Index: llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
@@ -53,6 +53,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BuildLibCalls.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 using namespace llvm;
@@ -769,6 +770,7 @@
     Value *MSP =
         M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(),
                                Int8PtrTy, Int8PtrTy, IntPtr, (void *)nullptr);
+    inferLibFuncAttributes(*M->getFunction("memset_pattern16"), *TLI);
 
     // Otherwise we should form a memset_pattern16.  PatternValue is known to be
     // an constant array of 16-bytes.  Plop the value into a mergable global.
Index: llvm/trunk/test/Transforms/LoopIdiom/basic.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopIdiom/basic.ll
+++ llvm/trunk/test/Transforms/LoopIdiom/basic.ll
@@ -564,3 +564,7 @@
 ; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* %X, i8 0, i64 10000, i32 1, i1 false)
 ; CHECK: ret void
 }
+
+; Validate that "memset_pattern" has the proper attributes.
+; CHECK: declare void @memset_pattern16(i8* nocapture, i8* nocapture readonly, i64) [[ATTRS:#[0-9]+]]
+; CHECK: [[ATTRS]] = { argmemonly }