[swift-dev] Changing ELF layout

Saleem Abdulrasool compnerd at compnerd.org
Sun Sep 17 12:15:01 CDT 2017

On Sat, Sep 16, 2017 at 6:19 PM, John McCall <rjmccall at apple.com> wrote:

> > On Sep 16, 2017, at 6:06 PM, Saleem Abdulrasool via swift-dev <
> swift-dev at swift.org> wrote:
> > Hello,
> >
> > I'd like to propose that we change the locations that we use to store
> the type metadata, protocol conformances, type references, reflection
> strings, field metadata, and associated types.
> >
> > I think that it is possible to simplify the design for the linker tables
> by changing section names and relying on the linker to perform the work
> necessary to generate the tables so that they can be walked later.
> >
> > Switching sections would mean that we would lose interoperability with
> previously built libraries.  Given that there is ABI stability work going
> on for at least the Darwin target, I figure that this would be the best
> time to do this.
> >
> > Would this be acceptable?  Is compatibility something that we need to
> worry about?
> Compatibility is not something that we're currently promising.  I think
> this is a fine time to be working on this problem.
> It's not clear from your proposal whether you're just proposing changing
> sections or whether you're interested in more invasive changes to metadata
> emission.  Can you be more specific.


Right now, we have two special object files which must be included in a
certain order to ensure that the sections that I mentioned earlier are
bounded and grouped.  However, this is unneessary.  As long as the section
name is a valid C identifier, the linker will group and bound the sections
with special variables that it will synthesize
(__{start,stop}_[SectionName]).  This will allow us to replace the two file
approach with a single file approach.  Furthermore, it will allow the file
to be injected anywhere (it drops the need for the files to appear in a
specific order).  Finally, it simplifies the logic so that we can write the
entire thing in C rather than having to roll the begin/end content in

I think that this would help reduce some of the complexity of the ELF
emission.  In particular, it would mean that we would change the following:

.swift3_typeref -> swift3_type_references
.swift3_reflstr -> swift3_reflection_strings
.swift3_fieldmd -> swift3_field_metadata
.swift3_assocty -> swift3_associated_types

.swift2_protocol_conformances -> swift2_protocol_conformances
.swift2_type_metadata -> swift2_type_metadata

AFAIK, ELF does not impose section name limits, so, Im not sure if there is
anything to gain from the shortened names.

While we are changing these things around, it seems that it may be a good
idea to also change the PE/COFF emission to use section grouping (as
specified within the specification) so that we can have similar handling on
both the ELF and COFF sides.

In the case of PE/COFF, the only change would be the augmentation of the
grouping specifier ($B) on the existing names.  The names already are
within the specification limits (COFF limits section names to 8
characters).  This would allow begin/end markers to be constructed.

> John.

Saleem Abdulrasool
compnerd (at) compnerd (dot) org
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