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

[Simon Atanasyan]

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