[wasm] Implement stepping in wasm code

src/wasm/wasm-interpreter.h

Index: src/wasm/wasm-interpreter.h
diff --git a/src/wasm/wasm-interpreter.h b/src/wasm/wasm-interpreter.h
index 7162f22b320770350c7fd5216af5bb09537ce054..f0167d03e7a9c3b76c18f27e7094ed9d8990ef8a 100644
--- a/src/wasm/wasm-interpreter.h
+++ b/src/wasm/wasm-interpreter.h
@@ -120,6 +120,13 @@ class V8_EXPORT_PRIVATE WasmInterpreter {
   //                       +------------- Finish -------------> FINISHED
   enum State { STOPPED, RUNNING, PAUSED, FINISHED, TRAPPED };
 
+  // Tells a thread to pause after certain instructions.

[titzer, 2017/01/23 10:10:10]

It should also be possible to implement this outside the interpreter, simply by
iterating the code and installing breakpoints at returns and calls. I'm not sure
if it's simpler until we try, though.

[Clemens Hammacher, 2017/01/23 12:25:34]

This is basically what JS does: Flood the function(s) with breakpoints, and
later clear them again. We would have to be careful tough to only clear those
breakpoints related to stepping.
This flag seemed simpler to me, as it allows to just set the flag and continue
execution. JS has a similar thing (hook_on_function_call_) to efficiently break
on each function call.

+  enum BreakFlag : uint8_t {
+    None = 0,
+    AfterReturn = 1 << 0,
+    AfterCall = 1 << 1
+  };
+
   // Representation of a thread in the interpreter.
   class V8_EXPORT_PRIVATE Thread {
     // Don't instante Threads; they will be allocated as ThreadImpl in the
@@ -150,6 +157,9 @@ class V8_EXPORT_PRIVATE WasmInterpreter {
     // TODO(wasm): Implement this once we support multiple threads.
     // bool SetBreakpoint(const WasmFunction* function, int pc, bool enabled);
     // bool GetBreakpoint(const WasmFunction* function, int pc);
+
+    void AddBreakFlags(uint8_t flags);
+    void ClearBreakFlags();
   };
 
   WasmInterpreter(const ModuleBytesEnv& env, AccountingAllocator* allocator);