Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -883,6 +883,9 @@
   if (auto *Calloc = foldMallocMemset(CI, B, *TLI))
     return Calloc;
 
+  if (CI->getCalledFunction()->isIntrinsic())
+    return nullptr;
+
   // memset(p, v, n) -> llvm.memset(align 1 p, v, n)
   Value *Val = B.CreateIntCast(CI->getArgOperand(1), B.getInt8Ty(), false);
   B.CreateMemSet(CI->getArgOperand(0), Val, CI->getArgOperand(2), 1);
@@ -2219,7 +2222,8 @@
       return optimizeLog(CI, Builder);
     case Intrinsic::sqrt:
       return optimizeSqrt(CI, Builder);
-    // TODO: Use foldMallocMemset() with memset intrinsic.
+    case Intrinsic::memset:
+      return optimizeMemSet(CI, Builder);
     default:
       return nullptr;
     }
@@ -2509,4 +2513,4 @@
 
 FortifiedLibCallSimplifier::FortifiedLibCallSimplifier(
     const TargetLibraryInfo *TLI, bool OnlyLowerUnknownSize)
-    : TLI(TLI), OnlyLowerUnknownSize(OnlyLowerUnknownSize) {}
+    : TLI(TLI), OnlyLowerUnknownSize(OnlyLowerUnknownSize) {}
\ No newline at end of file
Index: test/Transforms/InstCombine/memset-1.ll
===================================================================
--- test/Transforms/InstCombine/memset-1.ll
+++ test/Transforms/InstCombine/memset-1.ll
@@ -102,6 +102,15 @@
   ret i8* %memset
 }
 
+define i8* @memset_intrinstic_unchanged(i8* %ptr) #0 {
+; CHECK-LABEL: @memset_intrinstic_unchanged(
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* align 1 [[PTR:%.*]], i8 0, i32 10, i1 false)
+; CHECK-NEXT:    ret i8* [[PTR]]
+;
+  call void @llvm.memset.p0i8.i32(i8* %ptr, i8 0, i32 10, i32 1, i1 false)
+  ret i8* %ptr
+}
+
 attributes #0 = { nounwind ssp uwtable }
 attributes #1 = { nounwind }
 attributes #2 = { nounwind readnone }