[ReachingDefs] make ReachingDefDefaultVal smaller

[craig.topper]

LGTM

[yubing]

@craig.topper hi, Craig. why we don't set it as std::numeric_limits<int>::min() previously?

[jroelofs]

In D148198#4264234, @yubing wrote:

@craig.topper hi, Craig. why we don't set it as std::numeric_limits<int>::min() previously?

I think it can only go as small as -(1 << 30), since the pointer traits in ReachingDef takes the last two bits.

We have seen values between -(1<<21) and -(1<<20) in some workloads tripping the Defs must be sorted and unique assertion, so it may be prudent to set it to at least -(1<<22).

llvm/include/llvm/CodeGen/ReachingDefAnalysis.h

private:
   using MBBRegUnitDefs = TinyPtrVector<ReachingDef>;
   /// All reaching defs of all reg units for a given MBB
   using MBBDefsInfo = std::vector<MBBRegUnitDefs>;
   /// All reaching defs of all reg units for a all MBBs
   using MBBReachingDefsInfo = SmallVector<MBBDefsInfo, 4>;
   MBBReachingDefsInfo MBBReachingDefs;
 
   /// Default values are 'nothing happened a long time ago'.
-  const int ReachingDefDefaultVal = -(1 << 20);
+  const int ReachingDefDefaultVal = -(1 << 21);
 
   using InstSet = SmallPtrSetImpl<MachineInstr*>;
   using BlockSet = SmallPtrSetImpl<MachineBasicBlock*>;
 
 public:
   static char ID; // Pass identification, replacement for typeid
 
   ReachingDefAnalysis() : MachineFunctionPass(ID) {