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

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

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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?

Thanks,
- Tom

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[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