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

lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp

Index: lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
diff --git a/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp b/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
index 2c155aaf40dde487d467941d6f5bbb3e0cec5962..27c49b845dc09b9bb80dc0612fd0f8cd5efc921b 100644
--- a/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
+++ b/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
@@ -19,11 +19,14 @@
 #include "InstPrinter/ARMInstPrinter.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/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/TargetRegistry.h"
 
@@ -211,8 +214,30 @@ static MCStreamer *createMCStreamer(const Target &T, StringRef TT,
   // @LOCALMOD-BEGIN
   MCStreamer *Streamer = createARMELFStreamer(Ctx, MAB, OS, Emitter, false,
                            NoExecStack, TheTriple.getArch() == Triple::thumb);
-  if (TheTriple.isOSNaCl())
+  if (TheTriple.isOSNaCl()) {
     Streamer->EmitBundleAlignMode(4);
+    const char *NoteName = ".note.NaCl.ABI.arm";

[Mark Seaborn, 2013/05/09 21:54:11]

Since this is duplicated between ARM/x86 (and doesn't cover MIPS), how about
moving it to a common location so that it can be reused?  And then cover MIPS.
:-)

Can you add a comment to explain the purpose of this ELF note?

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

Why not:

const char NoteName[] = "...."

And then you can actually use sizeof() in EmitIntValue...

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

I didn't end up with a named const char array for this after moving to a common
location, so just went with strlen everywhere

+    const char *NoteNamespace = "NaCl";
+    const char *NoteArch = "arm";
+    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

[Mark Seaborn, 2013/05/09 21:54:11]

'maybe' -> 'Maybe'.  Make this a TODO?

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

Done.

+    // there haven't been any other sections switched to yet.
+    Streamer->SwitchSection(Note);
+    Streamer->EmitIntValue(5, 4); // sizeof("NaCl")

[Mark Seaborn, 2013/05/09 21:54:11]

Use strlen(NoteNamespace)

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

Done.

+    Streamer->EmitIntValue(4, 4); // sizeof("arm")

[Mark Seaborn, 2013/05/09 21:54:11]

ditto

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

Done.

+    Streamer->EmitIntValue(1, 4); // NT_VERSION

[Mark Seaborn, 2013/05/09 21:54:11]

Can you #define this in Support/ELF.h?

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

I could. I'm actually not sure if this is 1 because it's NT_VERSION or if it's 1
because we are using ABI version 1. see
https://code.google.com/p/nativeclient/issues/detail?id=2971

[Mark Seaborn, 2013/05/10 19:52:45]

It's NT_VERSION.  See gold/nacl.h in binutils:

            if ((elfcpp::Swap<32, big_endian>::readval(note_data + 0)
                 == sizeof "NaCl")
                && (elfcpp::Swap<32, big_endian>::readval(note_data + 4)
                    == nacl_abi_name_.size() + 1)
                && (elfcpp::Swap<32, big_endian>::readval(note_data + 8)
                    == elfcpp::NT_VERSION))

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

Done.

+    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
 }