Writing NewStore to MemRefs for correct isSafeToMergeLoad working.

Change-Id: Ib8e33d71716123e45d7a6b70d4df40c257ff0282

Author: aratajew

IGC/Compiler/CISACodeGen/MemOpt.cpp

@@ -1142,13 +1142,28 @@ bool MemOpt::mergeStore(StoreInst* LeadingStore,
     for (auto& I : StoresToMerge) {
         StoreInst* ST = cast<StoreInst>(std::get<0>(I));
         Value* Ptr = ST->getPointerOperand();
+        // Stores merged in the previous iterations can get merged again, so we need
+        // to update ToOpt vector to avoid null instruction in there
+        ToOpt.erase(std::remove(ToOpt.begin(), ToOpt.end(), ST), ToOpt.end());
         ST->eraseFromParent();
         RecursivelyDeleteTriviallyDeadInstructions(Ptr);
-        // Mark it as already merged.
-        std::get<2>(I)->first = nullptr;
-        // Checking zero distance is intentionally omitted here due to the first
-        // memory access won't be able to be checked again.
-        std::get<2>(I)->second -= 1;
+
+        if (std::get<2>(I)->first == TailingStore)
+            // Writing NewStore to MemRefs for correct isSafeToMergeLoad working.
+            // For example if MemRefs contains this sequence: S1, S2, S3, L5, L6, L7, S4, L4
+            // after stores merge MemRefs contains : L5, L6, L7, S1234, L4 and loads are
+            // merged to L567, final instructions instructions sequence is L567, S1234, L4.
+            // Otherwise the sequence could be merged to sequence L4567, S1234 with
+            // unordered L4,S4 accesses.
+            std::get<2>(I)->first = NewStore;
+        else {
+            // Mark it as already merged.
+            std::get<2>(I)->first = nullptr;
+            // Checking zero distance is intentionally omitted here due to the first
+            // memory access won't be able to be checked again.
+            std::get<2>(I)->second -= 1;
+        }
+
     }
 
     return true;