<div dir="ltr">I want to bring this again because I think it's important.<div><br></div><div>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.</div><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Feb 9, 2015 at 4:33 AM, Denis Protivensky <span dir="ltr"><<a href="mailto:dprotivensky@accesssoftek.com" target="_blank">dprotivensky@accesssoftek.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
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.<br>
<br>
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.<br>
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().<span><font color="#888888"><br>
<br>
- Denis.</font></span><div><div><br>
<br>
<div>On 02/08/2015 08:42 PM, Shankar
Easwaram <a href="mailto:shankarke@gmail.com" target="_blank"><shankarke@gmail.com></a> wrote:<br>
</div>
<blockquote type="cite">
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?<br>
<p><font>
<br>
> On Feb 8, 2015, at 04:50, Simon Atanasyan
<a href="mailto:simon@atanasyan.com" target="_blank"><simon@atanasyan.com></a> wrote:<br>
><br>
> Nick,<br>
><br>
> Right now I try to implement exactly the same approach. I
split a<br>
> relocation record on up to three separate relocations.
But there are<br>
> two problems on this way.<br>
><br>
> The first one is how to represent some additional data
provided in the<br>
> relocation record. That is why I asked the initial
question in this<br>
> thread. The current solution is to extend the `Reference`
class and<br>
> add one more attribute to it.<br>
><br>
> The second problem is how to handle these three
relocations. Now the<br>
> `TargetRelocationHandler::applyRelocation()` method
called one-by-one<br>
> for each reference. That is not applied to the MIPS N64
ABI<br>
> relocations. In that case we need to calculate the first
one, save the<br>
> result, calculate the second one using result from the
first step,<br>
> repeat that for the third relocations and only at the end
check the<br>
> final result on overflow and apply it to the location. My
current idea<br>
> is to allow a target specific code to handle the loop
over atom's<br>
> references now hardcoded in the
`AtomSection<ELFT>::write` method.<br>
><br>
>> On Sun, Feb 8, 2015 at 3:21 AM, Nick Kledzik
<a href="mailto:kledzik@apple.com" target="_blank"><kledzik@apple.com></a> wrote:<br>
>> Simon,<br>
>><br>
>> 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.<br>
>><br>
>> Or am I misunderstanding the issue?<br>
>><br>
>> -Nick<br>
>><br>
>><br>
>>> On Feb 5, 2015, at 12:22 PM, Simon Atanasyan
<a href="mailto:simon@atanasyan.com" target="_blank"><simon@atanasyan.com></a> wrote:<br>
>>> Hi,<br>
>>><br>
>>> I need an advice on implementation of a very
specific kind of relocations<br>
>>> used by MIPS N64 ABI. As usual the main problem
is how to pass target specific<br>
>>> data over Native/YAML conversion barrier.<br>
>>><br>
>>> In this ABI relocation record r_info field in
fact consists of five subfields:<br>
>>> * r_sym - symbol index<br>
>>> * r_ssym - special symbol<br>
>>> * r_type3 - third relocation type<br>
>>> * r_type2 - second relocation type<br>
>>> * r_type - first relocation type<br>
>>><br>
>>> Up to three these relocations applied one by one.
The first relocation uses<br>
>>> an addendum from the relocation record. Each
subsequent relocation takes as<br>
>>> its addend the result of the previous operation.
Only the final operation<br>
>>> actually modifies the location relocated. The
first relocation uses as<br>
>>> a reference symbol specified by the r_sym field.
The third relocation<br>
>>> assumes NULL symbol.<br>
>>><br>
>>> The most interesting case is the second
relocation. It uses the special<br>
>>> symbol value given by the r_ssym field. This
field can contain four<br>
>>> predefined values:<br>
>>> * RSS_UNDEF - zero value<br>
>>> * RSS_GP - value of gp symbol<br>
>>> * RSS_GP0 - gp0 value taken from the
.MIPS.options or .reginfo section<br>
>>> * RSS_LOC - address of location being relocated<br>
>>><br>
>>> So the problem is how to store these four
constants in the<br>
>>> lld::Reference object.<br>
>>> The RSS_UNDEF is obviously not a problem. To
represent the RSS_GP value I can<br>
>>> set an AbsoluteAtom created for the "_gp" as the
reference's target. But what<br>
>>> about RSS_GP0 and RSS_LOC? I am considering the
following approaches but cannot<br>
>>> select the best one:<br>
>>><br>
>>> a) Create AbsoluteAtom for each of these cases
and set them as the<br>
>>> reference's target.<br>
>>> The problem is that these atoms are fake and
should not go to the<br>
>>> symbol table.<br>
>>> One more problem is to select unique names for
these atoms.<br>
>>> b) Use two high bits of
lld::Reference::_kindValue field to encode<br>
>>> RSS_xxx value.<br>
>>> Then decode these bits in the RelocationHandler
to calculate result<br>
>>> of relocation.<br>
>>> In that case the problem is how to represent a
relocation kind<br>
>>> value in YAML format.<br>
>>> The simple
xxxRelocationStringTable::kindStrings[] array will not satisfy
us.<br>
>>> c) Add one more field to the lld::Reference
class. Something like the<br>
>>> DefinedAtom::CodeModel<br>
>>> field.<br>
>>><br>
>>> Any advices, ideas, and/or objections are much
appreciated.<br>
>>><br>
>>> --<br>
>>> Simon Atanasyan<br>
>>> _______________________________________________<br>
>>> LLVM Developers mailing list<br>
>>> <a href="mailto:LLVMdev@cs.uiuc.edu" target="_blank">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu" target="_blank">http://llvm.cs.uiuc.edu</a><br>
>>> <a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><br>
><br>
><br>
><br>
> --<br>
> Simon Atanasyan<br>
</font><br>
</p>
</blockquote>
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