[NFC][DSE] Clean up KnownNoReads and MemorySSAScanLimit in DSE

[yurai007]

Summary

Another simple cleanups set in DSE. CheckCache is removed since 1f1145006b32 and in consequence KnownNoReads is useless.
Also updated description of MemorySSAScanLimit which default value is 150 instead 100.

[xgupta]

LGTM, Thanks!

[fhahn]

LGTM, thanks!

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

 static cl::opt<bool>
 EnablePartialStoreMerging("enable-dse-partial-store-merging",
   cl::init(true), cl::Hidden,
   cl::desc("Enable partial store merging in DSE"));
 
 static cl::opt<unsigned>
     MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden,
                        cl::desc("The number of memory instructions to scan for "
-                                "dead store elimination (default = 100)"));
+                                "dead store elimination (default = 150)"));
 static cl::opt<unsigned> MemorySSAUpwardsStepLimit(
     "dse-memoryssa-walklimit", cl::init(90), cl::Hidden,
     cl::desc("The maximum number of steps while walking upwards to find "
              "MemoryDefs that may be killed (default = 90)"));
 
 static cl::opt<unsigned> MemorySSAPartialStoreLimit(
     "dse-memoryssa-partial-store-limit", cl::init(5), cl::Hidden,
     cl::desc("The maximum number candidates that only partially overwrite the "
getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess,
 
     SmallSetVector<MemoryAccess *, 32> WorkList;
     auto PushMemUses = [&WorkList](MemoryAccess *Acc) {
       for (Use &U : Acc->uses())
         WorkList.insert(cast<MemoryAccess>(U.getUser()));
     };
     PushMemUses(EarlierAccess);
 
-    // Optimistically collect all accesses for reads. If we do not find any
-    // read clobbers, add them to the cache.
-    SmallPtrSet<MemoryAccess *, 16> KnownNoReads;
-    if (!EarlierMemInst->mayReadFromMemory())
-      KnownNoReads.insert(EarlierAccess);
     // Check if EarlierDef may be read.
     for (unsigned I = 0; I < WorkList.size(); I++) {
       MemoryAccess *UseAccess = WorkList[I];
 
       LLVM_DEBUG(dbgs() << "   " << *UseAccess);
       // Bail out if the number of accesses to check exceeds the scan limit.
       if (ScanLimit < (WorkList.size() - I)) {
         LLVM_DEBUG(dbgs() << "\n    ...  hit scan limit\n");
         return None;
       }
       --ScanLimit;
       NumDomMemDefChecks++;
-      KnownNoReads.insert(UseAccess);
 
       if (isa<MemoryPhi>(UseAccess)) {
         if (any_of(KillingDefs, [this, UseAccess](Instruction *KI) {
               return DT.properlyDominates(KI->getParent(),
                                           UseAccess->getBlock());
             })) {
           LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n");
           continue;