llvm dev - Feb 2015 - [LLVMdev] [lld] Representation of lld::Reference with a fake target

Can we remove Native format support? I'd like to get input from anyone who
wants to keep the current Native format in LLD.

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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I am fine with it. I hope you are not planning to remove YAML.

On 2/6/2015 4:54 PM, Rui Ueyama 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.
>
> 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

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation

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I'm not planning to remove YAML. YAML is important for testing.

On Fri, Feb 6, 2015 at 3:57 PM, Shankar Easwaran <shankare at
codeaurora.org>
wrote:
>  I am fine with it. I hope you are not planning to remove YAML.
>
>
> On 2/6/2015 4:54 PM, Rui Ueyama 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.
>
> On Thu, Feb 5, 2015 at 2:03 PM, Shankar Easwaran <shankare at
codeaurora.org> <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 listLLVMdev at cs.uiuc.edu        
http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
by the Linux Foundation
>
>
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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
>

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.

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.

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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