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

Mon Feb 9 11:29:43 PST 2015

I want to bring this again because I think it's important.

It doesn't make sense to store and load intermediate atoms/references in
the middle of link, is it? Teaching LLD about how to read from YAML or
Native object file is fine, and we probably want to teach LLD about how to
emit a re-linkable object file in YAML or Native file. But the round-trip
tests do more than that, right? The tests require more -- dumping linker
internal state to a file and reading it correctly.

On Mon, Feb 9, 2015 at 4:33 AM, Denis Protivensky <
dprotivensky at accesssoftek.com> wrote:

>  A bit off topic: ARM Group relocations define a logical set of consequent
> instructions to be relocated to form one single address. For such
> relocations a 1 to 1 relation is also met, so no need of special processing
> in applyRelocation.
>
> Concerning native format: it also introduced unneeded code complexity to
> me when I wanted to set calculated relocation addend back into the
> Reference object in the relocation handler to simplify further processing.
> Since it crashed in tests because of native format conversion, I needed to
> keep calculated addend to pass it along the handlers instead of just
> calling ref.addend().
>
> - Denis.
>
>
> On 02/08/2015 08:42 PM, Shankar Easwaram <shankarke at gmail.com>
> <shankarke at gmail.com> wrote:
>
> ARM has a concept of Group relocations, which applies relocations
> considering them as one set may be we should consider extending apply
> relocation. What do you think?
>
>
> > On Feb 8, 2015, at 04:50, Simon Atanasyan <simon at atanasyan.com>
> <simon at atanasyan.com> wrote:
> >
> > Nick,
> >
> > Right now I try to implement exactly the same approach. I split a
> > relocation record on up to three separate relocations. But there are
> > two problems on this way.
> >
> > The first one is how to represent some additional data provided in the
> > relocation record. That is why I asked the initial question in this
> > thread. The current solution is to extend the `Reference` class and
> > add one more attribute to it.
> >
> > The second problem is how to handle these three relocations. Now the
> > `TargetRelocationHandler::applyRelocation()` method called one-by-one
> > for each reference. That is not applied to the MIPS N64 ABI
> > relocations. In that case we need to calculate the first one, save the
> > result, calculate the second one using result from the first step,
> > repeat that for the third relocations and only at the end check the
> > final result on overflow and apply it to the location. My current idea
> > is to allow a target specific code to handle the loop over atom's
> > references now hardcoded in the `AtomSection<ELFT>::write` method.
> >
> >> On Sun, Feb 8, 2015 at 3:21 AM, Nick Kledzik <kledzik at apple.com>
> <kledzik at apple.com> wrote:
> >> Simon,
> >>
> >> Sounds like the instruction relocating is so complicated that ELF
> requires up to three relocation records to encode how to fix up one
> instruction.  If that is the case, why not do the same and have up to three
> Reference objects on the same atom offset to record the same info?  I
> thought that was model that the ELF part of lld was using - there is a
> one-to-one mapping of ELF reloc to lld References.
> >>
> >> Or am I misunderstanding the issue?
> >>
> >> -Nick
> >>
> >>
> >>> On Feb 5, 2015, at 12:22 PM, Simon Atanasyan <simon at atanasyan.com>
> <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
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
> >
> > --
> > Simon Atanasyan
>
>
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>
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