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

lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp

 //===-- X86MCTargetDesc.cpp - X86 Target Descriptions ---------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file provides X86 specific target descriptions.
 //
 //===----------------------------------------------------------------------===//
 
 #include "X86MCTargetDesc.h"
 #include "InstPrinter/X86ATTInstPrinter.h"
 #include "InstPrinter/X86IntelInstPrinter.h"
 #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"
 
 #define GET_REGINFO_MC_DESC
 #include "X86GenRegisterInfo.inc"
 
 #define GET_INSTRINFO_MC_DESC
 #include "X86GenInstrInfo.inc"
 
@@ skipping 323 matching lines @@
                                     bool RelaxAll,
                                     bool NoExecStack) {
   Triple TheTriple(TT);
 
   if (TheTriple.isOSDarwin() || TheTriple.getEnvironment() == Triple::MachO)
     return createMachOStreamer(Ctx, MAB, _OS, _Emitter, RelaxAll);
 
   if (TheTriple.isOSWindows() && TheTriple.getEnvironment() != Triple::ELF)
     return createWinCOFFStreamer(Ctx, MAB, *_Emitter, _OS, RelaxAll);
 
   // @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
 }
 
 static MCInstPrinter *createX86MCInstPrinter(const Target &T,
                                              unsigned SyntaxVariant,
                                              const MCAsmInfo &MAI,
                                              const MCInstrInfo &MII,
                                              const MCRegisterInfo &MRI,
@@ skipping 56 matching lines @@
                                            createMCStreamer);
   TargetRegistry::RegisterMCObjectStreamer(TheX86_64Target,
                                            createMCStreamer);
 
   // Register the MCInstPrinter.
   TargetRegistry::RegisterMCInstPrinter(TheX86_32Target,
                                         createX86MCInstPrinter);
   TargetRegistry::RegisterMCInstPrinter(TheX86_64Target,
                                         createX86MCInstPrinter);
 }