Subzero: Fix an Om1 crash from memset lowering.

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 3786 matching lines @@
 
   // Unlikely, but nothing to do if it does happen
   if (IsCountConst && CountValue == 0)
     return;
 
   // TODO(ascull): if the count is constant but val is not it would be possible
   // to inline by spreading the value across 4 bytes and accessing subregs e.g.
   // eax, ax and al.
   if (IsCountConst && IsValConst) {
     Variable *Base = legalizeToReg(Dest);
+    // Add a FakeUse in case Base is ultimately not used, e.g. it falls back to
+    // calling memset().  Otherwise Om1 register allocation fails because this
+    // infinite-weight variable has a definition but no uses.
+    Context.insert(InstFakeUse::create(Func, Base));

[ascull, 2015/08/05 23:15:16]

I'm doing some refactoring as I'm writing memcpy so I'll see if I can get
rid of this fake use.

[Jim Stichnoth, 2015/08/05 23:22:30]

That sounds great, thanks!

I originally tried to delay initializing Base until its actual use, but that
turned out to be messy and tricky, and caused other lit tests to fail if I
accidentally initialized it more than once.

 
     // 3 is the awkward size as it is too small for the vector or 32-bit
     // operations and will not work with lowerLeftOvers as there is no valid
     // overlap.
     if (CountValue == 3) {
       Constant *Offset = nullptr;
       auto *Mem =
           Traits::X86OperandMem::create(Func, IceType_i16, Base, Offset);
       _store(Ctx->getConstantInt16((ValValue << 8) | ValValue), Mem);
 
@@ skipping 1585 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