Fix bug in flow-graph loop detection.

runtime/vm/flow_graph.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/intermediate_language.h"
 #include "vm/longjump.h"
@@ skipping 994 matching lines @@
         instr_it.RemoveCurrentFromGraph();
       }
     }
   }
 }
 
 
 // Find the natural loop for the back edge m->n and attach loop information
 // to block n (loop header). The algorithm is described in "Advanced Compiler
 // Design & Implementation" (Muchnick) p192.
-void FlowGraph::FindLoop(BlockEntryInstr* m, BlockEntryInstr* n) {
+BitVector* FlowGraph::FindLoop(BlockEntryInstr* m, BlockEntryInstr* n) {
   GrowableArray<BlockEntryInstr*> stack;
   BitVector* loop = new BitVector(preorder_.length());
 
   loop->Add(n->preorder_number());
   if (n != m) {
     loop->Add(m->preorder_number());
     stack.Add(m);
   }
 
   while (!stack.is_empty()) {
     BlockEntryInstr* p = stack.RemoveLast();
     for (intptr_t i = 0; i < p->PredecessorCount(); ++i) {
       BlockEntryInstr* q = p->PredecessorAt(i);
       if (!loop->Contains(q->preorder_number())) {
         loop->Add(q->preorder_number());
         stack.Add(q);
       }
     }
   }
-  n->set_loop_info(loop);
-  if (FLAG_trace_optimization) {
-    for (BitVector::Iterator it(loop); !it.Done(); it.Advance()) {
-      OS::Print("  B%" Pd "\n", preorder_[it.Current()]->block_id());
-    }
-  }
+  return loop;
 }
 
 
 ZoneGrowableArray<BlockEntryInstr*>* FlowGraph::ComputeLoops() {
   ZoneGrowableArray<BlockEntryInstr*>* loop_headers =
       new ZoneGrowableArray<BlockEntryInstr*>();
 
   for (BlockIterator it = postorder_iterator();
        !it.Done();
        it.Advance()) {
     BlockEntryInstr* block = it.Current();
     for (intptr_t i = 0; i < block->PredecessorCount(); ++i) {
       BlockEntryInstr* pred = block->PredecessorAt(i);
       if (block->Dominates(pred)) {
         if (FLAG_trace_optimization) {
           OS::Print("Back edge B%" Pd " -> B%" Pd "\n", pred->block_id(),
                     block->block_id());
         }
-        FindLoop(pred, block);
-        loop_headers->Add(block);
+        BitVector* loop_info = FindLoop(pred, block);
+        // Loops that share the same loop header are treated as one loop.
+        BlockEntryInstr* header = NULL;
+        for (intptr_t i = 0; i < loop_headers->length(); ++i) {
+          if ((*loop_headers)[i] == block) {
+            header = (*loop_headers)[i];
+            break;
+          }
+        }
+        if (header != NULL) {
+          header->loop_info()->AddAll(loop_info);
+        } else {
+          block->set_loop_info(loop_info);
+          loop_headers->Add(block);
+        }
       }
     }
   }
-
+  if (FLAG_trace_optimization) {
+    for (intptr_t i = 0; i < loop_headers->length(); ++i) {
+      BlockEntryInstr* header = (*loop_headers)[i];
+      OS::Print("Loop header B%" Pd "\n", header->block_id());
+      for (BitVector::Iterator it(header->loop_info());
+           !it.Done();
+           it.Advance()) {
+        OS::Print("  B%" Pd "\n", preorder_[it.Current()]->block_id());
+       }
+    }
+  }
   return loop_headers;
 }
 
 
 void FlowGraph::Bailout(const char* reason) const {
   const Function& function = parsed_function_.function();
   const Error& error = Error::Handle(
       LanguageError::NewFormatted(Error::Handle(),  // No previous error.
                                   Script::Handle(function.script()),
                                   function.token_pos(),
@@ skipping 123 matching lines @@
 }
 
 
 bool BlockEffects::IsSideEffectFreePath(BlockEntryInstr* from,
                                         BlockEntryInstr* to) const {
   return available_at_[to->postorder_number()]->Contains(
       from->postorder_number());
 }
 
 }  // namespace dart