[lld] r293963 - Update comments.

[Sean Silva via llvm-commits]

On Thu, Feb 2, 2017 at 3:26 PM, Rui Ueyama via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: ruiu
> Date: Thu Feb  2 17:26:12 2017
> New Revision: 293963
>
> URL: http://llvm.org/viewvc/llvm-project?rev=293963&view=rev
> Log:
> Update comments.
>
> Modified:
>     lld/trunk/ELF/LinkerScript.cpp
>
> Modified: lld/trunk/ELF/LinkerScript.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
> LinkerScript.cpp?rev=293963&r1=293962&r2=293963&view=diff
> ============================================================
> ==================
> --- lld/trunk/ELF/LinkerScript.cpp (original)
> +++ lld/trunk/ELF/LinkerScript.cpp Thu Feb  2 17:26:12 2017
> @@ -696,9 +696,28 @@ static bool shouldSkip(const BaseCommand
>    return Assign->Name != ".";
>  }
>
> -// Orphan sections are sections present in the input files which are not
> -// explicitly placed into the output file by the linker script. This just
> -// places them in the order already decided in OutputSections.
> +// Orphan sections are sections present in the input files which are
> +// not explicitly placed into the output file by the linker script.
> +//
> +// When the control reaches this function, Opt.Commands contains
> +// output section commands for non-orphan sections only. This function
> +// adds new elements for orphan sections to Opt.Commands so that all
> +// sections are explicitly handled by Opt.Commands.
> +//
> +// Writer<ELFT>::sortSections has already sorted output sections.
> +// What we need to do is to scan OutputSections vector and
> +// Opt.Commands in parallel to find orphan sections. If there is an
> +// output section that doesn't have a corresponding entry in
> +// Opt.Commands, we will insert a new entry to Opt.Commands.
> +//
> +// There is some ambiguity as to where exactly a new entry should be
> +// inserted, because Opt.Commands contains not only output section
> +// commands but other types of commands such as symbol assignment
> +// expressions. There's no correct answer here due to the lack of the
> +// formal specification of the linker script. We use heuristics to
> +// determine whether a new output command should be added before or
> +// after another commands. For the details, look at shouldSkip
> +// function.
>

Can you mention compatibility with BFD/gold? IIRC orphan section handling
was one of the issues with linking a working FreeBSD kernel. Also IIRC the
GNU linkers actually have a well-defined default behavior (See
--orphan-handling=MODE in
https://sourceware.org/binutils/docs/ld/Options.html#Options and
https://sourceware.org/binutils/docs/ld/Orphan-Sections.html#Orphan-Sections
)

-- Sean Silva


>  template <class ELFT> void LinkerScript<ELFT>::placeOrphanSections() {
>    // The OutputSections are already in the correct order.
>    // This loops creates or moves commands as needed so that they are in
> the
>
>
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