[LLVMdev] LLD improvement plan

Fri May 29 01:14:28 PDT 2015

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.

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.

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
>>
>>
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>>
>>
>
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