[turbofan] Visit reachable nodes at least once in loop analysis

Loop analysis never visited nodes whose marks happened to not change
when visiting their outputs in backwards propagation. This CL ensures
that each reachable node is visited at least once.

Change-Id: I70cd73737c0abe8151d5e23bc50525599fa3ea6a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2581538Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71938}

src/compiler/loop-analysis.cc

@@ -25,7 +25,8 @@ namespace compiler {
 // Temporary information for each node during marking.
 struct NodeInfo {
   Node* node;
-  NodeInfo* next;       // link in chaining loop members
+  NodeInfo* next;  // link in chaining loop members
+  bool backwards_visited;
 };
 
 
@@ -61,7 +62,7 @@ class LoopFinderImpl {
         end_(graph->end()),
         queue_(zone),
         queued_(graph, 2),
-        info_(graph->NodeCount(), {nullptr, nullptr}, zone),
+        info_(graph->NodeCount(), {nullptr, nullptr, false}, zone),
         loops_(zone),
         loop_num_(graph->NodeCount(), -1, zone),
         loop_tree_(loop_tree),
@@ -192,7 +193,7 @@ class LoopFinderImpl {
     while (!queue_.empty()) {
       tick_counter_->TickAndMaybeEnterSafepoint();
       Node* node = queue_.front();
-      info(node);
+      info(node).backwards_visited = true;
       queue_.pop_front();
       queued_.Set(node, false);
 
@@ -224,10 +225,18 @@ class LoopFinderImpl {
         Node* input = node->InputAt(i);
         if (IsBackedge(node, i)) {
           // Only propagate the loop mark on backedges.
-          if (SetBackwardMark(input, loop_num)) Queue(input);
+          if (SetBackwardMark(input, loop_num) ||
+              !info(input).backwards_visited) {
+            Queue(input);
+          }
         } else {
           // Entry or normal edge. Propagate all marks except loop_num.
-          if (PropagateBackwardMarks(node, input, loop_num)) Queue(input);
+          // TODO(manoskouk): Add test that needs backwards_visited to function
+          // correctly, probably using wasm loop unrolling when it is available.
+          if (PropagateBackwardMarks(node, input, loop_num) ||
+              !info(input).backwards_visited) {
+            Queue(input);
+          }
         }
       }
     }