[llvm-dev] LLD status update and performance chart

Tue Dec 13 12:01:59 PST 2016

> On Dec 13, 2016, at 11:51 AM, Rui Ueyama <ruiu at google.com> wrote:
> 
> On Tue, Dec 13, 2016 at 11:37 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Dec 13, 2016, at 11:30 AM, Rui Ueyama <ruiu at google.com <mailto:ruiu at google.com>> wrote:
>> 
>> On Tue, Dec 13, 2016 at 11:23 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> 
>>> On Dec 13, 2016, at 11:08 AM, Rui Ueyama <ruiu at google.com <mailto:ruiu at google.com>> wrote:
>>> 
>>> On Tue, Dec 13, 2016 at 11:02 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>> 
>>>> On Dec 13, 2016, at 10:06 AM, Rui Ueyama <ruiu at google.com <mailto:ruiu at google.com>> wrote:
>>>> 
>>>> On Tue, Dec 13, 2016 at 9:28 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>> 
>>>> > On Dec 13, 2016, at 5:55 AM, Rafael Avila de Espindola via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>> >
>>>> > Sean Silva via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> writes:
>>>> >> This will also greatly facilitate certain measurements I'd like to do
>>>> >> w.r.t. different strategies for avoiding memory costs for input files (esp.
>>>> >> minor faults and dTLB costs). I've almost gotten to the point of
>>>> >> implementing this just to do those measurements.
>>>> >
>>>> > If you do please keep it local. The bare minimum we have of library
>>>> > support is already disproportionately painful and prevents easier sharing
>>>> > with COFF. We should really not add more until the linker is done.
>>>> 
>>>> This is so much in contrast with the LLVM development, I find it quite hard to see this as an acceptable position on llvm-dev.
>>>> 
>>>> LLD is a subproject of the LLVM project, but as a product, LLD itself is not LLVM nor Clang, so some technical decisions that make sense to them are not directly be applicable or even inappropriate. As a person who spent almost two years on the old LLD and 1.5 years on the new LLD, I can say that Rafael's stance on focusing on making a good linker first really makes sense. I can easily imagine that if we didn't focus on that, we couldn't make this much progress over the past 1.5 year and would be stagnated at a very basic level. Do you know if I'm a person who worked really hard on the old (and probably "modular" whatever it means) linker so hard? I'm speaking based on the experience. If you have an concrete idea how to construct a linker from smaller modules,  please tell me. I still don't get what you want. We can discuss concrete proposals, but "making it (more) modular" is too vague and not really a proposal, so it cannot be a productive discussion.
>>>> 
>>>> That said, I think the current our "API" to allow users call our linker's main function hit the sweet spot. I know at least a few LLVM-based language developers who want to eliminate external dependencies and embed a linker to their compilers. That's a reasonable usage, and I think allowing them to pass a map from filename to MemoryBuffer objects makes sense, too. That would be done without affecting the overall linker architecture. I don't oppose to that idea, and if someone wrote a patch, I'm fine with that.
>>> 
>>> 
>>> I’m totally willing to believe you that it is not possible to write the fastest ELF linker on earth (or in the universe) with a library based and reusable components approach. But clang is not the fastest C/C++ compiler available, and LLVM is not the fastest compiler framework either!
>>> 
>>> So as a project, it seems to me that LLVM has not put the tradeoff on the speed/efficiency historically when it was to the detriment of layering/component/modularity/reusability/…
>>> 
>>> Writing the fastest linker possible is nice goal, I regret that a LLVM subproject is putting this goal above layering/component/modularity/reusability/… though.
>>> 
>>> I've never mentioned that creating the fastest linker is the only goal.
>> 
>> I believe this has clearly been put *ahead* the other design aspects I mentioned, isn’t it?
>> 
>>> Medhi, please tell how you would *actually* layer linkers with fine-grained components.
>> 
>> That’s not a bait I’m gonna bite.
>> 
>> That's not a bait... I guess you are proposing a different architecture, so you need to explain it.
> 
> That’s a bait in the sense that I’m not having two months to dig into the Elf lld and write a design document/proposal for the sole purpose of making a point. And me not doing this does not impact and is not relevant to the discussion at stance..
> 
> If it takes two months for you to investigate and make a proposal, why are you so confident about the conclusion of the investigation that what you think (I still don't get what it is) is doable? I actually dug into the old LLD for two years (not months) with the hope that there's a good way of make it work but failed.

You can’t be serious. You’re bait is asking for me proposing a different architecture. I’m not biting.
This is *not* the baseline. For instance, you said from the start that you won’t return an Error from APIs and instead call exit(). I don’t need to propose a “linker architecture” for that.
The main contention point is how any library design guidelines is rejected from the start on the principle that it’ll slow down the linker.

— 
Mehdi

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