[MIPS] Modify LongBranch expansion to work with sandboxing

include/llvm/MC/MCExpr.h

 //===- MCExpr.h - Assembly Level Expressions --------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MC_MCEXPR_H
@@ skipping 199 matching lines @@
     VK_Mips_GPOFF_LO,
     VK_Mips_GOT_DISP,
     VK_Mips_GOT_PAGE,
     VK_Mips_GOT_OFST,
     VK_Mips_HIGHER,
     VK_Mips_HIGHEST,
     VK_Mips_GOT_HI16,
     VK_Mips_GOT_LO16,
     VK_Mips_CALL_HI16,
     VK_Mips_CALL_LO16,
+    // @LOCALMOD-START
+    VK_Mips_NACL_LONG_BRANCH_HI16,

[Mark Seaborn, 2013/10/24 23:38:43]

This produces:
native_client/pnacl/git/llvm/lib/MC/MCExpr.cpp:179:10: warning: enumeration
value ‘VK_Mips_NACL_LONG_BRANCH_HI16’ not handled in switch [-Wswitch]

Maybe you could fix that warning?

I think you should also update MipsInstPrinter.cpp, otherwise its asm printer
will do 'llvm_unreachable("Invalid kind!")'.  Can you add a test for the
assembly printing, please?

More general design question:

GNU 'as' has some support for writing relocations like this:

        lui $at, %hi(label2 - label1)
        addiu $at, $at, %lo(label2 - label1)
// or
        lui $at, (label2 - label1) >> 16
        addiu $at, $at, (label2 - label1) & 0xffff
// (though the latter is not exactly what you want here)
label1:
        ...
label2:

Is there a reason that wouldn't work for MIPS in LLVM?  Does LLVM/MIPS support
expressions like that now?  If not, would adding support for that be better than
adding ad-hoc relocation types?

[petarj, 2013/11/21 18:23:42]

Done.

+    VK_Mips_NACL_LONG_BRANCH_LO16,
+    // @LOCALMOD-END
 
     VK_COFF_IMGREL32 // symbol@imgrel (image-relative)
   };
 
 private:
   /// The symbol being referenced.
   const MCSymbol *Symbol;
 
   /// The symbol reference modifier.
   const VariantKind Kind;
@@ skipping 247 matching lines @@
   virtual void fixELFSymbolsInTLSFixups(MCAssembler &) const = 0;
 
   static bool classof(const MCExpr *E) {
     return E->getKind() == MCExpr::Target;
   }
 };
 
 } // end namespace llvm
 
 #endif