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

[Oleg Ranevskyy via llvm-dev]

Thank you for looking into this.

--discard-locals produces the same set of symbols.
--discard-all removes the unnamed .L symbols as well as other local symbols.

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
> >
>
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