnv* env, const byte* start, const byte* end) {
   }
 }
 
-
 // Analyzes loop bodies for static assignments to locals, which helps in
 // reducing the number of phis introduced at loop headers.
 class LoopAssignmentAnalyzer : public WasmDecoder {
@@ -1641,7 +1614,7 @@ class LoopAssignmentAnalyzer : public WasmDecoder {
         new (zone_) BitVector(function_env_->total_locals, zone_);
     // Keep a stack to model the nesting of expressions.
     std::vector<int> arity_stack;
-    arity_stack.push_back(OpcodeArity(function_env_, pc_));
+    arity_stack.push_back(OpcodeArity(pc_));
     pc_ += OpcodeLength(pc_);
 
     // Iteratively process all AST nodes nested inside the loop.
@@ -1650,16 +1623,16 @@ class LoopAssignmentAnalyzer : public WasmDecoder {
       int arity = 0;
       int length = 1;
       if (opcode == kExprSetLocal) {
-        uint32_t index;
-        LocalOperand(pc_, &index, &length);
+        LocalIndexOperand operand(this, pc_);
         if (assigned->length() > 0 &&
-            static_cast<int>(index) < assigned->length()) {
+            static_cast<int>(operand.index) < assigned->length()) {
           // Unverified code might have an out-of-bounds index.
-          assigned->Add(index);
+          assigned->Add(operand.index);
         }
         arity = 1;
+        length = 1 + operand.length;
       } else {
-        arity = OpcodeArity(function_env_, pc_);
+        arity = OpcodeArity(pc_);
         length = OpcodeLength(pc_);
       }