[llvm-dev] Removing pointers from MCInstrDesc for less relocations

[Nicolai Hähnle via llvm-dev]

On 09.05.2016 05:19, Benjamin Kramer wrote:
> On Mon, May 9, 2016 at 5:35 AM, Nicolai Hähnle <llvm-dev at lists.llvm.org> wrote:
>> Hi everybody,
>>
>> I noticed today that my libLLVM-3.9svn.so has a ~1.7MB .data.rel.ro segment
>> - i.e. data that needs to be touched by the dynamic linker even though it's
>> ultimately read-only, and data that cannot be shared between multiple
>> processes using LLVM.
>>
>> It turns out that a solid ~1.3MB of that data is in the tablegen'd
>> MCInstrDesc tables - there a pointers for ImplicitUses, ImplicitDefs, and
>> OpInfo that need to be relocated.
>>
>> This can be fixed of course by having target-global arrays for those
>> structures referenced by MCInstrInfo (and hence TargetInstrInfo), and only
>> storing offsets into those global arrays in MCInstrDesc.
>>
>> The downside is that several relevant accessors need to be augmented with
>> MCInstrInfo parameters, but in the long run it seems worth it to me.
>>
>> Any objections or suggestions? Is this something that people can agree on,
>> i.e. do patches going in this direction have a good chance of being
>> accepted?
>
> Sounds good to me, I'm happy to accept any patch that reduces LLVM binary size
> without sacrificing performance. Passing MCInstrInfo isn't a big deal.

Thanks. Though note that it wouldn't affect the total binary size - 
instead, it would move data from .data.rel.ro to .rodata, which means 
the total binary size remains roughly the same, but more data can be 
directly mmapped from disk.

> Is moving ImplicitUses/ImplicitDefs/OpInfo enough to get all relocs
> out of the instruction
> tables? Then the compiler could put them into a read-only section and
> we don't have to
> page them in at startup anymore.

Yes, that does the trick - I've previously done that in other places to 
get reloc sizes down.

Cheers,
Nicolai