Implement memcpy for mini_installer

On ARM64 builds of mini_installer the compiler generates calls to
memcpy. Since we don't link mini_installer with the CRT (to guarantee
that it is kept small) we have to implement this function.

Bug: 931856
Change-Id: I3e1909b6ebf6255e0da981f20282562c33547a9b
Reviewed-on: https://chromium-review.googlesource.com/c/1476527
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633843}

chrome/installer/mini_installer/mini_installer_exe_main.cc

@@ -4,6 +4,7 @@
 
 #include <stdint.h>
 
+#include "build/build_config.h"
 #include "chrome/installer/mini_installer/mini_installer.h"
 
 // http://blogs.msdn.com/oldnewthing/archive/2004/10/25/247180.aspx
@@ -27,6 +28,9 @@ extern "C" int WINAPI wWinMain(HINSTANCE /* instance */,
 }
 #endif
 
+// We don't link with the CRT so we have to implement CRT functions that the
+// compiler generates calls to.
+
 // VC Express editions don't come with the memset CRT obj file and linking to
 // the obj files between versions becomes a bit problematic. Therefore,
 // simply implement memset.
@@ -56,4 +60,22 @@ void* memset(void* dest, int c, size_t count) {
     *scan++ = static_cast<uint8_t>(c);
   return dest;
 }
+
+#if defined(_DEBUG) && defined(ARCH_CPU_ARM64)
+// The compiler generates calls to memcpy for ARM64 debug builds so we need to
+// supply a memcpy implementation in that configuration.
+// See comments above for why we do this incantation.
+#ifdef __clang__
+__attribute__((used))
+#else
+#pragma function(memcpy)
+#endif
+void* memcpy(void* destination, const void* source, size_t count) {
+  auto* dst = reinterpret_cast<uint8_t*>(destination);
+  auto* src = reinterpret_cast<const uint8_t*>(source);
+  while (count--)
+    *dst++ = *src++;
+  return destination;
+}
+#endif
 }  // extern "C"