diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 261e9a37ecb55..d44e14a35cac8 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -109,7 +109,9 @@ void MCObjectStreamer::addSpecialFragment(MCFragment *Frag) {
 void MCObjectStreamer::appendContents(ArrayRef<char> Contents) {
   ensureHeadroom(Contents.size());
   assert(FragSpace >= Contents.size());
-  llvm::copy(Contents, getCurFragEnd());
+  // As this is performance-sensitive code, explicitly use std::memcpy.
+  // Optimization of std::copy to memmove is unreliable.
+  std::memcpy(getCurFragEnd(), Contents.begin(), Contents.size());
   CurFrag->FixedSize += Contents.size();
   FragSpace -= Contents.size();
 }