Subzero: Slight improvement to phi lowering.

src/IceInst.cpp

 //===- subzero/src/IceInst.cpp - High-level instruction implementation ----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 165 matching lines @@
                     Liveness *Liveness, LiveBeginEndMap *LiveBegin,
                     LiveBeginEndMap *LiveEnd) {
   assert(!isDeleted());
 
   Dead = false;
   if (Dest) {
     SizeT VarNum = Liveness->getLiveIndex(Dest->getIndex());
     if (Live[VarNum]) {
       if (!isDestNonKillable()) {
         Live[VarNum] = false;
-        if (LiveBegin) {
+        if (LiveBegin && Liveness->getRangeMask(Dest->getIndex())) {
           LiveBegin->push_back(std::make_pair(VarNum, InstNumber));
         }
       }
     } else {
       if (!hasSideEffects())
         Dead = true;
     }
   }
   if (Dead)
     return false;
@@ skipping 19 matching lines @@
           // twice.  ICE only allows a variable to have a single
           // liveness interval in a basic block (except for blocks
           // where a variable is live-in and live-out but there is a
           // gap in the middle).  Therefore, this lowered sequence
           // needs to represent a single conservative live range for
           // t.  Since the instructions are being traversed backwards,
           // we make sure LiveEnd is only set once by setting it only
           // when LiveEnd[VarNum]==0 (sentinel value).  Note that it's
           // OK to set LiveBegin multiple times because of the
           // backwards traversal.
-          if (LiveEnd) {
+          if (LiveEnd && Liveness->getRangeMask(Var->getIndex())) {
             // Ideally, we would verify that VarNum wasn't already
             // added in this block, but this can't be done very
             // efficiently with LiveEnd as a vector.  Instead,
             // livenessPostprocess() verifies this after the vector
             // has been sorted.
             LiveEnd->push_back(std::make_pair(VarNum, InstNumber));
           }
         }
       }
     }
@@ skipping 737 matching lines @@
     // preserve these.
     return true;
   }
   if (!Dest->hasReg() && !SrcVar->hasReg() &&
       Dest->getStackOffset() == SrcVar->getStackOffset())
     return true;
   return false;
 }
 
 } // end of namespace Ice