[MIPS] Modify LongBranch expansion to work with sandboxing

include/llvm/MC/MCExpr.h

Index: include/llvm/MC/MCExpr.h
diff --git a/include/llvm/MC/MCExpr.h b/include/llvm/MC/MCExpr.h
index a2c5bd3f766135b8288145d47e9796c9db39ed27..31fba811130066041bb74c636ae9d1d42c8672cf 100644
--- a/include/llvm/MC/MCExpr.h
+++ b/include/llvm/MC/MCExpr.h
@@ -217,6 +217,10 @@ public:
     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)
   };