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

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

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.
>>
>
>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.
>
>Note I think what's missing is some general blurb in our coding
>conventions
>> as to how much of C++11 we are supposed to use in non-infrastructure
>parts
>> of GCC (I expect things like hash-table.h to use more C++ features
>than,
>> say, tree-ssa-alias.c).
>>
>> 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.
>
>
>> 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? 

Richard. 

>Jason