Compute the loop nest depth of each CfgNode and weight Variables by it.

src/IceTargetLoweringX86BaseImpl.h

 //===- subzero/src/IceTargetLoweringX86BaseImpl.h - x86 lowering -*- C++ -*-==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 309 matching lines @@
       return;
     Func->placePhiStores();
     if (Func->hasError())
       return;
     Func->deletePhis();
     if (Func->hasError())
       return;
     Func->dump("After Phi lowering");
   }
 
+  // Run this early so it can be used to focus optimizations on potentially hot
+  // code.
+  // TODO(stichnot,ascull): currently only used for regalloc not expensive high
+  // level optimizations which could be focused on potentially hot code.
+  Func->computeLoopNestDepth();
+  Func->dump("After loop nest depth analysis");
+
   // Address mode optimization.
   Func->getVMetadata()->init(VMK_SingleDefs);
   Func->doAddressOpt();
 
   // Find read-modify-write opportunities.  Do this after address mode
   // optimization so that doAddressOpt() doesn't need to be applied to RMW
   // instructions as well.
   findRMW();
   Func->dump("After RMW transform");
 
@@ skipping 20 matching lines @@
   // Disable constant blinding or pooling for load optimization.
   {
     BoolFlagSaver B(RandomizationPoolingPaused, true);
     doLoadOpt();
   }
   Func->genCode();
   if (Func->hasError())
     return;
   Func->dump("After x86 codegen");
 
-  // Register allocation.  This requires instruction renumbering and full
-  // liveness analysis.
+  // Register allocation. This requires instruction renumbering and full
+  // liveness analysis. Loops must be identified before liveness so variable
+  // use weights are correct.
   Func->renumberInstructions();
   if (Func->hasError())
     return;
   Func->liveness(Liveness_Intervals);
   if (Func->hasError())
     return;
   // Validate the live range computations.  The expensive validation call is
   // deliberately only made when assertions are enabled.
   assert(Func->validateLiveness());
   // The post-codegen dump is done here, after liveness analysis and associated
@@ skipping 4982 matching lines @@
   }
   // the offset is not eligible for blinding or pooling, return the original
   // mem operand
   return MemOperand;
 }
 
 } // end of namespace X86Internal
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H