[DSE][NFC] Introduce "doesn't overwrite" return code for isOverwrite

Add OR_None code to indicate that there is no overwrite. This has no any effect for current uses but will be used in one of the next patches building support for PHI translation.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D105098

Author: Evgeniy Brevnov

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -256,6 +256,7 @@ enum OverwriteResult {
   OW_End,
   OW_PartialEarlierWithFullLater,
   OW_MaybePartial,
+  OW_None,
   OW_Unknown
 };
 
@@ -849,6 +850,7 @@ struct DSEState {
   /// Return OW_MaybePartial if \p KillingI does not completely overwrite
   /// \p DeadI, but they both write to the same underlying object. In that
   /// case, use isPartialOverwrite to check if \p KillingI partially overwrites
+  /// \p DeadI. Returns 'OR_None' if \p KillingI is known to not overwrite the
   /// \p DeadI. Returns 'OW_Unknown' if nothing can be determined.
   OverwriteResult isOverwrite(const Instruction *KillingI,
                               const Instruction *DeadI,
@@ -911,8 +913,16 @@ struct DSEState {
 
     // If we can't resolve the same pointers to the same object, then we can't
     // analyze them at all.
-    if (DeadUndObj != KillingUndObj)
+    if (DeadUndObj != KillingUndObj) {
+      // Non aliasing stores to different objects don't overlap. Note that
+      // if the killing store is known to overwrite whole object (out of
+      // bounds access overwrites whole object as well) then it is assumed to
+      // completely overwrite any store to the same object even if they don't
+      // actually alias (see next check).
+      if (AAR == AliasResult::NoAlias)
+        return OW_None;
       return OW_Unknown;
+    }
 
     // If the KillingI store is to a recognizable object, get its size.
     uint64_t KillingUndObjSize = getPointerSize(KillingUndObj, DL, TLI, &F);
@@ -966,9 +976,8 @@ struct DSEState {
       return OW_MaybePartial;
     }
 
-    // Can reach here only if accesses are known not to overlap. There is no
-    // dedicated code to indicate no overlap so signal "unknown".
-    return OW_Unknown;
+    // Can reach here only if accesses are known not to overlap.
+    return OW_None;
   }
 
   bool isInvisibleToCallerAfterRet(const Value *V) {
@@ -1368,7 +1377,7 @@ struct DSEState {
                               KillingOffset, DeadOffset);
         // If Current does not write to the same object as KillingDef, check
         // the next candidate.
-        if (OR == OW_Unknown)
+        if (OR == OW_Unknown || OR == OW_None)
           continue;
         else if (OR == OW_MaybePartial) {
           // If KillingDef only partially overwrites Current, check the next
@@ -1377,6 +1386,7 @@ struct DSEState {
           // which are less likely to be removable in the end.
           if (PartialLimit <= 1) {
             WalkerStepLimit -= 1;
+            LLVM_DEBUG(dbgs() << "   ... reached partial limit ... continue with next access\n");
             continue;
           }
           PartialLimit -= 1;