Index: llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -231,10 +231,27 @@
     }
     }
   }
-  if (auto *CB = dyn_cast<CallBase>(Inst))
+
+  if (auto *CB = dyn_cast<CallBase>(Inst)) {
+    LibFunc LF;
+    if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
+      switch (LF) {
+      case LibFunc_strncpy:
+        if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
+          return MemoryLocation(CB->getArgOperand(0),
+                                LocationSize::precise(Len->getZExtValue()),
+                                CB->getAAMetadata());
+        LLVM_FALLTHROUGH;
+      default:
+        break;
+      }
+    }
+
     // All the supported TLI functions so far happen to have dest as their
     // first argument.
     return MemoryLocation::getAfter(CB->getArgOperand(0));
+  }
+
   return MemoryLocation();
 }
 
@@ -1114,8 +1131,13 @@
       LibFunc LF;
       if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
         switch (LF) {
-        case LibFunc_strcpy:
         case LibFunc_strncpy:
+          if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2)))
+            return MemoryLocation(CB->getArgOperand(0),
+                                  LocationSize::precise(Len->getZExtValue()),
+                                  CB->getAAMetadata());
+          LLVM_FALLTHROUGH;
+        case LibFunc_strcpy:
         case LibFunc_strcat:
         case LibFunc_strncat:
           return {MemoryLocation::getAfter(CB->getArgOperand(0))};
Index: llvm/test/Transforms/DeadStoreElimination/memset-and-strncpy.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/DeadStoreElimination/memset-and-strncpy.ll
@@ -0,0 +1,79 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -inferattrs -dse -S | FileCheck %s
+
+declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i1) nounwind
+declare i8* @strncpy(i8* noalias nocapture, i8* noalias nocapture readonly, i64)
+
+; Test that strncpy/memset overwriting each other is optimized out
+
+; strncpy -> memset, full overwrite
+define void @test1(i8* noalias %out, i8* noalias %in) {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT:%.*]], i8 42, i64 100, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 100)
+  tail call void @llvm.memset.p0i8.i64(i8* %out, i8 42, i64 100, i1 false)
+  ret void
+}
+
+; strncpy -> memset, partial overwrite
+define void @test2(i8* noalias %out, i8* noalias %in) {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT:%.*]], i8* [[IN:%.*]], i64 100)
+; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT]], i8 42, i64 99, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 100)
+  tail call void @llvm.memset.p0i8.i64(i8* %out, i8 42, i64 99, i1 false)
+  ret void
+}
+
+; strncpy -> memset, different destination
+define void @test3(i8* noalias %out1, i8* noalias %out2, i8* noalias %in) {
+; CHECK-LABEL: @test3(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT1:%.*]], i8* [[IN:%.*]], i64 100)
+; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT2:%.*]], i8 42, i64 100, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %call = tail call i8* @strncpy(i8* %out1, i8* %in, i64 100)
+  tail call void @llvm.memset.p0i8.i64(i8* %out2, i8 42, i64 100, i1 false)
+  ret void
+}
+
+
+; memset -> strncpy, full overwrite
+define void @test4(i8* noalias %out, i8* noalias %in) {
+; CHECK-LABEL: @test4(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT:%.*]], i8* [[IN:%.*]], i64 100)
+; CHECK-NEXT:    ret void
+;
+  tail call void @llvm.memset.p0i8.i64(i8* %out, i8 42, i64 100, i1 false)
+  %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 100)
+  ret void
+}
+
+; memset -> strncpy, partial overwrite
+define void @test5(i8* noalias %out, i8* noalias %in) {
+; CHECK-LABEL: @test5(
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, i8* [[OUT:%.*]], i64 99
+; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* align 1 [[TMP1]], i8 42, i64 1, i1 false)
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT]], i8* [[IN:%.*]], i64 99)
+; CHECK-NEXT:    ret void
+;
+  tail call void @llvm.memset.p0i8.i64(i8* %out, i8 42, i64 100, i1 false)
+  %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 99)
+  ret void
+}
+
+; memset -> strncpy, different destination
+define void @test6(i8* noalias %out1, i8* noalias %out2, i8* noalias %in) {
+; CHECK-LABEL: @test6(
+; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* [[OUT1:%.*]], i8 42, i64 100, i1 false)
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT2:%.*]], i8* [[IN:%.*]], i64 100)
+; CHECK-NEXT:    ret void
+;
+  tail call void @llvm.memset.p0i8.i64(i8* %out1, i8 42, i64 100, i1 false)
+  %call = tail call i8* @strncpy(i8* %out2, i8* %in, i64 100)
+  ret void
+}