[LLVMdev] Adding fixups and relocations late in code generation

[Jim Grosbach]

On Oct 10, 2011, at 10:11 AM, Jack Carter wrote:

> Gang,
> 
> I'm tasked with direct object generation for Mips and am trying to not
> hack the code.
> 
> I how exactly does one set an expression to be PC relative and if the
> compiler can resolve it, not produce a relocation?
> 

Fixups are created for all expressions that may need a relocation. The MC layer evaluates them as best it can (MCAssembler::EvaluateFixup()) and calls the target hook RecordRelocation() if it cannot evaluate it directly and needs a relocation in the object file.


> In our case, the backend produces an expression for the branch which is
> the target label. I make a call from the .td for the branch instruction
> which calls a routine in MipsMCCodeEmmitter.cpp. That routine 
> 
> ##############################################################
> unsigned MipsMCCodeEmitter::
> getBranchTargetOpValue(const MCInst &MI, unsigned OpNo,
>        SmallVectorImpl<MCFixup> &Fixups) const {
> 
>  const MCOperand &MO = MI.getOperand(OpNo);
> 
>  if (MO.isReg()) {
>    unsigned Reg = MO.getReg();
> 
>    unsigned RegNo = getMipsRegisterNumbering(Reg);
>    return RegNo;
> 
>  } else if (MO.isImm()) {
>    return static_cast<unsigned>(MO.getImm());
>  } else if (MO.isFPImm()) {
>    return static_cast<unsigned>(APFloat(MO.getFPImm())
>           .bitcastToAPInt().getHiBits(32).getLimitedValue());
>  } else if (MO.isExpr()) {
>      const MCExpr *p_expr = MO.getExpr();
>      Fixups.push_back(MCFixup::Create(0, p_expr,
> MCFixupKind(Mips::fixup_Mips_Branch_PCRel)));
>  }
>  return 0;
> }
> 
> ############################################
> 
> Later in MipsAsmBackend.cpp:ApplyFixup() I find the value is the offset
> of the target value from the beginning of the section and not the delta
> from the branch address.
> 

I'm not sure I follow. Wouldn't that still require a relocation from the start of the section?

> I also get a relocation request even though the label should be
> resolved.
> 

The logic for which cases are considered resolved and which require relocations is in MCAssembler::EvaluateFixup(). It's currently not factored all that well, with some target-specific knowledge embedded in the generic code. It's entirely possible we need to create a target hook there to handle any special cases.

> So, I must not be telling the compiler what to do when setting up the
> relocation. What am I doing wrong here?
> 
> As a hack, I calculate the delta in ApplyFixup() but it should have been
> done before ApplyFixup(). In ApplyFixup() I believe I just need to
> adjust the final value for Mips idiosyncrasies like shifting the delta 2
> bits to the right.
> 
> I would really like to become fluent in clean llvm expression building.
> 
> Thanks,
> 
> Jack
> 
> 
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