Index: lib/Target/X86/X86.td
===================================================================
--- lib/Target/X86/X86.td
+++ lib/Target/X86/X86.td
@@ -273,6 +273,13 @@
           "fast-shld-rotate", "HasFastSHLDRotate", "true",
           "SHLD can be used as a faster rotate">;
 
+// String operations (e.g. REP MOVS) are fast. See "REP String Enhancement" in
+// the Intel Software Development Manual.
+def FeatureFastString
+    : SubtargetFeature<
+          "fast-string", "HasFastString", "true",
+          "REP MOVS/STOS are fast">;
+
 //===----------------------------------------------------------------------===//
 // X86 processors supported.
 //===----------------------------------------------------------------------===//
@@ -498,6 +505,7 @@
   FeatureAVX2,
   FeatureBMI,
   FeatureBMI2,
+  FeatureFastString,
   FeatureFMA,
   FeatureLZCNT,
   FeatureMOVBE,
Index: lib/Target/X86/X86InstrInfo.td
===================================================================
--- lib/Target/X86/X86InstrInfo.td
+++ lib/Target/X86/X86InstrInfo.td
@@ -897,6 +897,7 @@
 def HasFastMem32 : Predicate<"!Subtarget->isUnalignedMem32Slow()">;
 def HasFastLZCNT : Predicate<"Subtarget->hasFastLZCNT()">;
 def HasFastSHLDRotate : Predicate<"Subtarget->hasFastSHLDRotate()">;
+def HasFastString : Predicate<"Subtarget->hasFastString()">;
 def HasMFence    : Predicate<"Subtarget->hasMFence()">;
 
 //===----------------------------------------------------------------------===//
Index: lib/Target/X86/X86SelectionDAGInfo.cpp
===================================================================
--- lib/Target/X86/X86SelectionDAGInfo.cpp
+++ lib/Target/X86/X86SelectionDAGInfo.cpp
@@ -229,7 +229,12 @@
     return SDValue();
 
   MVT AVT;
-  if (Align & 1)
+  if (Subtarget.hasFastString())
+    // If the target has fast strings, then it's at least as fast to use
+    // REP MOVSB instead of REP MOVS{W,D,Q}, and it avoids having to handle
+    // BytesLeft.
+    AVT = MVT::i8;
+  else if (Align & 1)
     AVT = MVT::i8;
   else if (Align & 2)
     AVT = MVT::i16;
Index: lib/Target/X86/X86Subtarget.h
===================================================================
--- lib/Target/X86/X86Subtarget.h
+++ lib/Target/X86/X86Subtarget.h
@@ -232,6 +232,9 @@
   /// True if SHLD based rotate is fast.
   bool HasFastSHLDRotate;
 
+  /// True if the processor has fast REP MOVS.
+  bool HasFastString;
+
   /// True if the short functions should be padded to prevent
   /// a stall when returning too early.
   bool PadShortFunctions;
@@ -472,6 +475,7 @@
   bool hasFastVectorFSQRT() const { return HasFastVectorFSQRT; }
   bool hasFastLZCNT() const { return HasFastLZCNT; }
   bool hasFastSHLDRotate() const { return HasFastSHLDRotate; }
+  bool hasFastString() const { return HasFastString; }
   bool hasSlowDivide32() const { return HasSlowDivide32; }
   bool hasSlowDivide64() const { return HasSlowDivide64; }
   bool padShortFunctions() const { return PadShortFunctions; }
Index: lib/Target/X86/X86Subtarget.cpp
===================================================================
--- lib/Target/X86/X86Subtarget.cpp
+++ lib/Target/X86/X86Subtarget.cpp
@@ -303,6 +303,7 @@
   HasFastVectorFSQRT = false;
   HasFastLZCNT = false;
   HasFastSHLDRotate = false;
+  HasFastString = false;
   HasSlowDivide32 = false;
   HasSlowDivide64 = false;
   PadShortFunctions = false;
Index: test/CodeGen/X86/memcpy-struct-by-value.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/memcpy-struct-by-value.ll
@@ -0,0 +1,15 @@
+; RUN: llc -mtriple=x86_64-linux-gnu -mattr=-fast-string < %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=NOFAST
+; RUN: llc -mtriple=x86_64-linux-gnu -mattr=+fast-string < %s -o - | FileCheck %s --check-prefix=ALL --check-prefix=FAST
+
+%struct.large = type { [4096 x i8] }
+
+declare void @foo(%struct.large* align 8 byval) nounwind
+
+define void @test1(%struct.large* nocapture %x) nounwind {
+  call void @foo(%struct.large* align 8 byval %x)
+  ret void
+
+; ALL-LABEL: test1:
+; NOFAST: rep;movsq
+; FAST: rep;movsb
+}