[csa] More conservative propagation of flag marking blocks needing frames

Specifically, don't propage "needs_frame" up through non-deferred -> deferred
block transitions where there are multiple edges from the non-deferred to
deferred code.

LOG=N
R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2606893002
Cr-Commit-Position: refs/heads/master@{#41972}

src/compiler/frame-elider.cc

@@ -114,13 +114,36 @@ bool FrameElider::PropagateIntoBlock(InstructionBlock* block) {
     }
   }
 
-  // Propagate towards start ("upwards") if there are successors and all of
-  // them need a frame.
-  for (RpoNumber& succ : block->successors()) {
-    if (!InstructionBlockAt(succ)->needs_frame()) return false;
+  // Propagate towards start ("upwards")
+  bool need_frame_successors = false;
+  if (block->SuccessorCount() == 1) {
+    // For single successors, propagate the needs_frame information.
+    need_frame_successors =
+        InstructionBlockAt(block->successors()[0])->needs_frame();
+  } else {
+    // For multiple successors, each successor must only have a single
+    // predecessor (because the graph is in edge-split form), so each successor
+    // can independently create/dismantle a frame if needed. Given this
+    // independent control, only propagate needs_frame if all non-deferred
+    // blocks need a frame.
+    for (RpoNumber& succ : block->successors()) {
+      InstructionBlock* successor_block = InstructionBlockAt(succ);
+      DCHECK_EQ(1, successor_block->PredecessorCount());
+      if (!successor_block->IsDeferred()) {
+        if (successor_block->needs_frame()) {
+          need_frame_successors = true;
+        } else {
+          return false;
+        }
+      }
+    }
+  }
+  if (need_frame_successors) {
+    block->mark_needs_frame();
+    return true;
+  } else {
+    return false;
   }
-  block->mark_needs_frame();
-  return true;
 }