[Interpreter] Enable tracing of bytecode handler dispatches.

src/interpreter/interpreter.cc

Index: src/interpreter/interpreter.cc
diff --git a/src/interpreter/interpreter.cc b/src/interpreter/interpreter.cc
index 5084300dfee60294b3af19832d026410b5834c6f..c4fd9c04c4c3de74f44fcdb60eb2c1a8a23d077f 100644
--- a/src/interpreter/interpreter.cc
+++ b/src/interpreter/interpreter.cc
@@ -4,6 +4,8 @@
 
 #include "src/interpreter/interpreter.h"
 
+#include <fstream>
+
 #include "src/ast/prettyprinter.h"
 #include "src/code-factory.h"
 #include "src/compiler.h"
@@ -33,6 +35,14 @@ void Interpreter::Initialize() {
   Zone zone(isolate_->allocator());
   HandleScope scope(isolate_);
 
+  if (FLAG_trace_ignition_dispatches) {
+    static const int kBytecodeCount = static_cast<int>(Bytecode::kLast) + 1;
+    bytecode_dispatch_count_table_.Reset(
+        new uintptr_t[kBytecodeCount * kBytecodeCount]);
+    memset(bytecode_dispatch_count_table_.get(), 0,
+           sizeof(uintptr_t) * kBytecodeCount * kBytecodeCount);
+  }
+
   // Generate bytecode handlers for all bytecodes and scales.
   for (OperandScale operand_scale = OperandScale::kSingle;
        operand_scale <= OperandScale::kMaxValid;
@@ -148,9 +158,11 @@ bool Interpreter::MakeBytecode(CompilationInfo* info) {
 }
 
 bool Interpreter::IsDispatchTableInitialized() {
-  if (FLAG_trace_ignition || FLAG_trace_ignition_codegen) {
-    // Regenerate table to add bytecode tracing operations
-    // or to print the assembly code generated by TurboFan.
+  if (FLAG_trace_ignition || FLAG_trace_ignition_codegen ||
+      FLAG_trace_ignition_dispatches) {
+    // Regenerate table to add bytecode tracing operations,
+    // print the assembly code generated by TurboFan,
+    // or instrument handlers with dispatch counters.
     return false;
   }
   return dispatch_table_[0] != nullptr;
@@ -178,6 +190,52 @@ const char* Interpreter::LookupNameOfBytecodeHandler(Code* code) {
   return nullptr;
 }
 
+void Interpreter::WriteDispatchCounters() {
+  std::ofstream stream(FLAG_trace_ignition_dispatches_output_file);
+  static const int kBytecodeCount = static_cast<int>(Bytecode::kLast) + 1;
+
+  // Output is a JSON-encoded object of objects.
+  //
+  // The keys on the top level object are source bytecodes,
+  // and corresponding value are objects. Keys on these last are the
+  // destinations of the dispatch and the value associated is a counter for
+  // the correspondent source-destination dispatch chain.
+  //
+  // Only non-zero counters are written to file, but an entry in the top-level
+  // object is always present, even if the value is empty because all counters
+  // for that source are zero.
+
+  stream << '{';
+
+  for (int from_index = 0; from_index < kBytecodeCount; ++from_index) {
+    if (from_index > 0) stream << ",\n ";
+
+    Bytecode from_bytecode = Bytecodes::FromByte(from_index);
+    stream << "\"" << Bytecodes::ToString(from_bytecode) << "\": {";
+
+    bool emitted_first = false;
+    for (int to_index = 0; to_index < kBytecodeCount; ++to_index) {
+      uintptr_t counter =
+          bytecode_dispatch_count_table_[from_index * kBytecodeCount +
+                                         to_index];
+      if (counter > 0) {
+        if (emitted_first) {
+          stream << ", ";
+        } else {
+          emitted_first = true;
+        }
+
+        Bytecode to_bytecode = Bytecodes::FromByte(to_index);
+        stream << '"' << Bytecodes::ToString(to_bytecode) << "\": " << counter;
+      }
+    }
+
+    stream << "}";
+  }
+
+  stream << '}';
+}
+
 // LdaZero
 //
 // Load literal '0' into the accumulator.