[LLVMdev] [lld] Representation of lld::Reference with a fake target

[Rui Ueyama]

Yes, it is always a pain when we have to keep information over the
round-trip conversion from/to Native. Native format is really able to
handle only fields we define to base Atom or Reference and not a subclass
of them. This is, well, hard. This is like a roadblock that is always there
when we do something with Reference. I think this needs fixing.

I once proposed or at least suggested that we remove Native file format
from LLD. I still think that's not a bad idea at all -- no one is using
Native and thus delivers no value currently, but it imposes burden to
developers. This is not a good situation.

If we still want to maintain Native, conversion from/to Native needs to be
easier to support. I can think of a few reasons why supporting Native is
hard.

1. It is not extendable. It only recognizes base classes and not subclasses
of Atom or References.

2. The file format is designed to be mmap()'able, so that you can just mmap
the entire file and cast a struct to read it. It'd be very fast indeed --
but it can't support any optional field or something.

If I would design from scratch, I'd probably use a serializable data
structure like Protocol Buffers or Thrust to represent Atoms and
References, so that all that conversion is automatically done. They are
fast enough and easy to use.

That being said, I think the short term solution for your need is to just
add a new field to Reference.

On Thu, Feb 5, 2015 at 12:22 PM, Simon Atanasyan <simon at atanasyan.com>
wrote:

> Hi,
>
> I need an advice on implementation of a very specific kind of relocations
> used by MIPS N64 ABI. As usual the main problem is how to pass target
> specific
> data over Native/YAML conversion barrier.
>
> In this ABI relocation record r_info field in fact consists of five
> subfields:
> * r_sym   - symbol index
> * r_ssym  - special symbol
> * r_type3 - third relocation type
> * r_type2 - second relocation type
> * r_type  - first relocation type
>
> Up to three these relocations applied one by one. The first relocation uses
> an addendum from the relocation record. Each subsequent relocation takes as
> its addend the result of the previous operation. Only the final operation
> actually modifies the location relocated. The first relocation uses as
> a reference symbol specified by the r_sym field. The third relocation
> assumes NULL symbol.
>
> The most interesting case is the second relocation. It uses the special
> symbol value given by the r_ssym field. This field can contain four
> predefined values:
> * RSS_UNDEF - zero value
> * RSS_GP    - value of gp symbol
> * RSS_GP0   - gp0 value taken from the .MIPS.options or .reginfo section
> * RSS_LOC   - address of location being relocated
>
> So the problem is how to store these four constants in the
> lld::Reference object.
> The RSS_UNDEF is obviously not a problem. To represent the RSS_GP value I
> can
> set an AbsoluteAtom created for the "_gp" as the reference's target. But
> what
> about RSS_GP0 and RSS_LOC? I am considering the following approaches but
> cannot
> select the best one:
>
> a) Create AbsoluteAtom for each of these cases and set them as the
> reference's target.
>    The problem is that these atoms are fake and should not go to the
> symbol table.
>    One more problem is to select unique names for these atoms.
> b) Use two high bits of lld::Reference::_kindValue field to encode
> RSS_xxx value.
>    Then decode these bits in the RelocationHandler to calculate result
> of relocation.
>    In that case the problem is how to represent a relocation kind
> value in YAML format.
>    The simple xxxRelocationStringTable::kindStrings[] array will not
> satisfy us.
> c) Add one more field to the lld::Reference class. Something like the
> DefinedAtom::CodeModel
>    field.
>
> Any advices, ideas, and/or objections are much appreciated.
>
> --
> Simon Atanasyan
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150205/1e25f827/attachment.html>