[LiveDebugValues] Initialized variable to avoid msan reports

[vitalybuka]

Summary

Reproducible with =-1 and assert: https://reviews.llvm.org/P8309

[jmorse]

LGTM, thanks for catching that, it is indeed broken. I think there's only one path through the code that ends up reading that uninitialized, and the output from it shouldn't be used, so this is a safe way to fix it.

[vitalybuka]

remove fixme

[vitalybuka]

use -1

[jmorse]

Also LGTM

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.h

public:
   SmallSet<Register, 8> SPAliases;
 
   /// Unique-ification of spill. Used to number them -- their LocID number is
   /// the index in SpillLocs minus one plus NumRegs.
   UniqueVector<SpillLoc> SpillLocs;
 
   // If we discover a new machine location, assign it an mphi with this
   // block number.
-  unsigned CurBB = 0; // FIXME: https://reviews.llvm.org/D150420
+  unsigned CurBB = -1;
 
   /// Cached local copy of the number of registers the target has.
   unsigned NumRegs;
 
   /// Number of slot indexes the target has -- distinct segments of a stack
   /// slot that can take on the value of a subregister, when a super-register
   /// is written to the stack.
   unsigned NumSlotIdxes;
private:
   // objects, as we can't have null expressions.
   const DIExpression *EmptyExpr;
 
   /// Object to track machine locations as we step through a block. Could
   /// probably be a field rather than a pointer, as it's always used.
   MLocTracker *MTracker = nullptr;
 
   /// Number of the current block LiveDebugValues is stepping through.
-  unsigned CurBB;
+  unsigned CurBB = -1;
 
   /// Number of the current instruction LiveDebugValues is evaluating.
   unsigned CurInst;
 
   /// Variable tracker -- listens to DBG_VALUEs occurring as InstrRefBasedImpl
   /// steps through a block. Reads the values at each location from the
   /// MLocTracker object.
   VLocTracker *VTracker = nullptr;