[PATCH] Add DarwinInputGraph

[Rui Ueyama]

There are two points.

1. The core linker already knows about ELF or other platform specific
features. A few examples are a tentative symbol, a section group, or a weak
external symbol. It would be very difficult to do to implement them outside
of the core.

Also, what the core linker implements is the Unix linker's semantics, which
is different from, for example, Windows. It's not platform neutral as a
whole.

(We can assume a linker for an embedded system or something, whose feature
is pretty limited. Say, it doesn't support an archive file and a shared
library file. Now, all of sudden, archive files and shared library files
become "platform dependent" feature. Should we move the code handling them
out of the core? No, we don't want to do that. Why do we think the set of
features that we currently have in the core is "platform neutral"? There's
actually no obvious reason.)

And having platform dependent feature in the core linker is not immediately
a bad thing, as I noted as (3) in the previous mail. What makes code more
maintainable for all of us is the right thing to do, whether that's a patch
to the core or a platform specific tree.

2. It's totally OK to get nothing from rewriting the InputGraph. It even
shouldn't change the functionality. It's refactoring to improve code
quality.

On Wed Oct 22 2014 at 6:16:29 PM Shankar Easwaran <shankare at codeaurora.org>
wrote:

> On 10/22/2014 8:01 PM, Rui Ueyama wrote:
>
> On Tue Oct 21 2014 at 9:38:57 PM Nick Kledzik <kledzik at apple.com> <kledzik at apple.com> wrote:
>
>
>  On Oct 21, 2014, at 6:31 PM, Rui Ueyama <ruiu at google.com> <ruiu at google.com> wrote:
>
> Let's not add a back reference to the input element. It will make the
> complicated data structure even worse and would raise new issues (for
> example, I could imagine that it's not clear whether the back reference
> should point to a parsed file in an archive file or should point to the
> entire archive file.)
>
> Before discussing possible alternatives, it's worth discussing the issues
> that the current InputGraph has. Here's my take.
>
> 1. We have too many classes to represent the concept of "file".
>
> FileNode in the InputGraph is different from the File class that
> represents parsed file. And the InputGraph has various kinds of FileNodes
> -- FileNode, SimpleFileNode, <flavor>FileNode, etc. That needless
> distinction makes hard to handle files.
>
> FileNode needs to be merged with the File class. Or if we are going to
> replace the InputGraph with a simpler data structure, it should be removed.
>
> 2. We have too many (other) classes.
>
> We have too many classes for the InputGraph that are simply overdesigned.
> Command line argument is not an easy stuff, but it shouldn't need that
> amount of code and number of classes. We have created too deep class
> hierarchy there.
>
> 3. Making the core linker platform neutral is not an ultimate goal, but
> the InputGraph is designed for that.
>
> The InputGraph made made the core linker more or less platform neutral.
> That's basically a good thing. We'd like to keep the core part as platform
> neutral as possible because it would help us maintain the code. However,
> neutrality is not a goal that we need to achieve at any cost. If separating
> platform neutral code from dependent code will mess up the entire code
> base, we simply shouldn't do that. (It's worth noting that the InputGraph
> failed to hide everything already, one example is the ELF section group.)
>
> We should teach the core linker a little bit more about platform specific
> features and removes complexity from the InputGraph.
>
> I agree on all the above.
>
> Looking at how core linking uses InputGraph, it is always through the
> LinkingContext and:
>   Resolver.cpp needs an iterator (getNextFile()) and a way to notify when
> it used a file (notifyProgress())
>   Driver.cpp needs a way to iterate files (inputElements) to parse them in
> parallel
>
> So this could be simplified to hide/implement the “graph” in the
> LinkingContext.  That is, the LinkingContext subclasses each use the data
> structures natural for their needs.  The drivers just call addFile() on the
> LinkingContext, and the gnu driver calls startGroup()/endGroup() on the
> LinkingContext.  The Resolver could call a forEachFile() method on
> LinkingContext which takes a lambda on what to do with each file.  The
> lambda can return if the Resolver used the file (to track whether the
> current group should be scanned again). The parallel parsing currently done
> in the Driver could be a utility method in the base LinkingContext class
> that each concrete LinkingContext uses.
>
>
>  Why does the GNU ld driver have to ask the LinkingContext to construct a
> startGroup/endGroup? The GNU driver should be capable of understanding all
> the command line options for GNU ld, so it can construct a data structure
> directly.
>
> forEachFile() doesn't seem different from what we have now. It's a
> getNextFile(). I don't think it will improve the situation.
>
> It feels to me that we don't need a sophisticated technique here. I'd
> suggest making the Resolver to handle ELF groups to eliminate
> notifyProgress. I'd even remove InputGraph entirely and replace it with a
> std::vector<FileOrGroup> or something like that where FileOrGroup is a
> class representing a file or a ELF group.
>
>  I dont think the Core resolver should not understand anything about ELF.
>
> Even changing this to a std::vector<FileOrGroup> doesnot change what can
> be achieved with the new functionality compared to what we have now.
>
> As this is one of the major things that we need to handle, I think We
> could move this discusssion to a hallway discussion during the LLVM
> developers meeting, if you are all ok.
>
>  On Tue, Oct 21, 2014 at 5:33 PM, Shankar Easwaram <shankarke at gmail.com> <shankarke at gmail.com>
> wrote:
>
>
>  If we could have lld::file have a back reference to the input element
> (or) use the file ordinal to access the input element in the input graph
> you could just add inputs on a need basis ?
>
> Let's discuss alternative proposals as part of this thread ?
>
> I am also interested in moving the input file which could be a linker
> script from the driver to the registry too in addition.
>
> We still need to have the getNextFile as it made the resolver more flavor
> neutral.
>
> Shankar easwaran
>
>
>
> On Oct 21, 2014, at 17:58, Rui Ueyama <ruiu at google.com> <ruiu at google.com> wrote:
>
> On Tue, Oct 21, 2014 at 3:41 PM, Michael Spencer <bigcheesegs at gmail.com> <bigcheesegs at gmail.com>
> wrote:
>
>
>  On Mon, Oct 20, 2014 at 9:21 PM, Rui Ueyama <ruiu at google.com> <ruiu at google.com> wrote:
>
>  I'm not still sure whether or not "InputGraph" thing was after all the
>
>  right way to abstract the input file list. That often makes easy things
> hard. In this case, sorting a list of input files became that hard. When I
> proposed the idea I was thinking that would make this kind of things easier
> to handle. I don't think the current shape of the API is not desirable at
> least. Or the fundamental idea was not very good.
>
>  One thing we probably should consider is, in my opinion, we are
>
>  pushing too hard to separate all ports. We are trying to write any
> architecture-dependent code into architecture-specific file. But the
> natural border of API doesn't always fit to the architecture
> dependent/independent border. We should probably relax that constraint a
> bit where that makes sense and write code that's short and easier to
> understand.
>
> I agree. The current InputGraph code makes modifying the input very
> difficult to do. Part of the reason for the model in the first place
> was to make this easy. I'm currently working on adding #pragma lib
> like support for ELF (PS4 specific stuff) and have found it basically
> impossible to add a new input file in the correct position in the
> graph with the current model.
>
>
>  To deal with the issue of the InputGraph I have a few ugly hacks in
> PECOFF/LinkerGeneratedSymbolFile.h. There are a few virtual .a files there
> that generate magical symbols on the fly to control the core linker in an
> obscure way. These hacks need to be removed, but because the InputFile API
> exists in between, it's impossible to do.
>
> The InputGraph API design needs to be revisited and probably be rewritten.
>
>
>
>
>
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>
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