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

Always execute the implicit return if we fell off the function bytes.
This is not considered an additional "step" as it is not executing a
wasm instruction.
Otherwise, we might pause at an invalid position (one after the
function bytes).

R=titzer@chromium.org
BUG=v8:5822

Review-Url: https://codereview.chromium.org/2650293003
Cr-Commit-Position: refs/heads/master@{#42730}

src/wasm/wasm-interpreter.cc

@@ -1183,15 +1183,7 @@ class ThreadImpl {
 #define PAUSE_IF_BREAK_FLAG(flag) \
   if (V8_UNLIKELY(break_flags_ & WasmInterpreter::BreakFlag::flag)) max = 0;
 
-      if (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()))
-          return;
-        decoder.Reset(code->start, code->end);
-        PAUSE_IF_BREAK_FLAG(AfterReturn);
-        continue;
-      }
+      DCHECK_GT(limit, pc);
 
       const char* skip = "        ";
       int len = 1;
@@ -1635,6 +1627,14 @@ class ThreadImpl {
       }
 
       pc += len;
+      if (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()))
+          return;
+        decoder.Reset(code->start, code->end);
+        PAUSE_IF_BREAK_FLAG(AfterReturn);
+      }
     }
     // Set break_pc_, even though we might have stopped because max was reached.
     // We don't want to stop after executing zero instructions next time.

test/cctest/wasm/test-run-wasm-interpreter.cc

@@ -219,7 +219,7 @@ TEST(Breakpoint_I32Add) {
 }
 
 TEST(Step_I32Mul) {
-  static const int kTraceLength = 5;
+  static const int kTraceLength = 4;
   byte code[] = {WASM_I32_MUL(WASM_GET_LOCAL(0), WASM_GET_LOCAL(1))};
 
   WasmRunner<int32_t, uint32_t, uint32_t> r(kExecuteInterpreted);

test/inspector/debugger/wasm-stepping-expected.txt

@@ -77,7 +77,7 @@ Paused at wasm://wasm/wasm-ccfaf3fa/wasm-ccfaf3fa-0:2:2:   >nop
 Step action: stepInto
 Paused at wasm://wasm/wasm-ccfaf3fa/wasm-ccfaf3fa-0:3:0: >end
 Step action: stepInto
-Paused at wasm://wasm/wasm-ccfaf3fa/wasm-ccfaf3fa-0:0:0: >func $wasm_A
+Paused at wasm://wasm/wasm-ccfaf3fa/wasm-ccfaf3fa-1:9:6:       >br 1
 Step action: resume
 exports.main returned!
 Finished!