[lld] r243232 - ELF2: Avoid calling std::sort to make output deterministic.

[Rui Ueyama]

On Mon, Jul 27, 2015 at 5:00 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015-Jul-27, at 16:37, Rui Ueyama <ruiu at google.com> wrote:
> >
> > On Mon, Jul 27, 2015 at 4:18 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
> > On Mon, Jul 27, 2015 at 2:02 PM Rui Ueyama <ruiu at google.com> wrote:
> > On Mon, Jul 27, 2015 at 12:01 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> > > On 2015-Jul-25, at 17:50, Rui Ueyama <ruiu at google.com> wrote:
> > >
> > > Author: ruiu
> > > Date: Sat Jul 25 19:50:15 2015
> > > New Revision: 243232
> > >
> > > URL: http://llvm.org/viewvc/llvm-project?rev=243232&view=rev
> > > Log:
> > > ELF2: Avoid calling std::sort to make output deterministic.
> >
> > Using `std::stable_sort` should also make it deterministic.  Is that
> > better than switching to a `std::map`?
> >
> > Yeah, I think the new code is slightly more readable than the previous
> one. At least it's shorter than before.
> >
> >
> > "Yeah" doesn't seem to agree with the rest of your email, so I'm left
> confused.
> >
> > Why wouldn't switching to std::stable_sort be better?
> >
> > std::map is horribly slow in essentially every way, and so I struggle to
> believe it is the best tool for the job even if the code using it is very
> compact.
> >
> > std::stable_sort would actually be slower than the new code. What we are
> doing here is bin chunks into a few sections, so putting everything into
> one array and stable-sorting them and scanning them again doesn't make much
> sense. We used std::stable_sort in COFF before but I removed that in
> r239292 for that reason.
>
> Looking again, you're just using `Map` as a cache here, right?  If
> you're not iterating over the map, you can probably skip the
> red-black tree by using a (Small?)DenseMap.
>

Yes, we can do that. Use of std::map was just my negligence. I'll replace
them with a DenseMap.

>
> >
> > >
> > > Modified:
> > >    lld/trunk/ELF/Writer.cpp
> > >
> > > Modified: lld/trunk/ELF/Writer.cpp
> > > URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.cpp?rev=243232&r1=243231&r2=243232&view=diff
> >
> > >
> ==============================================================================
> > > --- lld/trunk/ELF/Writer.cpp (original)
> > > +++ lld/trunk/ELF/Writer.cpp Sat Jul 25 19:50:15 2015
> > > @@ -10,6 +10,7 @@
> > > #include "Writer.h"
> > > #include "Chunks.h"
> > > #include "Driver.h"
> > > +#include <map>
> > >
> > > using namespace llvm;
> > > using namespace llvm::ELF;
> > > @@ -58,24 +59,14 @@ void OutputSection::addChunk(Chunk *C) {
> > >   Header.sh_size = Off;
> > > }
> > >
> > > -static int compare(const Chunk *A, const Chunk *B) {
> > > -  return A->getSectionName() < B->getSectionName();
> > > -}
> > > -
> > > // Create output section objects and add them to OutputSections.
> > > template <class ELFT> void Writer<ELFT>::createSections() {
> > > -  std::vector<Chunk *> Chunks = Symtab->getChunks();
> > > -  if (Chunks.empty())
> > > -    return;
> > > -  std::sort(Chunks.begin(), Chunks.end(), compare);
> > > -
> > > -  Chunk *Prev = nullptr;
> > > -  OutputSection *Sec = nullptr;
> > > -  for (Chunk *C : Chunks) {
> > > -    if (Prev == nullptr || Prev->getSectionName() !=
> C->getSectionName()) {
> > > +  std::map<StringRef, OutputSection *> Map;
> > > +  for (Chunk *C : Symtab->getChunks()) {
> > > +    OutputSection *&Sec = Map[C->getSectionName()];
> > > +    if (!Sec) {
> > >       Sec = new (CAlloc.Allocate()) OutputSection(C->getSectionName());
> > >       OutputSections.push_back(Sec);
> > > -      Prev = C;
> > >     }
> > >     Sec->addChunk(C);
> > >   }
> > >
> > >
> > > _______________________________________________
> > > llvm-commits mailing list
> > > llvm-commits at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150727/058fd3bc/attachment.html>