[cfe-dev] Compiling with both -flto and -g -- is it supposed to work ?

[David Blaikie via cfe-dev]

On Thu, Jun 11, 2020 at 7:56 PM Philippe Blain
<levraiphilippeblain at gmail.com> wrote:
>
> Hi David,
>
> > Le 9 juin 2020 à 11:16, David Blaikie <dblaikie at gmail.com> a écrit :
> >
> > Not sure how this works on MacOS where object files are needed for
> > debugging - on Linux lto works with -g because debug info is linked
> > into the final binary. (CC'd some Apple folks who might know how this
> > is supposed to work)
>
> Thanks.
>
> I ended up making additional tests, and adding `-v` to the clang invocation
> on  a simple hello-world type project made me realize
> that I need to add `-Wl,object_path_lto,lto.o` to the linking command
> if doing compilation and liking in separate steps (as is usually
> the case with Make- and Automake- based build system!)
>
> This flag is automatically added by the driver if doing compilation+linking in a single step,
> and if it is not passed to the linker, `man ld` reveals that the temporary object file used
> for link time optimization is deleted after the linking step is finished, so the executable contains
> a reference to `/tmp/lto.o` which does not exist anymore when debugging actually starts
> (I guess this path is hardcoded in ld64's code, I haven't checked that.)
>
> Adding the flag as above keeps `lto.o` in the build directory, and debuggers are able to read
> the debugging symbols. Also, `dsymutil`can then be run manually to create the dSYM bundle (which
> is also done automatically if compiling and linking in a single step).
>
> I'm not sure anything can be done to improve the situation... maybe add some notes in the
> LTO documentation ? I could try to do that.

Yeah - wherever you think extra info might've helped hasten your
discovery would be great, thanks!