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

[Eugene Leviant via llvm-commits]

четверг, 28 июля 2016 г. пользователь Rui Ueyama написал:

> On Thu, Jul 28, 2016 at 1:22 PM, Eugene Leviant <evgeny.leviant at gmail.com
> <javascript:_e(%7B%7D,'cvml','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.
>

May be it's possible to just implement Factory::register(name,
type, attrs), where attrs is a combination of input section attributes? I
wonder what should be done if input sections are not compatible, i.e.
PROGBITS/NOBITS? Raise error?

>
>

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