Subzero: Fix a regalloc bug involving too-aggressive AllowRegisterOverlap.

src/IceTargetLoweringX8632.cpp

 //===- subzero/src/IceTargetLoweringX8632.cpp - x86-32 lowering -----------===//
 //
 //                        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 TargetLoweringX8632 class, which
@@ skipping 2597 matching lines @@
         (From->getType() == IceType_f32 || From->getType() == IceType_f64))
       // On x86, FP constants are lowered to mem operands.
       NeedsReg = true;
     if (NeedsReg) {
       From = copyToReg(From, RegNum);
     }
     return From;
   }
   if (Variable *Var = llvm::dyn_cast<Variable>(From)) {
     // We need a new physical register for the operand if:
-    //   Mem is not allowed and Var->getRegNum() is unknown, or
+    //   Mem is not allowed and Var isn't guaranteed a physical
+    //   register, or
     //   RegNum is required and Var->getRegNum() doesn't match.
-    if ((!(Allowed & Legal_Mem) && !Var->hasReg()) ||
+    bool WillHaveRegister =
+        (Var->hasReg() || Var->getWeight() == RegWeight::Inf);
+    if ((!(Allowed & Legal_Mem) && !WillHaveRegister) ||
         (RegNum != Variable::NoRegister && RegNum != Var->getRegNum())) {
       Variable *Reg = copyToReg(From, RegNum);
       if (RegNum == Variable::NoRegister) {
         Reg->setPreferredRegister(Var, AllowOverlap);
       }
       From = Reg;
     }
     return From;
   }
   llvm_unreachable("Unhandled operand kind in legalize()");
@@ skipping 204 matching lines @@
     for (SizeT i = 0; i < Size; ++i) {
       Str << "\t.byte\t" << (((unsigned)Data[i]) & 0xff) << "\n";
     }
     Str << "\t.size\t" << MangledName << ", " << Size << "\n";
   }
   Str << "\t" << (IsInternal ? ".local" : ".global") << "\t" << MangledName
       << "\n";
 }
 
 } // end of namespace Ice