[PATCH] D22683: [ELF] Symbol assignment within input section list

[Rui Ueyama via llvm-commits]

On Thu, Jul 28, 2016 at 1:22 PM, Eugene Leviant <evgeny.leviant at gmail.com>
wrote:

> evgeny777 added inline comments.
>
> ================
> Comment at: ELF/LinkerScript.cpp:67
> @@ +66,3 @@
> +
> +  // This section should never be written.
> +  Hdr.sh_type = SHT_NOBITS;
> ----------------
> ruiu wrote:
> > It's a bit misleading. This section is written, but because it is empty,
> there's nothing to be written in reality.
> Not exactly. If you don't set SHT_NOBITS, lld will *attempt* to write the
> section and fail because file pointer is set to null.


Well, it's philosophical question whether .bss sections are "written" to
file or not. What I was trying to say is to ask you to make this comment
clear as to why we are doing this. We set NOBITS because it's a dummy
section just to anchor symbols that doesn't have contents.


> ================
> Comment at: ELF/LinkerScript.cpp:179-182
> @@ +178,6 @@
> +      for (const std::unique_ptr<BaseCommand> &Base2 : Cmd->Commands)
> +        if (auto *Assignment = dyn_cast<SymbolAssignment>(Base2.get()))
> +          PendingSymbols.push_back(Assignment);
> +        else
> +          AddMultiple(Cmd->Name,
> cast<InputSectionDescription>(Base2.get()));
> +
> ----------------
> ruiu wrote:
> > I do not understand why you needed a concept of "pending" here. You can
> add a SymbolInputSection as soon as you see a new SymbolAssignment, can't
> you?
> Imagine the symbol is declared before any input section rule, like in the
> sample below:
>
> ```
> .foo : { begin_foo = .; *(.foo) }
> ```
>
> Which output section will own it? You can't create output section given
> symbol assignment command, because you know only section name, not section
> attributes, which can be obtained only from InputSectionBase<ELFT>.


This is why I think we shouldn't use Factory to create output sections in
the linker script. Factory has a logic to create output sectinos in a smart
way, but in the linker script, we need to create output sections just as
instructed by scripts. As soon as we see SECTIONS command, we should be
able to create an output section. It's related to
https://reviews.llvm.org/D22749.


>
> Repository:
>   rL LLVM
>
> https://reviews.llvm.org/D22683
>
>
>
>
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