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

[Michael Spencer]

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
>>
>
>
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