Re: [PATCH, master+11][gdb/build] Add CXX_DIALECT to CXX

[Pedro Alves <pedro@palves.net>]

On 2021-09-23 3:27 p.m., Simon Marchi wrote:
> 
> 
> On 2021-09-08 8:15 a.m., Tom de Vries wrote:
>> Hi,
>>
>> The problem reported in PR28318 is that when using a gcc version that (while
>> supporting c++11) does not support c++11 by default, the configure test for
>> std::thread support will fail.
>>
>> If gdb would use the default AX_CXX_COMPILE_STDCXX from autoconf, then we'd
>> have:
>> ...
>> CXX="g++ -std=gnu++11"
>> ...
>> and the test for std::thread support would succeed.
>>
>> Instead, gdb uses a custom AX_CXX_COMPILE_STDCXX (see commit 0bcda685399)
>> which does:
>> ...
>> CXX=g++
>> CXX_DIALECT=-std=gnu++11
>> ...
>>
>> We could add $CXX_DIALECT to the test for std::thread support, but that would
>> have to be repeated for each added c++ support test.
>>
>> Instead, fix this by doing:
>> ...
>> CXX="g++ -std=gnu++11"
>> CXX_DIALECT=-std=gnu++11
>> ...
>>
>> The code added in gdb/ax_cxx_compile_stdcxx.m4 is copied from the default
>> AX_CXX_COMPILE_STDCXX from autoconf.
> 
> From autoconf-archive?
> 
> At first glance, it looks like CXX_DIALECT would not be needed anymore,
> since it's only used to be appended to CXX in Makefiles.  So now it
> would end up twice in the command-line, which is not harmful but not
> useful.
> 
> However, if we look at the original explanation of why we have this
> local modification:
> 
>   We need to tweak AX_CXX_COMPILE_STDCXX a bit though.  Pristine
>   upstream AX_CXX_COMPILE_STDCXX appends -std=gnu++11 to CXX directly.
>   That doesn't work for us, because the top level Makefile passes CXX
>   down to subdirs, and that overrides whatever gdb/Makefile may set CXX
>   to.  The result would be that a make invocation from the build/gdb/
>   directory would use "g++ -std=gnu++11" as expected, while a make
>   invocation at the top level would not.
> 
>   So instead of having AX_CXX_COMPILE_STDCXX set CXX directly, tweak it
>   to AC_SUBST a separate variable -- CXX_DIALECT -- and use '$(CXX)
>   (CXX_DIALECT)' to compile/link.
> 
>   https://gitlab.com/gnutools/binutils-gdb/-/commit/0bcda68539948828795564b35a497dc69c27f768
> 
> So it sounds like we need both (and your patch is correct):
> 
>  - the switch in CXX for the std::thread tests (and other tests)
>  - the switch in CXX_DIALECT so it can be appended in Makefiles, to
>    counteract the fact that the top-level Makefile overrides CXX

Thanks for raising this aspect.  I had been meaning to reply raising it, and ask
where that original motivation is still a concern, as it wasn't clear
from the patch description.

Pedro Alves

> 
> This should probably be explained in the comment, because that situation
> is strange, and people might wonder why we end up with two -std=...
> switches, and might try to "fix" it.
> 
> Simon
>