[LLVMdev] llvm-mc and endianess.

[Dominique Torette]

For what concern, the ARM architecture the endianess problem seem to be managed 'by hand' in ' ARMELFStreamer::emitInst()', not by 'EmitInstruction'.
The third argument to createELFObjectWriter() only set a boolean attribute that is retrieved in LittleEndian' local variable.
This local variable is then used, with some length information, to re-order the instruction's bytes.
Then, my understanding is that I have to define my own 'emitInst()' that perform explicitly the bytes swapping.
Could someone confirms this analysis?
 

  virtual void emitInst(uint32_t Inst, char Suffix) {
    unsigned Size;
    char Buffer[4];
    const bool LittleEndian = getContext().getAsmInfo()->isLittleEndian();

    switch (Suffix) {
    case '\0':
      Size = 4;

      assert(!IsThumb);
      EmitARMMappingSymbol();
      for (unsigned II = 0, IE = Size; II != IE; II++) {
        const unsigned I = LittleEndian ? (Size - II - 1) : II;
        Buffer[Size - II - 1] = uint8_t(Inst >> I * CHAR_BIT);
      }

      break;
    case 'n':
    case 'w':
      Size = (Suffix == 'n' ? 2 : 4);

      assert(IsThumb);
      EmitThumbMappingSymbol();
      for (unsigned II = 0, IE = Size; II != IE; II = II + 2) {
        const unsigned I0 = LittleEndian ? II + 0 : (Size - II - 1);
        const unsigned I1 = LittleEndian ? II + 1 : (Size - II - 2);
        Buffer[Size - II - 2] = uint8_t(Inst >> I0 * CHAR_BIT);
        Buffer[Size - II - 1] = uint8_t(Inst >> I1 * CHAR_BIT);
      }

      break;
    default:
      llvm_unreachable("Invalid Suffix");
    }

    MCELFStreamer::EmitBytes(StringRef(Buffer, Size));
  }

-----Original Message-----
From: Richard Sandiford [mailto:rsandifo at linux.vnet.ibm.com] 
Sent: jeudi 6 mars 2014 16:31
To: Dominique Torette
Cc: LLVMdev at cs.uiuc.edu
Subject: Re: [LLVMdev] llvm-mc and endianess.

Dominique Torette <Dominique.Torette at spacebel.be> writes:
> Hi,
>
> As a first step to port the LLVM chain on an in-house big-endian 
> processor, I'm integrating the native assembler as a new '-assemble 
> -arch=' in llvm-mc.
> All work quite well, I have a correct output ELF format except that 
> generated code is little-endian.
> I've understood that the endianess of the LLVM chain is controlled by 
> the DataLayout class, but it appear to me that llvm-mc does not make 
> use of such class.
> I've seen a backend (CPU0,
> http://jonathan2251.github.io/lbd/genobj.html) that defines two 
> different targets and performs the byte swapping as part of the 
> EmitInstruction'. Is it the right way?
> Could somebody confirm my understanding and give me some tips about 
> endianess in llvm-mc?

At the MC level you need to make your *AsmInfo constructor set:

  IsLittleEndian = false;

Also make sure you pass false to the third argument to createELFObjectWriter().

FWIW there are several in-tree targets that support big-endian, such as ARM, MIPS and PowerPC.  SystemZ is big-endian only.
It might help to compare with one of those.

Thanks,
Richard

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