[lld] r314192 - Speed up SymbolTable::insert().

[Rui Ueyama via llvm-commits]

Yes, I should've mention that in the commit message.

And this is probably a sign that we should split StringRef::find into the
fast pass (which works on short strings) and the slow pass (which does
BM-ish string search) and move the fast pass from StringRef.cpp to
StringRef.h, so that they are inlined and optimized by compilers. I'll do
that when I have time.

On Thu, Oct 12, 2017 at 2:09 PM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:

> I can reproduce the speedup on various benchmarks.
>
> But note that it is not just a faster algorithm. The old version would
> produce a different result for foo at bar@@zed. That is fine for us, but we
> couldn't do the same as a special case for small needles in StringRef.
>
> Thanks,
> Rafael
>
> Rui Ueyama via llvm-commits <llvm-commits at lists.llvm.org> writes:
>
> > Author: ruiu
> > Date: Mon Sep 25 21:17:13 2017
> > New Revision: 314192
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=314192&view=rev
> > Log:
> > Speed up SymbolTable::insert().
> >
> > SymbolTable::insert() is a hot path function. When linking a clang debug
> > build, the function is called 3.7 million times. The total amount of
> "Name"
> > string contents is 300 MiB. That means this `Name.find("@@")` scans
> almost
> > 300 MiB of data. That's far from negligible.
> >
> > StringRef::find(StringRef) uses a sophisticated algorithm, but the
> > function is slow for a short needle. This patch replaces it with
> > StringRef::find(char).
> >
> > This patch alone speeds up a clang debug build link time by 0.5 seconds
> > from 8.2s to 7.7s. That's 6% speed up. It seems too good for this tiny
> > change, but looks like it's real.
> >
> > Modified:
> >     lld/trunk/ELF/SymbolTable.cpp
> >
> > Modified: lld/trunk/ELF/SymbolTable.cpp
> > URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
> SymbolTable.cpp?rev=314192&r1=314191&r2=314192&view=diff
> > ============================================================
> ==================
> > --- lld/trunk/ELF/SymbolTable.cpp (original)
> > +++ lld/trunk/ELF/SymbolTable.cpp Mon Sep 25 21:17:13 2017
> > @@ -206,8 +206,12 @@ static uint8_t getMinVisibility(uint8_t
> >  std::pair<Symbol *, bool> SymbolTable::insert(StringRef Name) {
> >    // <name>@@<version> means the symbol is the default version. In that
> >    // case <name>@@<version> will be used to resolve references to
> <name>.
> > -  size_t Pos = Name.find("@@");
> > -  if (Pos != StringRef::npos)
> > +  //
> > +  // Since this is a hot path, the following string search code is
> > +  // optimized for speed. StringRef::find(char) is much faster than
> > +  // StringRef::find(StringRef).
> > +  size_t Pos = Name.find('@');
> > +  if (Pos != StringRef::npos && Pos + 1 < Name.size() && Name[Pos + 1]
> == '@')
> >      Name = Name.take_front(Pos);
> >
> >    auto P = Symtab.insert(
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
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