[wasm] Interpreter: Don't pause on invalid position

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 0c2886e7b1a21de066f2f187b26f576034dfa9df..39dda2a717df8ed293ccd1533c3ac75b20e587c6 100644
--- a/src/wasm/wasm-interpreter.cc
+++ b/src/wasm/wasm-interpreter.cc
@@ -1179,11 +1179,12 @@ class ThreadImpl {
   void Execute(InterpreterCode* code, pc_t pc, int max) {
     Decoder decoder(code->start, code->end);
     pc_t limit = code->end - code->start;
-    while (--max >= 0) {
+    while (pc >= limit || --max >= 0) {

[titzer, 2017/01/25 12:25:06]

Hmm, that's weird. Why are we checking that the pc is out of bounds? (>=)?

[Clemens Hammacher, 2017/01/25 12:33:33]

To run into the if 4 lines below. I can also make it "pc == limit", if that's
more readable.
My first attempt was to move the "pc >= limit" check into the kExprEnd case, but
it also happens after a kExprBr or BrIf.

 #define PAUSE_IF_BREAK_FLAG(flag) \
   if (V8_UNLIKELY(break_flags_ & WasmInterpreter::BreakFlag::flag)) max = 0;
 
       if (pc >= limit) {
+        DCHECK_EQ(pc, limit);
         // Fell off end of code; do an implicit return.
         TRACE("@%-3zu: ImplicitReturn\n", pc);
         if (!DoReturn(&code, &pc, &limit, code->function->sig->return_count()))