Re: [PATCH RFC] bootstrap: Update requirement to C++11.

[Richard Biener <richard.guenther@gmail.com>]

On May 15, 2020 11:53:42 PM GMT+02:00, Jason Merrill <jason@redhat.com> wrote:
>On 5/15/20 2:21 PM, Richard Biener wrote:
>> On May 15, 2020 7:30:38 PM GMT+02:00, Jason Merrill
><jason@redhat.com> wrote:
>>> On Fri, May 15, 2020 at 3:15 AM Richard Biener
>>> <richard.guenther@gmail.com>
>>> wrote:
>>>
>>>>> +# When bootstrapping with GCC, build stage 1 in C++11 mode to
>>> ensure
>>>> that a
>>>>> +# C++11 compiler can still start the bootstrap.
>>>>>   if test "$enable_bootstrap:$GXX" = "yes:yes"; then
>>>>> +  CXX="$CXX -std=gnu++11"
>>>>
>>>> So I just spotted this - since we're requiring a ISO C++11 compiler
>shouldn't
>>>> we build stage1 with -std=c++11 rather than gnu++11 (whatever the
>detailed
>>>> differences are here)?  Also not sure what level of -pedantic we'd
>need to
>>>> avoid GNU extensions even with -std=c++11.  Of course there are (I
>hope)
>>>> a lot less GNU extensions for C++ than there were for C and
>hopefully
>>>> no extra in gnu++11 compared to gnu++98 which we checked
>previously.
>
>Building stage 1 with -std=c++11 -pedantic-errors works with 8.3.1, but
>
>fails pretty badly with 4.8.5,
>
>>> When we first moved to C++ I tried using -std=c++98, but there were
>too
>>> many places where we were assuming that if we're building with GCC,
>we can
>>> use GNU C extensions.
>>>
>>> I'll see if that's still a problem for -std=c++11.
>
>It doesn't seem to be, so I've made that change.
>
>>>> There also does not seem to be a configure check which may present
>>>> users with a more useful error message than later cryptic fail of
>build?
>>>> I suppose we cannot simply check __cplusplus for this, can we?  Do
>>>> other common host compilers need additional options to enable
>C++11?
>>>
>>> Good point, I'll add that.
>
>This patch uses a test from the autoconf archive to add any needed 
>flags.  Tested with GCC 4.8.5 and clang 3.4.2 (with the above stage 1 
>-std=c++11 disabled).
>
>>>> Should we try to second guess such flags via configury?  For
>example
>>>> GCC 4.8 defaults to -std=gnu++98 and the above only seems to apply
>>>> to the bootstrap case so GCC 4.8 cannot be used to build cross
>>> compilers
>>>> without adjusting CC and CXX?
>>>
>>> Older GCC is still GCC and will get the flag automatically.
>> 
>> But yes:yes suggests that when building a cross compiler this doesn't
>apply?
>
>True, but the new test should cover that case.
>
>OK for trunk?

OK if there are no further comments over the weekend. 

Thanks, 
Richard.