[liftoff] Disable opcode merging when debugging

For debugging code, disable opcode merging. Otherwise, the effect of the
first merged opcode would not be observable when stepping.

R=thibaudm@chromium.org

Bug: v8:10350
Change-Id: Id656c9dee8f9676bf3d7881f3782e5ead76b5e71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306802
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68960}

src/wasm/baseline/liftoff-compiler.cc

@@ -1103,7 +1103,7 @@ class LiftoffCompiler {
                            kNoTrap)
       case kExprI32Eqz:
         DCHECK(decoder->lookahead(0, kExprI32Eqz));
-        if (decoder->lookahead(1, kExprBrIf)) {
+        if (decoder->lookahead(1, kExprBrIf) && !for_debugging_) {
           DCHECK(!has_outstanding_op());
           outstanding_op_ = kExprI32Eqz;
           break;
@@ -1213,7 +1213,7 @@ class LiftoffCompiler {
   template <WasmOpcode opcode>
   void EmitI32CmpOp(FullDecoder* decoder) {
     DCHECK(decoder->lookahead(0, opcode));
-    if (decoder->lookahead(1, kExprBrIf)) {
+    if (decoder->lookahead(1, kExprBrIf) && !for_debugging_) {
       DCHECK(!has_outstanding_op());
       outstanding_op_ = opcode;
       return;

test/inspector/debugger/wasm-stepping-no-opcode-merging-expected.txt

 Tests that Liftoff does not merge opcodes while stepping
-Warning: This test tests the current (incorrect) behaviour. It will be fixed in a follow-up CL.
 Setting breakpoint at offset 33.
 Paused at offset 33: [0]
 Paused at offset 35: [0, 0]
-Paused at offset 36: [0, 0]
+Paused at offset 36: [0, 1]
 Paused at offset 33: [-1]
 Paused at offset 35: [-1, -1]
-Paused at offset 36: [-1, -1]
+Paused at offset 36: [-1, 0]
 Paused at offset 38: [-1]
 Paused at offset 33: [13]
 Paused at offset 35: [13, 13]
-Paused at offset 36: [13, 13]
+Paused at offset 36: [13, 0]
 Paused at offset 38: [13]
 Finished.

test/inspector/debugger/wasm-stepping-no-opcode-merging.js

@@ -26,11 +26,6 @@ Protocol.Debugger.onPaused(printPauseLocationAndStep);
   WasmInspectorTest.instantiate(module_bytes);
   [, {params: {scriptId: wasm_script_id}}] = await Protocol.Debugger.onceScriptParsed(2);
 
-  // TODO(clemensb): Fix this.
-  InspectorTest.log(
-      'Warning: This test tests the current (incorrect) behaviour. ' +
-      'It will be fixed in a follow-up CL.');
-
   // Set a breakpoint at the beginning of 'fun'.
   const offset = fun.body_offset;
   InspectorTest.log(`Setting breakpoint at offset ${offset}.`);