ARM64: initial backend import

This adds a second implementation of the AArch64 architecture to LLVM,
accessible in parallel via the "arm64" triple. The plan over the
coming weeks & months is to merge the two into a single backend,
during which time thorough code review should naturally occur.

Everything will be easier with the target in-tree though, hence this
commit.

llvm-svn: 205090
diff --git a/llvm/lib/MC/MCObjectFileInfo.cpp b/llvm/lib/MC/MCObjectFileInfo.cpp
index e808d0c..3b011c8 100644
--- a/llvm/lib/MC/MCObjectFileInfo.cpp
+++ b/llvm/lib/MC/MCObjectFileInfo.cpp
@@ -22,6 +22,9 @@
   IsFunctionEHFrameSymbolPrivate = false;
   SupportsWeakOmittedEHFrame = false;
 
+  if (T.isOSDarwin() && T.getArch() == Triple::arm64)
+    SupportsCompactUnwindWithoutEHFrame = true;
+
   PersonalityEncoding = dwarf::DW_EH_PE_indirect | dwarf::DW_EH_PE_pcrel
     | dwarf::DW_EH_PE_sdata4;
   LSDAEncoding = FDEEncoding = FDECFIEncoding = dwarf::DW_EH_PE_pcrel;
@@ -146,7 +149,8 @@
 
   COFFDebugSymbolsSection = 0;
 
-  if (T.isMacOSX() && !T.isMacOSXVersionLT(10, 6)) {
+  if ((T.isMacOSX() && !T.isMacOSXVersionLT(10, 6)) ||
+      (T.isOSDarwin() && T.getArch() == Triple::arm64)) {
     CompactUnwindSection =
       Ctx->getMachOSection("__LD", "__compact_unwind",
                            MachO::S_ATTR_DEBUG,
@@ -154,6 +158,8 @@
 
     if (T.getArch() == Triple::x86_64 || T.getArch() == Triple::x86)
       CompactUnwindDwarfEHFrameOnly = 0x04000000;
+    else if (T.getArch() == Triple::arm64)
+      CompactUnwindDwarfEHFrameOnly = 0x03000000;
   }
 
   // Debug Information.
@@ -763,6 +769,7 @@
   // cellspu-apple-darwin. Perhaps we should fix in Triple?
   if ((Arch == Triple::x86 || Arch == Triple::x86_64 ||
        Arch == Triple::arm || Arch == Triple::thumb ||
+       Arch == Triple::arm64 ||
        Arch == Triple::ppc || Arch == Triple::ppc64 ||
        Arch == Triple::UnknownArch) &&
       (T.isOSDarwin() || T.isOSBinFormatMachO())) {