[LLVMdev] LLD improvement plan

[Sean Silva]

On Fri, May 29, 2015 at 1:14 AM, Rui Ueyama <ruiu at google.com> wrote:

> I want to make it clear that I didn't (at least intend to) compromise
> flexibility or beauty of design with short-term performance gain. I was
> trying to do simple things in a simple way for both humans and computers,
> and I believe I did that fairly well.  I'd even argue that the new design
> is cleaner and more expressive than before, because the "atom" model is in
> some part too detailed and restrictive on how to represent data and
> relations between symbols, particularly how to represent relocations. It
> also lacked capability of representing indivisible memory areas having
> multiple names.
>
> After I wrote up the first patch, I realized that the goal of the code is
> somewhat similar to what the atom model aims to achieve, with some
> differences. I assume that you have read the readme file for the new port.
> The differences are
>
>  -  An atom has only one name, but the new "chunk" can have one or more
> symbols referring that. But the actual difference is that chunks are
> agnostic of symbols referring them in the new design. I have separated
> actual data from symbols to get more flexibility. And that flexibility
> enabled me to achieve better performance by writing more abstract code
> which reads less data.
>
>  - In the atom model, we have detailed information about relocations,
> including relocation target, offset, etc, for each atom. In the new design,
> we don't have them. Instead, we have just a set of symbols for each chunk
> that needs to be resolved to include that input chunk properly. This is
> more abstract and flexible than the existing design.
>
> - The atom model reads too much data from files prematurely to construct a
> complete graph, while the new design avoided that. This is partly an
> implementation's issue, but partly unavoidable, because we actually needed
> to build more complex data structure.
>
> - And this might be stemmed from the implementation and not from the model
> itself, but the thing is that it's hard to write code for the atom model
> because their data types have too much detailed relations with other types.
> For example, any atom in the model has to have a "file" that an atom was
> created from. This makes it hard to append linker-generated data to output
> which don't have a source file (we ended up having a notion of "virtual
> input file" that doesn't do anything meaningful itself.). Another example
> is that, if you want to create a symbol on-demand, you've got to create a
> "virtual archive" file that returns a "virtual file" containing one
> "virtual atom" when the archive file is asked for that symbol. In the new
> design, it can be expressed in one line of code instead of multiple class
> definitions and object juggling. Also, because relocations are explicitly
> represented as "references" in the atom model, we've got to create
> platform-specific relocation objects even for linker-generated data if it
> refers some other symbols, and let a platform-specific relocation function
> to consume that data to apply relocations. That's less abstracted than the
> new design, in which all classes but the actual data type needs to know
> about relocations are agnostic about how relocations are represented and
> how to actually apply them.
>

These all sound like things that just indicate "we have some refactoring to
do", just like Duncan did for debug metadata, or David is doing for the
opaque pointer type, or how the Type system has been changed over the
years, or how clang's template parsing is changed to be compatible with
weird MSVC behavior. Is there something about the current situation with
LLD that made you think that refactoring was hopeless and required a
rewrite? If what we currently have doesn't fit our use cases, why not just
fix it?


> Besides them, I'd say from my experiences of working on the atom model,
> the new model's ability is not that different from the atom model. They are
> different, there are pros and cons, and I don't agree that the atom model
> is more flexible or conceptually better.
>

I don't understand this focus on "the atom model". "the atom model" is not
any particular thing. We can generalize the meaning of atom, we can make it
more narrow, we can remove responsibilities from Atom, we can add
responsibilities to Atom, we can do whatever is needed. As you yourself
admit, the "new model" is not that different from "the atom model". Think
of "the atom model" like SSA. LLVM IR is SSA; there is a very large amount
of freedom to decide on the exact design within that scope. "the atom
model" AFAICT just means that a core abstraction inside the linker is the
notion of an indivisible chunk. Our current design might need to be
changed, but starting from scratch only to arrive at the same basic idea
but now having to effectively maintain two codebases doesn't seem worth it.

A lot of the issue here is that we are falsely distinguishing
"section-based" and "atom-based". A suitable generalization of the notion
of "indivisible chunks" and what you can do with them covers both cases,
but traditional usage of sections makes the "indivisible chunks" be a lot
larger (and loses more information in doing so). But as
-ffunction-sections/-fdata-sections shows, there is not really any
fundamental difference.

-- Sean Silva


>
> On Thu, May 28, 2015 at 8:22 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> On Thu, May 28, 2015 at 6:25 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>
>>>
>>> On May 28, 2015, at 5:42 PM, Sean Silva <chisophugis at gmail.com> wrote:
>>>
>>> I guess, looking back at Nick's comment:
>>>
>>> "The atom model is a good fit for the llvm compiler model for all
>>> architectures.  There is a one-to-one mapping between llvm::GlobalObject
>>> (e.g. function or global variable) and lld:DefinedAtom."
>>>
>>> it seems that the primary issue on the ELF/COFF side is that currently
>>> the LLVM backends are taking a finer-grained atomicity that is present
>>> inside LLVM, and losing information by converting that to a coarser-grained
>>> atomicity that is the typical "section" in ELF/COFF.
>>> But doesn't -ffunction-sections -fdata-sections already fix this,
>>> basically?
>>>
>>> On the Mach-O side, the issue seems to be that Mach-O's notion of
>>> section carries more hard-coded meaning than e.g. ELF, so at the very least
>>> another layer of subdivision below what Mach-O calls "section" would be
>>> needed to preserve this information; currently symbols are used as a bit of
>>> a hack as this "sub-section" layer.
>>>
>>> I’m not sure what you mean here.
>>>
>>>
>>> So the problem seems to be that the transport format between the
>>> compiler and linker varies by platform, and each one has a different way to
>>> represent things, some can't represent everything we want to do, apparently.
>>>
>>> Yes!
>>>
>>>
>>> BUT it sounds like at least relocatable ELF semantics can, in principle,
>>> represent everything that we can imagine an "atom-based file
>>> format"/"native format" to want to represent. Just to play devil's
>>> advocate here, let's start out with the "native format" being relocatable
>>> ELF - on *all platforms*. Relocatable object files are just a transport
>>> format between compiler and linker, after all; who cares what we use? If
>>> the alternative is a completely new format, then bootstrapping from
>>> relocatable ELF is strictly less churn/tooling cost.
>>>
>>> People on the "atom side of the fence", what do you think? Is there
>>> anything that we cannot achieve by saying "native"="relocatable ELF"?
>>>
>>> 1) Turns out .o files are written once but read many times by the
>>> linker.  Therefore, the design goal of .o files should be that they are as
>>> fast to read/parse in the linker as possible.  Slowing down the compiler to
>>> make a .o file that is faster for the linker to read is a good trade off.
>>> This is the motivation for the native format - not that it is a universal
>>> format.
>>>
>>
>> I don't think that switching from ELF to something new can make linkers
>> significantly faster. We need to handle ELF files carefully not to waste
>> time on initial load, but if you do, reading data required for symbol
>> resolution from ELF file should be satisfactory fast (I did that for COFF
>> -- the current "atom-based ELF" linker is doing too much things in an
>> initial load, like read all relocation tables, splitting indivisble chunk
>> of data and connect them with "indivisible" edges, etc.) Looks like we read
>> symbol table pretty quickly in the new implementation, and the bottleneck
>> of it is now the time to insert symbols into the symbol hash table -- which
>> you cannot make faster by changing object file format.
>>
>> Speaking of the performance, if I want to make a significant difference,
>> I'd focus on introducing new symbol resolution semantics. Especially, the
>> Unix linker semantics is pretty bad for performance because we have to
>> visit files one by one serially and possibly repeatedly. It's not only bad
>> for parallelism but also for a single-thread case because it increase size
>> of data to be processed. This is I believe the true bottleneck of Unix
>> linkers. Tackling that problem seems to be most important to me, and "ELF
>> as a file format is slow" is still an unproved thing to me.
>>
>>
>>>
>>> 2) I think the ELF camp still thinks that linkers are “dumb”.  That they
>>> just collate .o files into executable files.  The darwin linker does a lot
>>> of processing/optimizing the content (e.g. Objective-C optimizing, dead
>>> stripping, function/data re-ordering).  This is why atom level granularity
>>> is needed.
>>>
>>
>> I think that all these things are doable (and are being done) using
>> -ffunction-sections.
>>
>>
>>>
>>> For darwin, ELF based .o files is not interesting.  It won’t be faster,
>>> and it will take a bunch of effort to figure out how to encode all the
>>> mach-o info into ELF.  We’d rather wait for a new native format.
>>>
>>
>>
>>> -Nick
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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>>>
>>>
>>
>
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