[LLVMdev] [lld][ELF] How to transfer st_other field value from input to output file

[Simon Atanasyan]

I was too optimistic. It is possible to use the contentTypes field for
handling STO_MICROMIPS and I have a working solution but the solution
is really ugly. This approach has at least two the following
shortcomings:

1. A MIPS ELF symbol can hold multiple STO_xxx flags stored in the
st_other field (STO_MIPS_PIC, STO_MIPS_MICROMIPS, STO_MIPS_MIPS16
...). Sometimes these flags can be even combined. If we use the
contentTypes field, we have to define a separate ContentType flag for
each such combination. So we get a combinatorics explosion.

2. If we handle MIPS specific ContentType flags together with other
flags, it is pollute the common ELF code. If we factor out the
processing of MIPS specific flags, we have to duplicate code because a
symbol with say STO_MICROMIPS flag should be processed (setup size,
permissions etc) the same way as a regular DefinedAtom::typeCode
symbol.

I considered to create a map symbol name => symbol flags, fill this
map while read object files, and use the map while write a linked
file. But I need to handle both local and global symbols and it is
possible to get symbols with the same name.

It looks like the only solution (if I do not miss anything else) is to
add one more filed to the DefinedAtom class to hold
target/architecture specific set of flags and modify Native and YAML
formats correspondingly. Interpretation of this field is completely
target/architecture dependent.

Any opinions?

On Thu, Nov 6, 2014 at 7:09 PM, Simon Atanasyan <simon at atanasyan.com> wrote:
> STO_MIPS16 and STO_MICROMIPS flags denote that the symbol use a
> different "compressed" instructions encoding. Both these flags can be
> combined with usual "visibility" flags.
>
> It looks like adding new flag into the contentTypes set might solve
> the problem. Thanks for the idea. I try to implement it.
>
> On Thu, Nov 6, 2014 at 6:52 PM, Shankar Easwaran
> <shankare at codeaurora.org> wrote:
>> One way to do that is to add new visibility / contentTypes (whatever is
>> relevant) added for each of the values st_other picks ?
>>
>> What are the other values st_other can take on MIPS ?
>>
>> On 11/6/2014 8:50 AM, Simon Atanasyan wrote:
>>> On MIPS st_other field in the ELF symbols table might contain some
>>> additional MIPS-specific flags besides visibility ones. These flags
>>> should be copied to the output linked file. If YAML => Native
>>> conversion is switched off, there is no problem. But in case of the
>>> conversion we lose st_other field values.
>>>
>>> So I need an advice how to keep this information. Is it a good idea to
>>> extend YAML and Native format to store these data? Is there any
>>> alternative solutions?

-- 
Simon Atanasyan