[maglev] Fix JumpLoop to the current basic block

Drive-by improve some tracing too.

Bug: v8:7700
Change-Id: I52546a19c15ad1a6bbac1b15cdf8fba33dab1cb7
Fixed: chromium:1361345
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3886873
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83116}

src/maglev/maglev-graph-builder.cc

@@ -76,6 +76,10 @@ MaglevGraphBuilder::MaglevGraphBuilder(LocalIsolate* local_isolate,
     int offset = offset_and_info.first;
     const compiler::LoopInfo& loop_info = offset_and_info.second;
     const compiler::BytecodeLivenessState* liveness = GetInLivenessFor(offset);
+    DCHECK_NULL(merge_states_[offset]);
+    if (FLAG_trace_maglev_graph_building) {
+      std::cout << "- Creating loop merge state at @" << offset << std::endl;
+    }
     merge_states_[offset] = MergePointInterpreterFrameState::NewForLoop(
         *compilation_unit_, offset, NumPredecessors(offset), liveness,
         &loop_info);
@@ -88,6 +92,11 @@ MaglevGraphBuilder::MaglevGraphBuilder(LocalIsolate* local_isolate,
       const compiler::BytecodeLivenessState* liveness =
           GetInLivenessFor(offset);
       DCHECK_EQ(NumPredecessors(offset), 0);
+      DCHECK_NULL(merge_states_[offset]);
+      if (FLAG_trace_maglev_graph_building) {
+        std::cout << "- Creating exception merge state at @" << offset
+                  << std::endl;
+      }
       merge_states_[offset] = MergePointInterpreterFrameState::NewForCatchBlock(
           *compilation_unit_, liveness, offset);
     }
@@ -2503,7 +2512,7 @@ void MaglevGraphBuilder::VisitJumpLoop() {
                                BytecodeOffset(iterator_.current_offset()),
                                compilation_unit_);
   BasicBlock* block =
-      target == iterator_.current_offset()
+      target == block_offset_
           ? FinishBlock<JumpLoop>(next_offset(), {}, &jump_targets_[target])
           : FinishBlock<JumpLoop>(next_offset(), {},
                                   jump_targets_[target].block_ptr());
@@ -2573,6 +2582,9 @@ void MaglevGraphBuilder::MergeDeadIntoFrameState(int target) {
     // If this merge is the last one which kills a loop merge, remove that
     // merge state.
     if (merge_states_[target]->is_unreachable_loop()) {
+      if (FLAG_trace_maglev_graph_building) {
+        std::cout << "! Killing loop merge state at @" << target << std::endl;
+      }
       merge_states_[target] = nullptr;
     }
   }

src/maglev/maglev-graph-builder.h

@@ -112,10 +112,11 @@ class MaglevGraphBuilder {
 
     // Set up edge-split.
     int predecessor_index = merge_state.predecessor_count() - 1;
-    if (merge_state.is_unmerged_loop()) {
+    if (merge_state.is_loop()) {
       // For loops, the JumpLoop block hasn't been generated yet, and so isn't
       // in the list of jump targets. IT's the last predecessor, so drop the
       // index by one.
+      DCHECK(merge_state.is_unmerged_loop());
       predecessor_index--;
     }
     BasicBlockRef* old_jump_targets = jump_targets_[offset].Reset();
@@ -244,8 +245,9 @@ class MaglevGraphBuilder {
                            graph()->last_block(), offset);
       }
       if (FLAG_trace_maglev_graph_building) {
-        auto detail =
-            merge_state->is_exception_handler() ? "exception handler" : "merge";
+        auto detail = merge_state->is_exception_handler() ? "exception handler"
+                      : merge_state->is_loop()            ? "loop header"
+                                                          : "merge";
         std::cout << "== New block (" << detail << ") ==" << std::endl;
       }