LLVM: Add ELF Note section to NaCl object files identifying them as such to gold

This is needed to switch the native linker to one based on upstream binutils
2.23
R=mseaborn@chromium.org
BUG= https://code.google.com/p/nativeclient/issues/detail?id=2971
also related to bug https://code.google.com/p/nativeclient/issues/detail?id=3424

Review URL: https://codereview.chromium.org/15067009

2 files changed, 75 insertions, 0 deletions

diff --git a/lib/MC/CMakeLists.txt b/lib/MC/CMakeLists.txt
index db882c020b..f2f78333b7 100644
--- a/lib/MC/CMakeLists.txt
+++ b/lib/MC/CMakeLists.txt
@@ -23,6 +23,7 @@ add_llvm_library(LLVMMC
   MCMachOStreamer.cpp
   MCMachObjectTargetWriter.cpp
   MCModule.cpp
+  MCNaCl.cpp
   MCNullStreamer.cpp
   MCObjectFileInfo.cpp
   MCObjectStreamer.cpp
diff --git a/lib/MC/MCNaCl.cpp b/lib/MC/MCNaCl.cpp
new file mode 100644
index 0000000000..4a6363d6c0
--- /dev/null
+++ b/lib/MC/MCNaCl.cpp
@@ -0,0 +1,74 @@
+//===- lib/MC/MCNaCl.cpp - NaCl-specific MC implementation ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/MC/MCNaCl.h"
+#include "llvm/ADT/Triple.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCSectionELF.h"
+#include "llvm/MC/MCStreamer.h"
+#include "llvm/Support/ELF.h"
+
+static const char NoteNamespace[] = "NaCl";
+
+namespace llvm {
+void initializeNaClMCStreamer(MCStreamer &Streamer, MCContext &Ctx,
+                              const Triple &TheTriple) {
+  assert(TheTriple.isOSNaCl());
+  const char *NoteName;
+  const char *NoteArch;
+  unsigned BundleAlign;
+  switch (TheTriple.getArch()) {
+    case Triple::arm:
+      NoteName = ".note.NaCl.ABI.arm";
+      NoteArch = "arm";
+      BundleAlign = 4;
+      break;
+    case Triple::mipsel:
+      NoteName = ".note.NaCl.ABI.mipsel";
+      NoteArch = "mipsel";
+      BundleAlign = 4;
+      break;
+    case Triple::x86:
+      NoteName = ".note.NaCl.ABI.x86-32";
+      NoteArch = "x86-32";
+      BundleAlign = 5;
+      break;
+    case Triple::x86_64:
+      NoteName = ".note.NaCl.ABI.x86-64";
+      NoteArch = "x86-64";
+      BundleAlign = 5;
+      break;
+    default:
+      report_fatal_error("Unsupported architecture for NaCl");
+  }
+
+  // Set bundle-alignment as required by the NaCl ABI for the target.
+  Streamer.EmitBundleAlignMode(BundleAlign);
+
+  // Emit an ELF Note section in its own COMDAT group which identifies NaCl
+  // object files to the gold linker, so it can use the NaCl layout.
+  const MCSection *Note = Ctx.getELFSection(
+      NoteName, ELF::SHT_NOTE, ELF::SHF_ALLOC | ELF::SHF_GROUP,
+      SectionKind::getReadOnly(), 0, NoteName);
+
+  // TODO(dschuff) This should probably use PushSection and PopSection, but
+  // PopSection will assert if there haven't been any other sections switched to
+  // yet.
+  Streamer.SwitchSection(Note);
+  Streamer.EmitIntValue(strlen(NoteNamespace) + 1, 4);
+  Streamer.EmitIntValue(strlen(NoteArch) + 1, 4);
+  Streamer.EmitIntValue(ELF::NT_VERSION, 4);
+  Streamer.EmitBytes(NoteNamespace);
+  Streamer.EmitIntValue(0, 1); // NUL terminator
+  Streamer.EmitValueToAlignment(4);
+  Streamer.EmitBytes(NoteArch);
+  Streamer.EmitIntValue(0, 1); // NUL terminator
+  Streamer.EmitValueToAlignment(4);
+}
+} // namespace llvm