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

[Benjamin Kramer via llvm-dev]

On Mon, May 9, 2016 at 5:56 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> 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.

Relocations also take space on disk, so there should be a reduction.

>
>> 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