[liftoff] Add some more code comments

Add some more code comments for code snippets that are not obvious,
especially if debug code is enabled.
The comments help when looking at Liftoff code for debugging code
generation issues.

R=thibaudm@chromium.org

Change-Id: I566bf2b05a454fb8addc030359969d36cb2cb707
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2108557Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66791}

src/wasm/baseline/liftoff-compiler.cc

@@ -488,6 +488,7 @@ class LiftoffCompiler {
   }
 
   void StackCheck(WasmCodePosition position) {
+    DEBUG_CODE_COMMENT("stack check");
     if (!FLAG_wasm_stack_checks || !env_->runtime_exception_support) return;
     out_of_line_code_.push_back(OutOfLineCode::StackCheck(
         position, __ cache_state()->used_registers,
@@ -540,6 +541,7 @@ class LiftoffCompiler {
 
     __ CodeEntry();
 
+    DEBUG_CODE_COMMENT("enter frame");
     __ EnterFrame(StackFrame::WASM_COMPILED);
     __ set_has_frame(true);
     pc_offset_stack_frame_construction_ = __ PrepareStackFrame();
@@ -552,6 +554,7 @@ class LiftoffCompiler {
     if (DidAssemblerBailout(decoder)) return;
 
     // Process parameters.
+    if (num_params) DEBUG_CODE_COMMENT("process parameters");
     __ SpillInstance(instance_reg);
     // Input 0 is the code target, 1 is the instance. First parameter at 2.
     uint32_t input_idx = kInstanceParameterIndex + 1;
@@ -562,6 +565,7 @@ class LiftoffCompiler {
     DCHECK_EQ(input_idx, descriptor_->InputCount());
 
     // Initialize locals beyond parameters.
+    if (num_params < __ num_locals()) DEBUG_CODE_COMMENT("init locals");
     if (SpillLocalsInitially(decoder, num_params)) {
       for (uint32_t param_idx = num_params; param_idx < __ num_locals();
            ++param_idx) {
@@ -632,6 +636,7 @@ class LiftoffCompiler {
       // In this mode, we never generate stack checks.
       DCHECK(!is_stack_check);
       __ CallTrapCallbackForTesting();
+      DEBUG_CODE_COMMENT("leave frame");
       __ LeaveFrame(StackFrame::WASM_COMPILED);
       __ DropStackSlotsAndRet(
           static_cast<uint32_t>(descriptor_->StackParameterCount()));
@@ -1480,6 +1485,7 @@ class LiftoffCompiler {
       return unsupported(decoder, kMultiValue, "multi-return");
     }
     if (num_returns > 0) __ MoveToReturnRegisters(decoder->sig_);
+    DEBUG_CODE_COMMENT("leave frame");
     __ LeaveFrame(StackFrame::WASM_COMPILED);
     __ DropStackSlotsAndRet(
         static_cast<uint32_t>(descriptor_->StackParameterCount()));