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

[Rafael Espíndola via llvm-dev]

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
>
>
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