[llvm-dev] Make LLD output COFF relocatable object file (like ELF's -r does). How much work is required to implement this?

[Reid Kleckner via llvm-dev]

On Tue, Oct 10, 2017 at 1:01 PM, Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> No. GHC **runtime** linker is much slower when handling *.a files (and
>> this is exactly the culprit of this whole story) since it goes through the
>> whole archive and links each object module separately doing all resolutions
>> and relocations and trampolines, than when linking already prelinked big
>> *.o file.
>>
>
> Looks like I still do not understand why a .a can be much slower than a
> prelinked .o. As far as I understand, "ld -r" doesn't reduce amount of data
> that much. It doesn't reduce the number of relocations, as relocations in
> input object files are basically passed through to the output. It doesn't
> reduce the number of symbols that much, as the combined object file
> contains a union of all symbols appeared in the input files. So, I think
> the amount of data in a .a is essentially the same as a prelinked .o. I
> wonder what can make a difference in speed.
>

I can't speak for Haskell, but ld -r can be useful for speeding up C++
links, because it acts as a pre-merging step for duplicate comdats.
Consider a library that uses many instantiations of the same template with
the same type. An archive will contain many copies of the template, but a
relocated object file will only contain one.
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