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

[Tom de Vries via Gdb-patches <gdb-patches@sourceware.org>]

On 9/27/21 2:59 PM, Tom de Vries via Gdb-patches wrote:
> On 9/23/21 4:27 PM, 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 $CXX_DIALECT to the test for std::thread support, but that would
>>> have to be repeated for each added c++ support test.
>>>
>>> Instead, fix this by doing:
>>> ...
>>> CXX="g++ -std=gnu++11"
>>> CXX_DIALECT=-std=gnu++11
>>> ...
>>>
>>> The code added in gdb/ax_cxx_compile_stdcxx.m4 is copied from the default
>>> AX_CXX_COMPILE_STDCXX from autoconf.
>> From autoconf-archive?
>>
> Yes, updated.
> 
>> At first glance, it looks like CXX_DIALECT would not be needed anymore,
>> since it's only used to be appended to CXX in Makefiles.  So now it
>> would end up twice in the command-line, which is not harmful but not
>> useful.
>>
>> However, if we look at the original explanation of why we have this
>> local modification:
>>
>>   We need to tweak AX_CXX_COMPILE_STDCXX a bit though.  Pristine
>>   upstream AX_CXX_COMPILE_STDCXX appends -std=gnu++11 to CXX directly.
>>   That doesn't work for us, because the top level Makefile passes CXX
>>   down to subdirs, and that overrides whatever gdb/Makefile may set CXX
>>   to.  The result would be that a make invocation from the build/gdb/
>>   directory would use "g++ -std=gnu++11" as expected, while a make
>>   invocation at the top level would not.
>>
>>   So instead of having AX_CXX_COMPILE_STDCXX set CXX directly, tweak it
>>   to AC_SUBST a separate variable -- CXX_DIALECT -- and use '$(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
>>
>> 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.
> Ack, I've rewritten the commit log to make this more clear (and copied
> part of you comments).
> 
> Any further comments?
> 

I'll commit this tomorrow, unless there are further comments.

Thanks,
- Tom


> 0001-gdb-build-Add-CXX_DIALECT-to-CXX.patch
> 
> [gdb/build] Add CXX_DIALECT to CXX
> 
> Say we use a gcc version that (while supporting c++11) does not support c++11
> by default, and needs an -std setting to enable it.
> 
> If gdb would use the default AX_CXX_COMPILE_STDCXX from autoconf-archive, then
> we'd have:
> ...
> CXX="g++ -std=gnu++11"
> ...
> 
> That mechanism however has the following problem (quoting from commit
> 0bcda685399):
> ...
> 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.
> ...
> 
> Commit 0bcda685399 fixes this by using a custom AX_CXX_COMPILE_STDCXX which
> does:
> ...
> CXX=g++
> CXX_DIALECT=-std=gnu++11
> ...
> 
> The problem reported in PR28318 is that using the custom instead of the
> default AX_CXX_COMPILE_STDCXX makes the configure test for std::thread
> support fail.
> 
> We could simply 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
> ...
> 
> This is somewhat awkward, since it results in -std=gnu++11 occuring twice in
> some situations:
> ...
> $ touch src/gdb/dwarf2/read.c
> $ ( cd build/gdb; make V=1 dwarf2/read.o )
> g++-4.8 -std=gnu++11 -x c++ -std=gnu++11 ...
> ...
> 
> However, both settings are needed:
>  - 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
> 
> The code added in gdb/ax_cxx_compile_stdcxx.m4 is copied from the default
> AX_CXX_COMPILE_STDCXX from autoconf-archive.
> 
> Tested on x86_64-linux.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28318
> 
> ---
>  gdb/ax_cxx_compile_stdcxx.m4 | 8 ++++++++
>  gdb/configure                | 8 ++++++++
>  gdbserver/configure          | 8 ++++++++
>  gdbsupport/configure         | 8 ++++++++
>  4 files changed, 32 insertions(+)
> 
> diff --git a/gdb/ax_cxx_compile_stdcxx.m4 b/gdb/ax_cxx_compile_stdcxx.m4
> index 413755a2e88..29d8e10bcc4 100644
> --- a/gdb/ax_cxx_compile_stdcxx.m4
> +++ b/gdb/ax_cxx_compile_stdcxx.m4
> @@ -94,6 +94,10 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
>           CXX="$ac_save_CXX"])
>        if eval test x\$$cachevar = xyes; then
>          CXX_DIALECT="$switch"
> +        CXX="$CXX $switch"
> +        if test -n "$CXXCPP" ; then
> +          CXXCPP="$CXXCPP $switch"
> +        fi
>          ac_success=yes
>          break
>        fi
> @@ -118,6 +122,10 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
>             CXX="$ac_save_CXX"])
>          if eval test x\$$cachevar = xyes; then
>            CXX_DIALECT="$switch"
> +          CXX="$CXX $switch"
> +          if test -n "$CXXCPP" ; then
> +            CXXCPP="$CXXCPP $switch"
> +          fi
>            ac_success=yes
>            break
>          fi
> diff --git a/gdb/configure b/gdb/configure
> index f0b1af4a6ea..98badb46cfd 100755
> --- a/gdb/configure
> +++ b/gdb/configure
> @@ -5841,6 +5841,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>        if eval test x\$$cachevar = xyes; then
>          CXX_DIALECT="$switch"
> +        CXX="$CXX $switch"
> +        if test -n "$CXXCPP" ; then
> +          CXXCPP="$CXXCPP $switch"
> +        fi
>          ac_success=yes
>          break
>        fi
> @@ -6160,6 +6164,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>          if eval test x\$$cachevar = xyes; then
>            CXX_DIALECT="$switch"
> +          CXX="$CXX $switch"
> +          if test -n "$CXXCPP" ; then
> +            CXXCPP="$CXXCPP $switch"
> +          fi
>            ac_success=yes
>            break
>          fi
> diff --git a/gdbserver/configure b/gdbserver/configure
> index b227167e270..f05c1a9b976 100755
> --- a/gdbserver/configure
> +++ b/gdbserver/configure
> @@ -5625,6 +5625,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>        if eval test x\$$cachevar = xyes; then
>          CXX_DIALECT="$switch"
> +        CXX="$CXX $switch"
> +        if test -n "$CXXCPP" ; then
> +          CXXCPP="$CXXCPP $switch"
> +        fi
>          ac_success=yes
>          break
>        fi
> @@ -5944,6 +5948,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>          if eval test x\$$cachevar = xyes; then
>            CXX_DIALECT="$switch"
> +          CXX="$CXX $switch"
> +          if test -n "$CXXCPP" ; then
> +            CXXCPP="$CXXCPP $switch"
> +          fi
>            ac_success=yes
>            break
>          fi
> diff --git a/gdbsupport/configure b/gdbsupport/configure
> index a9dd02c5b72..ae6047865ae 100755
> --- a/gdbsupport/configure
> +++ b/gdbsupport/configure
> @@ -6520,6 +6520,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>        if eval test x\$$cachevar = xyes; then
>          CXX_DIALECT="$switch"
> +        CXX="$CXX $switch"
> +        if test -n "$CXXCPP" ; then
> +          CXXCPP="$CXXCPP $switch"
> +        fi
>          ac_success=yes
>          break
>        fi
> @@ -6839,6 +6843,10 @@ eval ac_res=\$$cachevar
>  $as_echo "$ac_res" >&6; }
>          if eval test x\$$cachevar = xyes; then
>            CXX_DIALECT="$switch"
> +          CXX="$CXX $switch"
> +          if test -n "$CXXCPP" ; then
> +            CXXCPP="$CXXCPP $switch"
> +          fi
>            ac_success=yes
>            break
>          fi
>