[llvm-dev] Fwd: Skipping names of temporary symbols increased size of ARM binaries.

[Oleg Ranevskyy via llvm-dev]

Hi Rafael,

Could you advise what might be done next about this, please?
Would it be reasonable to discuss the issue in the binutils mailing list?

Thank you.

On Sat, Sep 19, 2015 at 10:02 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> Honestly this looks like a bug in the linker.
>
>
> Do they still show up with --discard-locals and/or --discard-all?
>
> On 18 September 2015 at 12:10, Oleg Ranevskyy via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > CC llvm-dev
> >
> > ---------- Forwarded message ----------
> >
> > Hello Duncan
> >
> > The size of ARM binaries created by clang has increased after r236642.
> > Would you be able to find some time to look at my findings and share your
> > thoughts about the problem, please?
> >
> > r236642 prevents emitting of temp label names into object files to save
> > memory. This is fine, the label names do not appear in the resulting
> > binaries. However, this creates some problems for the binutils linker,
> which
> > analyzes symbol names from the input object files and can decide to skip
> > some local compiler generated labels. Now it no longer sees the label
> names
> > and therefore puts them all into the final binary. I will demonstrate
> this
> > on an example.
> >
> > If we compile the attached main.cpp file for ARM
> >     clang++ -c -o main.o -O0 -g --target=armv7l-linux-gnueabihf main.cpp
> > and then look at the symbols
> >     readelf -s main.o
> > there will be a number of similar entries (showing one entry only here
> for
> > conciseness):
> >     Num:    Value  Size Type       Bind   Vis      Ndx   Name
> >       7:  00000062    0 NOTYPE  LOCAL  DEFAULT    9
> > These are the .Linfo_string<index> symbols whose names are skipped due to
> > r236642.
> >
> > If we now link it
> >     clang++ -o main.out --target=armv7l-linux-gnueabihf main.o
> > all the symbols get through to the final binary:
> >     readelf -s main.out
> >
> >     Num:    Value     Size Type         Bind      Vis           Ndx Name
> >     73:   0000006e    0    NOTYPE  LOCAL  DEFAULT   32
> >
> > The linker can't decide if the labels are the local ones to be left out
> not
> > seeing their names. Its bfd_is_local_label_name function returns false
> and
> > the labels are not skipped.
> >
> > Before r236642 the names were inserted into object files:
> >     readelf -s main.o
> >
> >     Num:    Value       Size  Type         Bind     Vis Ndx   Name
> >      23:    0000007a    0     NOTYPE  LOCAL  DEFAULT    9 .Linfo_string7
> >
> > bfd_is_local_label_name returns true if the name starts with ".L" and the
> > symbol is skipped.
> >
> > This is not critical for small projects but can create noticeable
> overhead
> > for the big ones.
> >
> > Any help will be much appreciated.
> > Thank you.
> > Oleg
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150929/3928347f/attachment.html>