Wrap back edge table in an iterator.

src/runtime.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 8529 matching lines @@
     // The top JS function is this one, the PC is somewhere in the
     // unoptimized code.
     JavaScriptFrameIterator it(isolate);
     JavaScriptFrame* frame = it.frame();
     ASSERT(frame->function() == *function);
     ASSERT(frame->LookupCode() == *unoptimized);
     ASSERT(unoptimized->contains(frame->pc()));
 
     // Use linear search of the unoptimized code's back edge table to find
     // the AST id matching the PC.
-    Address start = unoptimized->instruction_start();
-    unsigned target_pc_offset = static_cast<unsigned>(frame->pc() - start);
-    Address table_cursor = start + unoptimized->back_edge_table_offset();
-    uint32_t table_length = Memory::uint32_at(table_cursor);
-    table_cursor += kIntSize;
+    uint32_t target_pc_offset = frame->pc() - unoptimized->instruction_start();
     uint32_t loop_depth = 0;
-    for (unsigned i = 0; i < table_length; ++i) {
-      // Table entries are (AST id, pc offset) pairs.
-      uint32_t pc_offset = Memory::uint32_at(table_cursor + kIntSize);
-      if (pc_offset == target_pc_offset) {
-        ast_id = BailoutId(static_cast<int>(Memory::uint32_at(table_cursor)));
-        loop_depth = Memory::uint32_at(table_cursor + 2 * kIntSize);
+
+    for (FullCodeGenerator::BackEdgeTableIterator back_edges(*unoptimized);
+         !back_edges.Done();
+         back_edges.Next()) {
+      if (back_edges.pc_offset() == target_pc_offset) {
+        ast_id = back_edges.ast_id();
+        loop_depth = back_edges.loop_depth();
         break;
       }
-      table_cursor += FullCodeGenerator::kBackEdgeEntrySize;
     }
     ASSERT(!ast_id.IsNone());
+
     if (FLAG_trace_osr) {
       PrintF("[replacing on-stack at AST id %d, loop depth %d in ",
               ast_id.ToInt(), loop_depth);
       function->PrintName();
       PrintF("]\n");
     }
 
     // Try to compile the optimized code.  A true return value from
     // CompileOptimized means that compilation succeeded, not necessarily
     // that optimization succeeded.
@@ skipping 5584 matching lines @@
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags,
                                        "Runtime::PerformGC");
   }
 }
 
 
 } }  // namespace v8::internal