[wasm] Add a BytecodeIterator and use in non-performance-critical situations.

src/wasm/wasm-interpreter.cc

Index: src/wasm/wasm-interpreter.cc
diff --git a/src/wasm/wasm-interpreter.cc b/src/wasm/wasm-interpreter.cc
index d7e58796b730656babd09d961d2f189270c2829b..ed4dd64a5e07f418e629c668d449302143fb96a6 100644
--- a/src/wasm/wasm-interpreter.cc
+++ b/src/wasm/wasm-interpreter.cc
@@ -761,104 +761,101 @@ class ControlTransfers : public ZoneObject {
 
     std::vector<Control> control_stack;
     size_t value_depth = 0;
-    Decoder decoder(start, end);  // for reading operands.
-    const byte* pc = start + locals_encoded_size;
-
-    while (pc < end) {
-      WasmOpcode opcode = static_cast<WasmOpcode>(*pc);
-      TRACE("@%td: control %s (depth = %zu)\n", (pc - start),
+    for (BytecodeIterator i(start + locals_encoded_size, end); i.has_next();
+         i.next()) {
+      WasmOpcode opcode = i.current();
+      TRACE("@%u: control %s (depth = %zu)\n", i.pc_offset(),
             WasmOpcodes::OpcodeName(opcode), value_depth);
       switch (opcode) {
         case kExprBlock: {
-          TRACE("control @%td $%zu: Block\n", (pc - start), value_depth);
+          TRACE("control @%u $%zu: Block\n", i.pc_offset(), value_depth);
           CLabel* label = new (zone) CLabel(zone, value_depth);
-          control_stack.push_back({pc, label, nullptr});
+          control_stack.push_back({i.pc(), label, nullptr});
           break;
         }
         case kExprLoop: {
-          TRACE("control @%td $%zu: Loop\n", (pc - start), value_depth);
+          TRACE("control @%u $%zu: Loop\n", i.pc_offset(), value_depth);
           CLabel* label1 = new (zone) CLabel(zone, value_depth);
           CLabel* label2 = new (zone) CLabel(zone, value_depth);
-          control_stack.push_back({pc, label1, nullptr});
-          control_stack.push_back({pc, label2, nullptr});
-          label2->Bind(&map_, start, pc, false);
+          control_stack.push_back({i.pc(), label1, nullptr});
+          control_stack.push_back({i.pc(), label2, nullptr});
+          label2->Bind(&map_, start, i.pc(), false);
           break;
         }
         case kExprIf: {
-          TRACE("control @%td $%zu: If\n", (pc - start), value_depth);
+          TRACE("control @%u $%zu: If\n", i.pc_offset(), value_depth);
           value_depth--;
           CLabel* end_label = new (zone) CLabel(zone, value_depth);
           CLabel* else_label = new (zone) CLabel(zone, value_depth);
-          control_stack.push_back({pc, end_label, else_label});
-          else_label->Ref(&map_, start, {pc, value_depth, false});
+          control_stack.push_back({i.pc(), end_label, else_label});
+          else_label->Ref(&map_, start, {i.pc(), value_depth, false});
           break;
         }
         case kExprElse: {
           Control* c = &control_stack.back();
-          TRACE("control @%td $%zu: Else\n", (pc - start), value_depth);
-          c->end_label->Ref(&map_, start, {pc, value_depth, false});
+          TRACE("control @%u $%zu: Else\n", i.pc_offset(), value_depth);
+          c->end_label->Ref(&map_, start, {i.pc(), value_depth, false});
           value_depth = c->end_label->value_depth;
           DCHECK_NOT_NULL(c->else_label);
-          c->else_label->Bind(&map_, start, pc + 1, false);
+          c->else_label->Bind(&map_, start, i.pc() + 1, false);
           c->else_label = nullptr;
           break;
         }
         case kExprEnd: {
           Control* c = &control_stack.back();
-          TRACE("control @%td $%zu: End\n", (pc - start), value_depth);
+          TRACE("control @%u $%zu: End\n", i.pc_offset(), value_depth);
           if (c->end_label->target) {
             // only loops have bound labels.
             DCHECK_EQ(kExprLoop, *c->pc);
             control_stack.pop_back();
             c = &control_stack.back();
           }
-          if (c->else_label) c->else_label->Bind(&map_, start, pc + 1, true);
-          c->end_label->Ref(&map_, start, {pc, value_depth, false});
-          c->end_label->Bind(&map_, start, pc + 1, true);
+          if (c->else_label)
+            c->else_label->Bind(&map_, start, i.pc() + 1, true);
+          c->end_label->Ref(&map_, start, {i.pc(), value_depth, false});
+          c->end_label->Bind(&map_, start, i.pc() + 1, true);
           value_depth = c->end_label->value_depth + 1;
           control_stack.pop_back();
           break;
         }
         case kExprBr: {
-          BreakDepthOperand operand(&decoder, pc);
-          TRACE("control @%td $%zu: Br[arity=%u, depth=%u]\n", (pc - start),
+          BreakDepthOperand operand(&i, i.pc());
+          TRACE("control @%u $%zu: Br[arity=%u, depth=%u]\n", i.pc_offset(),
                 value_depth, operand.arity, operand.depth);
           value_depth -= operand.arity;
           control_stack[control_stack.size() - operand.depth - 1].Ref(
-              &map_, start, pc, value_depth, operand.arity > 0);
+              &map_, start, i.pc(), value_depth, operand.arity > 0);
           value_depth++;
           break;
         }
         case kExprBrIf: {
-          BreakDepthOperand operand(&decoder, pc);
-          TRACE("control @%td $%zu: BrIf[arity=%u, depth=%u]\n", (pc - start),
+          BreakDepthOperand operand(&i, i.pc());
+          TRACE("control @%u $%zu: BrIf[arity=%u, depth=%u]\n", i.pc_offset(),
                 value_depth, operand.arity, operand.depth);
           value_depth -= (operand.arity + 1);
           control_stack[control_stack.size() - operand.depth - 1].Ref(
-              &map_, start, pc, value_depth, operand.arity > 0);
+              &map_, start, i.pc(), value_depth, operand.arity > 0);
           value_depth++;
           break;
         }
         case kExprBrTable: {
-          BranchTableOperand operand(&decoder, pc);
-          TRACE("control @%td $%zu: BrTable[arity=%u count=%u]\n", (pc - start),
+          BranchTableOperand operand(&i, i.pc());
+          TRACE("control @%u $%zu: BrTable[arity=%u count=%u]\n", i.pc_offset(),
                 value_depth, operand.arity, operand.table_count);
           value_depth -= (operand.arity + 1);
-          for (uint32_t i = 0; i < operand.table_count + 1; ++i) {
-            uint32_t target = operand.read_entry(&decoder, i);
+          for (uint32_t j = 0; j < operand.table_count + 1; ++j) {
+            uint32_t target = operand.read_entry(&i, j);
             control_stack[control_stack.size() - target - 1].Ref(
-                &map_, start, pc + i, value_depth, operand.arity > 0);
+                &map_, start, i.pc() + j, value_depth, operand.arity > 0);
           }
           value_depth++;
           break;
         }
         default: {
-          value_depth = value_depth - OpcodeArity(pc, end) + 1;
+          value_depth = value_depth - OpcodeArity(i.pc(), end) + 1;
           break;
         }
       }
-
-      pc += OpcodeLength(pc, end);
     }
   }