[llvm-dev] LLD: creating linker-generated sections as input sections instead of output sections

[George Rimar via llvm-dev]

>This idea popped up in the review thread for https://reviews.llvm.org/D25627.

>
>Problem:
>
>Currently, LLD creates special sections that are not just concatenations of input sections but need link-time data generation, such as .got, .plt, interp, .mips.options, >etc., as output sections. We have OutputSectionBase subclasses (e.g. GotSection, PltSection, etc.) to create data. Even though this scheme works in most cases, >there are a few situations that doesn't work well as you may have noticed. Here are a issues.
>
 >- You cannot mix special sections with other types of sections.
>
>  For example, using linker scripts, you can instruct the linker put mergeable sections and non-mergeable sections into the same output section. Such script makes >sense. However, LLD cannot handle such script because string merging is the special mergeable output section's feature. The output section doesn't know how to >handle other types of sections, so you cannot feed non-mergeable sections to a mergeable output section.
>
> - It cannot handle linker scripts like this as pointed by Eugene.
>
>  .got { *(.got.plt) *(.got) }
>
>   In our current architecture, .got section is an output section, so it cannot be added to other output section. There's no clean way to handle this linker script.
>
>Proposal:
>
>Here's my idea: how about creating all special sections as input sections instead of output sections?
>
>GotSection, PltSection, etc. will be subclasses of InputSection that don't have corresponding input files. What they will do remain the same. They will be added to >OutputSections just like other regular sections are added. I think we could simplify OutputSection a lot -- OutputSection will probably become a dumb container >that just concatenates all input sections.
>
>This approach would solve the problems described above. Now that we create .got as an special input section with ".got" as a name, so they can naturally be added >to any output section. String merging occurs inside a special mergeable input section, so they can be added to any section, too.
>
>So, I think by moving the implementations from OutputSection to InputSection, we can solve many problems. I do not think of any obvious problem with the >approach.
>
>What do you think?

For me that sounds as interesting idea. My consern and guess that amount of code changes can be really large for that.
But generally I so not see real problems with this approach too.

George.

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