[libcamera-devel] [PATCH v3 4/7] libcamera: Don't unnecessarily link against libatomic

[Laurent Pinchart]

Hi Kieran,

On Sun, Mar 08, 2020 at 01:27:14AM +0200, Laurent Pinchart wrote:
> On Sat, Mar 07, 2020 at 11:01:06PM +0000, Kieran Bingham wrote:
> > On 07/03/2020 21:13, Laurent Pinchart wrote:
> > > libatomic provides atomic operations that are also available as compiler
> > > built-ins on most platforms. On some platforms (such as Sparc) linking
> > > against libatomic is required to use any atomic operation. libcamera
> > > links against libatomic if available for that reason.
> > > 
> > > This however makes libcamera link against libatomic even when not
> > > required, if libatomic is available. Restrict this to only the cases
> > > where libatomic is required with a link test.
> > 
> > Hrm, I thought we discussed and researched this topic when it went in.
> > 
> > I thought I recalled testing this and identifying that even if libatomic
> > was found on the system, and would be available to link - The toolchain
> > would take the internals as a preference and caused no binary change in
> > the output.
> > 
> > But I won't overly object to this change though.
> 
> You're right, I checked again:
> 
> --- libcamera.so.1.hex  2020-03-08 01:25:05.609314371 +0200
> +++ libcamera.so.2.hex  2020-03-08 01:25:13.060315088 +0200
> @@ -35886,7 +35886,7 @@
>  0008c600  6f 6e 00 00 4d 61 6e 75  61 6c 45 78 70 6f 73 75  |on..ManualExposu|
>  0008c610  72 65 00 00 4d 61 6e 75  61 6c 47 61 69 6e 00 00  |re..ManualGain..|
>  0008c620  4c 6f 63 61 74 69 6f 6e  00 00 00 00 76 30 2e 30  |Location....v0.0|
> -0008c630  2e 30 2b 31 31 31 36 2d  38 36 32 33 31 38 36 38  |.0+1116-86231868|
> +0008c630  2e 30 2b 31 31 31 37 2d  33 63 36 37 62 33 34 34  |.0+1117-3c67b344|
>  0008c640  00 00 00 00 42 75 66 66  65 72 00 00 53 65 72 69  |....Buffer..Seri|
>  0008c650  61 6c 69 7a 61 74 69 6f  6e 00 00 00 2e 2e 2f 2e  |alization...../.|
>  0008c660  2e 2f 73 72 63 2f 6c 69  62 63 61 6d 65 72 61 2f  |./src/libcamera/|
> 
> I'll drop this patch.

By the way I got the idea from
https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/70303d211886ae109dfc0a88948c362359bdd07c.
I wonder if it means they're doing it wrong too.

> > > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > 
> > Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> > 
> > > ---
> > >  meson.build               |  1 +
> > >  src/libcamera/meson.build |  2 +-
> > >  src/meson.build           | 17 +++++++++++++++++
> > >  3 files changed, 19 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/meson.build b/meson.build
> > > index c6e6a934e54e..a1e455249b0d 100644
> > > --- a/meson.build
> > > +++ b/meson.build
> > > @@ -24,6 +24,7 @@ libcamera_version = libcamera_git_version.split('+')[0]
> > >  , 
> > >  # Configure the build environment.
> > >  cc = meson.get_compiler('c')
> > > +cxx = meson.get_compiler('cpp')
> > >  config_h = configuration_data()
> > >  
> > >  if cc.has_header_symbol('execinfo.h', 'backtrace')
> > > diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> > > index 1b69bc0dee54..ac6f597c6188 100644
> > > --- a/src/libcamera/meson.build
> > > +++ b/src/libcamera/meson.build
> > > @@ -93,7 +93,7 @@ version_cpp = vcs_tag(command : [gen_version, meson.build_root()],
> > >  libcamera_sources += version_cpp
> > >  
> > >  libcamera_deps = [
> > > -    cc.find_library('atomic', required: false),
> > > +    libatomic_dep,
> > >      cc.find_library('dl'),
> > >      libudev,
> > >      dependency('threads'),
> > > diff --git a/src/meson.build b/src/meson.build
> > > index d818d8b86d93..abf755936f2b 100644
> > > --- a/src/meson.build
> > > +++ b/src/meson.build
> > 
> > Hrm ... is this the right place for this?
> > 
> > I guess it's not a 'bad' place for it.
> > Our top level meson.build is getting busier and busier... and this
> > presumably affects only 'src' code (hence being under src) ... but then
> > 'test/' misses out?
> > 
> > Still I guess test/ may be less likely to use atomics...
> 
> test/meson.build is included after src/meson.build, so libatomic_dep is
> available for it.
> 
> > > @@ -1,3 +1,20 @@
> > > +# On some platforms gcc doesn't provide built-ins for atomic operations,
> > > +# explicit linking against libatomic is required.
> > > +if cxx.links('''
> > > +    #include <atomic>
> > > +    #include <cstdint>
> > > +    int main(void)
> > > +    {
> > > +        std::atomic<std::uint32_t> u32(0);
> > > +        std::atomic<std::uint64_t> u64(0);
> > > +        return u32.load() + u64.load();
> > > +    }
> > > +''')
> > > +    libatomic_dep = dependency('', required : false)
> > > +else
> > > +    libatomic_dep = cc.find_library('atomic')
> > > +endif
> > > +
> > >  if get_option('android')
> > >      subdir('android')
> > >  endif

-- 
Regards,

Laurent Pinchart