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

lib/MC/MCNaCl.cpp

Index: lib/MC/MCNaCl.cpp
diff --git a/lib/MC/MCNaCl.cpp b/lib/MC/MCNaCl.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..b88d183453ec1d8eb493f7c10784bfc7c059e490
--- /dev/null
+++ b/lib/MC/MCNaCl.cpp
@@ -0,0 +1,76 @@
+//===- 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";

[Mark Seaborn, 2013/05/10 19:52:46]

static const char NoteNamespace[] = "NaCl";

That avoids a relocation.  Allegedly.

[Derek Schuff, 2013/05/10 22:59:26]

Done.

+
+namespace llvm {
+void initializeNaClMCStreamer(MCStreamer &Streamer, MCContext &Ctx,
+                              const Triple &TheTriple) {
+  assert(TheTriple.isOSNaCl());
+  errs() << "initializeNaClMCStreamer\n";

[Mark Seaborn, 2013/05/10 19:52:46]

Remove this debugging output

[Derek Schuff, 2013/05/10 22:59:26]

Done.

+  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,

[Mark Seaborn, 2013/05/10 19:52:46]

Nit: Indent by 4 rather than 6?

[Derek Schuff, 2013/05/10 22:59:26]

Done.

+        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(1, 4); // NT_VERSION or maybe just our version number
+  Streamer.EmitValueToAlignment(4);

[Mark Seaborn, 2013/05/10 19:52:46]

This one isn't needed, BTW.

[Derek Schuff, 2013/05/10 22:59:26]

Done.

+  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