[LLVMdev] Proposal for -filetype=obj full big endian support

[Jim Grosbach]

Hi Jack,

I would have expected the Mips backend to be using these routines (or more likely something on top of them) to emit the contents of the data and text sections and thus have the bit-twiddling done by these routines. I take it that's not happening?

Basically, this should already work without any additional infrastructure. If it's not, something is broken and bypassing the mechanisms they should be using to output data.

-Jim

On Dec 20, 2011, at 12:21 PM, Carter, Jack wrote:

> Jim,
> 
> I see routines that are already available to do the endianizing, but the data and text section contents don't have sizing for the individual elements as far as I can see so that I can endianize them. That is the part I am trying to solve, not the bit twiddling algorithm.
> 
> Did I miss something?
> 
> Jack
> ________________________________________
> From: Jim Grosbach [grosbach at apple.com]
> Sent: Tuesday, December 20, 2011 11:58 AM
> To: Carter, Jack
> Cc: List
> Subject: Re: [LLVMdev] Proposal for -filetype=obj full big endian support
> 
> Hi Jack,
> 
> Everything should be already using the sized Emit* routines directly rather than outputting individual bytes of larger entities piecemeal. Have you found that not to be the case?
> 
> The Emit* routines handle endianness for you, so you shouldn't have to do much beyond that. For example, here's the EmitInvValue() implementation that underlies things and does the endianness transform:
> 
> /// EmitIntValue - Special case of EmitValue that avoids the client having to
> /// pass in a MCExpr for constant integers.
> void MCStreamer::EmitIntValue(uint64_t Value, unsigned Size,
>                              unsigned AddrSpace) {
>  assert(Size <= 8 && "Invalid size");
>  assert((isUIntN(8 * Size, Value) || isIntN(8 * Size, Value)) &&
>         "Invalid size");
>  char buf[8];
>  const bool isLittleEndian = Context.getAsmInfo().isLittleEndian();
>  for (unsigned i = 0; i != Size; ++i) {
>    unsigned index = isLittleEndian ? i : (Size - i - 1);
>    buf[i] = uint8_t(Value >> (index * 8));
>  }
>  EmitBytes(StringRef(buf, Size), AddrSpace);
> }
> 
> -Jim
> 
> On Dec 19, 2011, at 5:17 PM, Carter, Jack wrote:
> 
>> Proposal for ELF text and data big endian support for direct object generation
>> 
>> Unless I am mistaken, currently big endian support in the MC layer for ELF
>> direct object generation is limited to ELF headers and tables like relocations.
>> Text and data section contents are still generated as individual bytes.
>> It looks as if the effort was started, but never completed.
>> 
>> The proposal is to extend the MCDataFragment class to include a container
>> of offset/size that one can traverse generating each element in the correct
>> endianess:
>> 
>>  // Pseudo code
>> for (iterator it = begin(), ie = end(); it != ie; ++it) {
>>     switch (it.getValueSize()) {
>>      default:
>>        assert(0 && "Invalid size!");
>>      case 1: OW->Write8 (uint8_t (it.getValue())); break;
>>      case 2: OW->Write16(uint16_t(it.getValue())); break;
>>      case 4: OW->Write32(uint32_t(it.getValue())); break;
>>      case 8: OW->Write64(uint64_t(it.getValue())); break;
>>      }
>>    }
>> 
>> Note: ObjectWriter has a set of endian correcting routines as use above.
>> 
>> The output routines like the snippet above would be in lib/MC/MCAssembler.cpp
>> 
>> There is an MCInstFragment, but it doesn't seem to be used. All text and data
>> seem to go through MCDataFragment.
>> 
>> The offset/size container would be filled at the
>> lib/Taget/<your target>/MCTargetDesc level. In <your targetMCCODEEmitter.cpp
>> for at least text. Some other source file for data.
>> 
>> It would be nice if someone could prove that the support is already there
>> and I don't have to do anything to get it, but if not, is the above
>> strawman reasonable? If so, I'll implement it and put it up for review.
>> 
>> Cheers,
>> 
>> Jack
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>