llvm dev - Mar 2015 - [LLVMdev] On LLD performance

On Fri, Mar 20, 2015 at 2:56 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> >> > https://people.freebsd.org/~davide/llvm/lld-03162015.svg
> >> > It seems now 85% of CPU time is spent inside
> >> > FileArchive::buildTableOfContents().
> >> > In particular, 35% of the samples are spent inserting into
> >> > unordered_map, so there's maybe something we can do
differently there
> >> > (e.g. , Rui's proposal of a concurrent map doesn't
seem that bad).
> >> >
> >>
> >> Why do we even need to build the table from name to member?
> >>
> >> Can't we just walk "archive->symbols()" and check
for each symbol if
> >> it is needed by the current link status?
> >
> >
> > Are you suggesting we do linear search instead of hash table lookup
each
> > time ArchiveFile::find(StringRef symbolName) is called?
>
>
> No, what I mean is that we only need to keep one hash of the current
> state of the link (which symbols are still undefined).
>
> Once you get to an archive, don't build a hashtable of every symbol it
> provides. Instead, walk the list of symbols it provides and if one of
> those is currently undefined in the global table, include it in the
> link.
>
I think that wouldn't work. When we find a member file in an archive that
resolves an undefined symbol, the file would add both undefined and defined
symbols. We would need to scan the list of symbols again.
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> I think that wouldn't work. When we find a member file in an archive
that
> resolves an undefined symbol, the file would add both undefined and defined
> symbols. We would need to scan the list of symbols again.
Yes, keep scanning until no member is added.

Cheers,
Rafael

Newly-added undefined symbols may be solved by other file in the same
archive, so you've got to restart scanning from beginning. You would visit
the same symbol many times. Is that faster?

On Fri, Mar 20, 2015 at 3:03 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
>  > I think that wouldn't work. When we find a member file in an
archive
> that
> > resolves an undefined symbol, the file would add both undefined and
> defined
> > symbols. We would need to scan the list of symbols again.
>
> Yes, keep scanning until no member is added.
>
> Cheers,
> Rafael
>
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