[Buildroot] __mcount on ARC

[Alexey Brodkin <Alexey.Brodkin@synopsys.com>]

Hi Zoltan,

On Mon, 2018-02-19 at 09:09 +0100, Thomas Petazzoni wrote:
> Hello,
> 
> +Alexey in Cc.
> 
> On Sun, 18 Feb 2018 19:45:08 +0100, Zoltan Gyarmati wrote:
> 
> > I'm investigating this [1] autobuild error, where the gpsd build fails
> > while profiling is enabled, due to undefined reference to `__mcount', on
> > ARC arch.

Thanks for doing this!

> > Ironically enough due to a bug in the gpsd build config the -pg flag is
> > not passed to the linker only to the compiler, but after patching that
> > one, the build issue remains.
> > According to this thread [2] this happened earlier with ARC but other
> > package, so is there anything particular about ARC regarding to
> > profiling? Should we just disable the gpsd profiling option on ARC?
> > 
> > [1]
> > https://urldefense.proofpoint.com/v2/url?u=http-3A__autobuild.buildroot.net_results_88870f5bf4aff557d8eac4e1cc5d3e397e607af0_&d=DwICAg&c=DPL6_X_6J
> > kXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656eViXO7breS55ytWkhpk5R81I&m=qUcbt2aQC5rv6A60vraf7le_bHJUKsNqmF82o_vwG88&s=Ex2r0cYgj4xqNS_I3TFKJaGy4ZSEyhGl1WyyW1
> > 880sY&e=
> > 
> > [2]
> > https://urldefense.proofpoint.com/v2/url?u=http-3A__buildroot-2Dbusybox.2317881.n4.nabble.com_autobuild-2Dbuildroot-2Dnet-2DBuild-2Dresults-2Dfor-
> > 2D2017-2D11-2D19-
> > 2Dtd180416.html&d=DwICAg&c=DPL6_X_6JkXFx7AXWqB0tg&r=lqdeeSSEes0GFDDl656eViXO7breS55ytWkhpk5R81I&m=qUcbt2aQC5rv6A60vraf7le_bHJUKsNqmF82o_vwG88&s=Ig
> > Fx-7wIxdakr0F3vfv1Nlf6kH6Z62gCs0QqtOh0TGI&e=

If you're talking about
http://autobuild.buildroot.net/results/69032f10edcfd65fd81d62a6854a4c06fa204d76/
then it's not the same, there uClibc was used which is known to be lacking
gprof support but in case of gpsd we deal with glibc so read-on...

> Alexey, could you comment on this ? I vaguely remember that profiling
> was not supported on ARC at some point, but I have no idea if this is
> still the case or not.

Looks like ARC port of glibc only defines "_mcount" but not "__mcount", see
------------------------------------->8---------------------------------
arc-linux-readelf -s libc.so.6 | grep mcount
   124: 000b397c     4 FUNC    GLOBAL DEFAULT   11 _mcount@@GLIBC_2.26
   423: 000df95c    44 FUNC    GLOBAL DEFAULT   11 _dl_mcount_wrapper_check@@GLIBC_2.26
  1584: 000df944    22 FUNC    GLOBAL DEFAULT   11 _dl_mcount_wrapper@@GLIBC_2.26
------------------------------------->8---------------------------------

and https://github.com/foss-for-synopsys-dwc-arc-processors/glibc/blob/vineet-glibc-master/sysdeps/arc/machine-gmon.h#L26

I'm an expert in neither glibc nor gprof so not sure what is a difference between
_mcount vs __mcount but for AArch64 thay set "strong_alias (__mcount, _mcount)" here:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/aarch64/mcount.c#l32

So maybe we need to do the same for ARC.

Adding Vineet's who's doing ARC port of glibc to maybe add this missing bit :)

-Alexey