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

lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp

Index: lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
diff --git a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
index c67bb944b6f5edfd9df0b8faf69f0a13c18c4cd9..03e000c54270f52a8dbd75724ed256288b353fc1 100644
--- a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
+++ b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
@@ -17,12 +17,15 @@
 #include "X86MCAsmInfo.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/MC/MCCodeGenInfo.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCInstrAnalysis.h"
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/MC/MCRegisterInfo.h"
+#include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/MachineLocation.h"
+#include "llvm/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/TargetRegistry.h"
@@ -369,8 +372,31 @@ static MCStreamer *createMCStreamer(const Target &T, StringRef TT,
   // @LOCALMOD-BEGIN

[eliben, 2013/05/09 21:54:59]

I wonder why the LOCALMOD is here, createELFStreamer seems to be the same
upstream

[Derek Schuff, 2013/05/10 18:35:30]

I removed this localmod entirely in the cleanup.

   MCStreamer *Streamer = createELFStreamer(Ctx, MAB, _OS, _Emitter,
                                            RelaxAll, NoExecStack);
-  if (TheTriple.isOSNaCl())
+  if (TheTriple.isOSNaCl()) {

[eliben, 2013/05/09 21:54:59]

Move this to a separate function with a descriptive name.

Looking around even more, perhaps a cleaner LOCALMOD would be to have our own
createNaClELFStreamer that only handles the ELF case, and have it registered in 
  TargetRegistry::RegisterMCObjectStreamer

if the OS is NaCl. This way we don't have to touch the upstreamer
createMCStreamer at all.

[Derek Schuff, 2013/05/10 18:35:30]

That actually doesn't work because registration only distinguishes between
(arch) targets and not OSes.
But we now have our own init...

     Streamer->EmitBundleAlignMode(5);
+    const char *NoteName = TheTriple.isArch32Bit() ?
+        ".note.NaCl.ABI.x86-32" : ".note.NaCl.ABI.x86-64";
+    const char *NoteNamespace = "NaCl";
+    const char *NoteArch = TheTriple.isArch32Bit() ? "x86-32" : "x86-64";
+    const MCSection *Note = Ctx.getELFSection(
+        NoteName, ELF::SHT_NOTE, ELF::SHF_ALLOC | ELF::SHF_GROUP,
+        SectionKind::getReadOnly(), 0, NoteName);
+
+    Streamer->reset();
+    // maybe should use pushSection and popSection but popSection will crash if
+    // there haven't been any other sections switched to yet.
+    Streamer->SwitchSection(Note);
+    Streamer->EmitIntValue(5, 4); // sizeof("NaCl")
+    Streamer->EmitIntValue(7, 4); // sizeof("x86-xx")
+    Streamer->EmitIntValue(1, 4); // NT_VERSION
+    Streamer->EmitValueToAlignment(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);
+  }
 
   return Streamer;
   // @LOCALMOD-END