Re: [PATCH] c++, driver: Fix -static-libstdc++ for targets without Bstatic/dynamic.

[Iain Sandoe <iain@sandoe.co.uk>]

> On 22 Dec 2022, at 21:15, Jason Merrill <jason@redhat.com> wrote:
> 
> On 12/4/22 11:30, Iain Sandoe wrote:
>> This fixes a long-standing problem on Darwin where we cannot independently set
>> -static-libstdc++ because the flag gets stripped by the g++ driver.
>> This patch is essentially the same as the one used for the 'D' driver and has
>> been in local use for some time.  It has also been tested on Linux.
>> OK for master?
>> backports?
>> thanks
>> Iain
>> -- >8 --
>> The current implementation for swapping between the static and shared c++
>> runtimes relies on the static linker supporting Bstatic/dynamic which is
>> not available for every target (Darwin's linker does not support this).
>> Specs substitution (%s) is an alternative solution for this (which is what
>> Darwin uses for Fortran, D and Objective-C).  However, specs substitution
>> requires that the '-static-libstdc++' be preserved in the driver's command
>> line.  The patch here arranges for this to be done when the configuration
>> determines that linker support for Bstatic/dynamic is missing.
> 
> Would it work to define LIBSTDCXX_STATIC instead?

Not without modifying the build of libstdc++.  When Darwin’s linker sees a convenience library with the
same name as a shared one, it will pick the shared one, so we would have to modify the build of libstdc++
to make the library named libstdc++-static.a or so (that was essentially what the Apple gcc-4.2.1 impl.
did AFAIR).

>  If not, the patch is OK.

Thanks.

> 
> Really there should be a way for lang_specific_driver to mark a flag as "validated" rather than prune it.

Yes - especially since we now already three drivers (c++, d, gm2) that need to do the same stuff for
handing libstdc++.
Iain

> 
>> Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
>> gcc/cp/ChangeLog:
>> 	* g++spec.cc (lang_specific_driver): Preserve -static-libstdc++ in
>> 	the driver command line for targets without -Bstatic/dynamic support
>> 	in their static linker.
>> ---
>>  gcc/cp/g++spec.cc | 5 +++++
>>  1 file changed, 5 insertions(+)
>> diff --git a/gcc/cp/g++spec.cc b/gcc/cp/g++spec.cc
>> index 3d3b042dd56..f95d7965355 100644
>> --- a/gcc/cp/g++spec.cc
>> +++ b/gcc/cp/g++spec.cc
>> @@ -234,7 +234,12 @@ lang_specific_driver (struct cl_decoded_option **in_decoded_options,
>>    	case OPT_static_libstdc__:
>>  	  library = library >= 0 ? 2 : library;
>> +#ifdef HAVE_LD_STATIC_DYNAMIC
>> +	  /* Remove -static-libstdc++ from the command only if target supports
>> +	     LD_STATIC_DYNAMIC.  When not supported, it is left in so that a
>> +	     back-end target can use outfile substitution.  */
>>  	  args[i] |= SKIPOPT;
>> +#endif
>>  	  break;
>>    	case OPT_stdlib_:
>