llvm dev - Apr 2016 - Adding fix-up information to an instruction created with BuildMI

I'm trying to work out how to add fix-up information to an instruction that
is built with BuildMI.

The instruction is taking a local label (MBB), also created in the same piece
code and needs to store its address. This address is fixed-up later in the
compilation process.

Here's the essential fragment of code. This is custom lowering of an
instruction (__builtin_setjmp), so I can't rely on the features provided by
DAG legalization to help with fix-ups. I need to mark these instructions as
needing later fix-ups here. I can't find any equivalent piece of code.

  MIB = BuildMI(*thisMBB, MI, DL, TII->get(SP::SETHIi), LabelReg)
            .addMBB(restoreMBB);
  // Need some code here to mark that a Sparc HI22 fix-up is needed on this
instruction

  MIB = BuildMI(*thisMBB, MI, DL, TII->get(SP::ADDri), LabelReg)
            .addMBB(restoreMBB)
            .addReg(LabelReg);
  // Need some code here to mark that a Sparc LO10 fix-up is needed on this
instruction

/ Chris.
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Hi Chris,

On 20 April 2016 at 06:22, Chris.Dewhurst via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>   MIB = BuildMI(*thisMBB, MI, DL, TII->get(SP::SETHIi), LabelReg)
>             .addMBB(restoreMBB);
>   // Need some code here to mark that a Sparc HI22 fix-up is needed on this
> instruction
There's a second "TargetFlags" argument to addMBB which encodes
information about the type of fixup needed. The rough sequence is:

1. You pass some "VK_*" argument there.
2. This gets attached to an MCExpr MCOperand when the MachineInstr ->
MCInst lowering takes place (SparcMCInstLower.cpp,
LowerSymbolOperand).
3. During MCCodeEmitter this gets converted into a fixup_* style fixup
(getMachineOpValue via a call to getFixupKind) and push_backed onto a
list of Fixups (separate from the instruction now).
4. Finally, this fixup either gets resolved in SparcAsmBackend
(hopefully this, for an MBB) or converted to an R_SPARC_* relocation
in SparcELFObjectWriter.cpp (goodness knows what would happen there,
since an MBB normally gets an assembler-local symbol).

So your main goal is going to be working out which kind of VK_* tag
you really need (see SparcMCExpr.h for the list). It *looks* like that
would be VK_Sparc_HI (& LO for your other inst), but that's just going
by the names -- I actually know basically nothing about Sparc.

Cheers.

Tim.