Subzero: Fix an Om1 crash from memset lowering.

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 245861cce9e7aac0533bc3f49570f1248a0c6168..73c0075ad26c5de9cca67979823ca1426c151ae5 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -3804,6 +3804,10 @@ void TargetX86Base<Machine>::lowerMemset(Operand *Dest, Operand *Val,
   // 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