[liftoff][debug] Implement StepOver at return locations

A StepOver at a return (either explicit return instruction, or implicit
return at the end of the function) should stop again in the caller
frame.

R=thibaudm@chromium.org

Bug: v8:10321
Change-Id: I313e6b612ac52e73b33ef07c6da1ced2aa0db600
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2110250
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66852}

src/wasm/wasm-debug.cc

@@ -709,6 +709,15 @@ class DebugInfoImpl {
     DCHECK(!it.done());
     DCHECK(it.frame()->is_wasm_compiled());
     WasmCompiledFrame* frame = WasmCompiledFrame::cast(it.frame());
+
+    // If we are at a return instruction, then any stepping action is equivalent
+    // to StepOut, and we need to flood the parent function.
+    if (IsAtReturn(frame)) {
+      it.Advance();
+      if (it.done() || !it.frame()->is_wasm_compiled()) return;
+      frame = WasmCompiledFrame::cast(it.frame());
+    }
+
     if (static_cast<int>(frame->function_index()) != flooded_function_index_) {
       FloodWithBreakpoints(frame->function_index(), isolate);
       flooded_function_index_ = frame->function_index();
@@ -862,6 +871,19 @@ class DebugInfoImpl {
     }
   }
 
+  bool IsAtReturn(WasmCompiledFrame* frame) {
+    DisallowHeapAllocation no_gc;
+    int position = frame->position();
+    NativeModule* native_module =
+        frame->wasm_instance().module_object().native_module();
+    uint8_t opcode = native_module->wire_bytes()[position];
+    if (opcode == kExprReturn) return true;
+    // Another implicit return is at the last kExprEnd in the function body.
+    int func_index = frame->function_index();
+    WireBytesRef code = native_module->module()->functions[func_index].code;
+    return static_cast<size_t>(position) == code.end_offset() - 1;
+  }
+
   NativeModule* const native_module_;
 
   // {mutex_} protects all fields below.

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

@@ -25,9 +25,33 @@ Paused at wasm://wasm/42af3c82:0:53
 Debugger.stepOver called
 Paused at wasm://wasm/42af3c82:0:54
 Debugger.stepOver called
+Paused at wasm://wasm/42af3c82:0:75
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:59
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:61
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:63
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:65
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:67
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:68
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:70
+Debugger.stepInto called
 Paused at wasm://wasm/42af3c82:0:73
-Debugger.resume called
+Debugger.stepInto called
 Paused at wasm://wasm/42af3c82:0:52
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:53
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:54
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:75
+Debugger.stepInto called
+Paused at wasm://wasm/42af3c82:0:59
 Debugger.resume called
 Paused at :0:24
 Debugger.resume called

test/inspector/debugger/wasm-stepping-liftoff.js

@@ -102,11 +102,14 @@ function instantiate(bytes) {
   // Step out of wasm_A, back to wasm_B.
   // TODO(clemensb/thibaudm): Replace by an actual 'stepOut'.
   for (let i = 0; i < 3; ++i) await waitForPauseAndStep('stepOver');
-  // Step over 10 times.
+  // Now step 10 times, until we are in wasm_A again.
-  // TODO(clemensb/thibaudm): Reenable once stepping over return works.
+  for (let i = 0; i < 10; ++i) await waitForPauseAndStep('stepInto');
-  //for (let i = 0; i < 10; ++i) await waitForPauseAndStep('stepOver');
+  // 3 more times, back to wasm_B.
-  // Resume three more times to continue over other breakpoints.
+  for (let i = 0; i < 3; ++i) await waitForPauseAndStep('stepInto');
-  for (let i = 0; i < 3; ++i) await waitForPauseAndStep('resume');
+  // Then just resume.
+  await waitForPauseAndStep('resume');
+  // TODO(clemensb/thibaudm): Figure out what this last break is (at ":0:24").
+  await waitForPauseAndStep('resume');
   InspectorTest.log('exports.main returned!');
 
   InspectorTest.log('Test stepping over a recursive call');