Subzero: Fix a regalloc eviction bug.

src/IceRegAlloc.cpp

 //===- subzero/src/IceRegAlloc.cpp - Linear-scan implementation -----------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the LinearScan class, which performs the
@@ skipping 327 matching lines @@
             Active.erase(I);
             Handled.push_back(Item);
           }
         }
         // Do the same for Inactive.
         for (UnorderedRanges::iterator I = Inactive.begin(), E = Inactive.end();
              I != E; I = Next) {
           Next = I;
           ++Next;
           LiveRangeWrapper Item = *I;
-          if (Item.Var->getRegNumTmp() == MinWeightIndex) {
+          // Note: The Item.overlaps(Cur) clause is not part of the
+          // description of AssignMemLoc() in the original paper.  But
+          // there doesn't seem to be any need to evict an inactive
+          // live range that doesn't overlap with the live range
+          // currently being considered.  It's especially bad if we
+          // would end up evicting an infinite-weight but
+          // currently-inactive live range.  The most common situation
+          // for this would be a scratch register kill set for call
+          // instructions.
+          if (Item.Var->getRegNumTmp() == MinWeightIndex &&
+              Item.overlaps(Cur)) {
             if (Func->getContext()->isVerbose(IceV_LinearScan)) {
               Str << "Evicting     ";
               Item.dump(Func);
               Str << "\n";
             }
             Item.Var->setRegNumTmp(Variable::NoRegister);
             Inactive.erase(I);
             Handled.push_back(Item);
           }
         }
@@ skipping 99 matching lines @@
   }
   Str << "++++++ Inactive:\n";
   for (UnorderedRanges::const_iterator I = Inactive.begin(), E = Inactive.end();
        I != E; ++I) {
     I->dump(Func);
     Str << "\n";
   }
 }
 
 } // end of namespace Ice