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

Rui Ueyama ruiu at google.com
Fri Feb 6 19:28:29 PST 2015 

Not all input files have to be able to represented in YAML/Native format.
There are many unrealistic use cases there. No one wants to write an
executable file in Native because there's no operating system that can run
that file. So is YAML. So is the combination of .so file and Native/YAML
unless we have an operating system whose loader is able to loads a YAML .so
file.

We might want to write a Native/YAML file as a re-linkable object file (in
GNU it's -r option), but that's an object file.

So it's totally okay if some input file type is not representable in
YAML/Native. Some use cases are not real. We can't force all developers to
spend their time to support unrealistic use cases.

On Fri, Feb 6, 2015 at 7:04 PM, Shankar Easwaran <shankarke at gmail.com>
wrote:

> The intermediate result is what is really written to disk when
> --output-filetype=yaml or native is chosen too.
>
>
> Writing to YAML/Reading back YAML is not doable when you convert input
> files to atoms because some of the input files are not representable in
> YAML format.
>
> On Fri, Feb 6, 2015 at 8:48 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> I think no one is opposing the idea of reading and writing YAML.
>>
>> The problem here is that why we need to force all developers to write
>> code to serialize intermediate data in the middle of link, which no one
>> except the round-trip passes needs.
>>
>> On Fri, Feb 6, 2015 at 6:41 PM, Shankar Easwaram <shankarke at gmail.com>
>> wrote:
>>
>>> Doing it for every input file is not useful as some of the input files
>>> are not represent able in YAML form. Examples are shared libraries.
>>>
>>> The reason I made the yaml pass be called before the writer was the
>>> intermediate result was more complete since all atoms have been resolved at
>>> that point and the state of all atoms are much sane.
>>>
>>> It was also easy to use the pass manager. the code was very small to
>>> achieve what we are trying to do that all the information to the writer is
>>> passed through references or atom properties.
>>>
>>> Shankar Easwaran
>>>
>>>
>>>
>>> On Feb 6, 2015, at 19:54, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>> On Fri, Feb 6, 2015 at 5:42 PM, Michael Spencer <bigcheesegs at gmail.com>
>>> wrote:
>>>
>>>> On Fri, Feb 6, 2015 at 5:31 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>> > There are two questions.
>>>> >
>>>> > Firstly, do you think the on-disk format needs to compatible with a
>>>> C++
>>>> > struct so that we can cast that memory buffer to the struct? That may
>>>> be
>>>> > super-fast but that also comes with many limitations. It's hard to
>>>> extend,
>>>> > for example. Every time we want to store variable-length objects we
>>>> need to
>>>> > define string-table-like data structure. And I'm not very sure that
>>>> it's
>>>> > fastest -- because mmap'able objects are not very compact on disk,
>>>> slow disk
>>>> > IO could be a bottleneck, if we compare that with more compact file
>>>> format.
>>>> > I believe Protobufs or Thrust are fast enough or even might be faster.
>>>>
>>>> I'm not sure here. Although I do question if the object files will
>>>> even need to be read from disk in your standard edit/compile/debug
>>>> loop or on a build server. I believe we'll need real data to determine
>>>> this.
>>>>
>>>> >
>>>> > Secondly, do you know why we are dumping post-linked object file to
>>>> Native
>>>> > format? If we want to have a different kind of *object* file format,
>>>> we
>>>> > would want to have a tool to convert an object file in an existing
>>>> file
>>>> > format (say, ELF) to "native", and teach LLD how read from the file.
>>>> > Currently we are writing a file in the middle of linking process,
>>>> which
>>>> > doesn't make sense to me.
>>>>
>>>> This is an artifact of having the native format before we had any
>>>> readers. I agree that it's weird and not terribly useful to write to
>>>> native format in the middle of the link, although I have found it
>>>> helpful to output yaml. There's no need to be able to read it back in
>>>> and resume though.
>>>>
>>>
>>> Even for YAML it doesn't make much sense to write it to a file and read
>>> it back from the file in the middle of the link, do it? I found that being
>>> able to output YAML is useful too, but round-trip is a different thing. In
>>> the middle of the process, we have bunch of additional information that
>>> doesn't exist in input files and doesn't have to be output to the link
>>> result. Ability to serialize that intermediate result is not useful.
>>>
>>> Shankar, you added these round-trip tests. Do you have any opinion?
>>>
>>> Ideally lld -r would be the tool we use to convert COFF/ELF/MachO to
>>>> the native format.
>>>>
>>>> - Michael Spencer
>>>>
>>>> >
>>>> > On Fri, Feb 6, 2015 at 5:02 PM, Michael Spencer <
>>>> bigcheesegs at gmail.com>
>>>> > wrote:
>>>> >>
>>>> >> On Fri, Feb 6, 2015 at 2:54 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>> >> > Can we remove Native format support? I'd like to get input from
>>>> anyone
>>>> >> > who
>>>> >> > wants to keep the current Native format in LLD.
>>>> >>
>>>> >> One of the original goals for LLD was to provide a new object file
>>>> >> format for performance. The reason it is not used currently is
>>>> because
>>>> >> we've yet to teach llvm to generate it, and we haven't done that
>>>> >> because it hasn't been finalized yet. The value it currently provides
>>>> >> is catching stuff like this, so we can fix it now instead of down the
>>>> >> road when we actually productize the native format.
>>>> >>
>>>> >> As for the specific implementation of the native format, I'm open to
>>>> >> an extensible format, but only if the performance cost is low.
>>>> >>
>>>> >> - Michael Spencer
>>>> >>
>>>> >> >
>>>> >> > On Thu, Feb 5, 2015 at 2:03 PM, Shankar Easwaran
>>>> >> > <shankare at codeaurora.org>
>>>> >> > wrote:
>>>> >> >>
>>>> >> >> The only way currently is to create a new reference, unless we can
>>>> >> >> think
>>>> >> >> of adding some target specific metadata information in the Atom
>>>> model.
>>>> >> >>
>>>> >> >> This has come up over and over again, we need something in the
>>>> Atom
>>>> >> >> model
>>>> >> >> to store information that is target specific.
>>>> >> >>
>>>> >> >> Shankar Easwaran
>>>> >> >>
>>>> >> >>
>>>> >> >> On 2/5/2015 2:22 PM, Simon Atanasyan 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.
>>>> >> >>>
>>>> >> >>
>>>> >> >>
>>>> >> >> --
>>>> >> >> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>>>> >> >> hosted
>>>> >> >> by the Linux Foundation
>>>> >> >>
>>>> >> >
>>>> >> >
>>>> >> > _______________________________________________
>>>> >> > LLVM Developers mailing list
>>>> >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>> >> >
>>>> >
>>>> >
>>>>
>>>
>>>
>>
>
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