[lld] r230732 - PECOFF: Do not add layout-after edges.

[Shankar Easwaran]

I thought there was a use of layout-after/layout-before edges to compute 
section size for handling COMDAT's w.r.t PECOFF.

If layout-after references are deleted, how does getSectionSize() work ?

Shankar Easwaran

On 2/26/2015 11:05 PM, Rui Ueyama wrote:
> Author: ruiu
> Date: Thu Feb 26 23:05:38 2015
> New Revision: 230732
>
> URL: http://llvm.org/viewvc/llvm-project?rev=230732&view=rev
> Log:
> PECOFF: Do not add layout-after edges.
>
> Previously we needed to create atoms as a doubly-linked link, but it's
> no longer needed. Also we don't use layout-after edges in PE/COFF.
> Creating such edges is just waste.
>
> Modified:
>      lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h
>      lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
>
> Modified: lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h?rev=230732&r1=230731&r2=230732&view=diff
> ==============================================================================
> --- lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h (original)
> +++ lld/trunk/lib/ReaderWriter/PECOFF/Atoms.h Thu Feb 26 23:05:38 2015
> @@ -351,27 +351,16 @@ void addLayoutEdge(T *a, U *b, uint32_t
>     a->addReference(std::unique_ptr<COFFReference>(ref));
>   }
>   
> -template <typename T, typename U> void connectWithLayoutEdge(T *a, U *b) {
> -  addLayoutEdge(a, b, lld::Reference::kindLayoutAfter);
> -  addLayoutEdge(b, a, lld::Reference::kindLayoutBefore);
> -}
> -
> -/// Connect atoms with layout-{before,after} edges. It usually serves two
> -/// purposes.
> -///
> -///   - To prevent atoms from being GC'ed (aka dead-stripped) if there is a
> -///     reference to one of the atoms. In that case we want to emit all the
> -///     atoms appeared in the same section, because the referenced "live" atom
> -///     may reference other atoms in the same section. If we don't add layout
> -///     edges between atoms, unreferenced atoms in the same section would be
> -///     GC'ed.
> -///   - To preserve the order of atmos. We want to emit the atoms in the
> -///     same order as they appeared in the input object file.
> +/// Connect atoms with layout-before edges. It prevents atoms from
> +/// being GC'ed (aka dead-stripped) if there is a reference to one of
> +/// the atoms in the same layout-before chain. In such case we want to
> +/// emit all the atoms appeared in the same chain, because the "live"
> +/// atom may reference other atoms in the same chain.
>   template <typename T> void connectAtomsWithLayoutEdge(std::vector<T *> &atoms) {
>     if (atoms.size() < 2)
>       return;
>     for (auto it = atoms.begin(), e = atoms.end(); it + 1 != e; ++it)
> -    connectWithLayoutEdge(*it, *(it + 1));
> +    addLayoutEdge(*(it + 1), *it, lld::Reference::kindLayoutBefore);
>   }
>   
>   } // namespace pecoff
>
> Modified: lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp?rev=230732&r1=230731&r2=230732&view=diff
> ==============================================================================
> --- lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp (original)
> +++ lld/trunk/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp Thu Feb 26 23:05:38 2015
> @@ -756,7 +756,7 @@ std::error_code FileCOFF::AtomizeDefined
>       if (atoms.size() > 0)
>         atoms[0]->setAlignment(getAlignment(section));
>   
> -    // Connect atoms with layout-before/layout-after edges.
> +    // Connect atoms with layout-before edges.
>       connectAtomsWithLayoutEdge(atoms);
>   
>       for (COFFDefinedFileAtom *atom : atoms) {
>
>
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