Re: isl 0.26 C++17 issues when bootstrapping with GCC

[Iain Sandoe <iain@sandoe.co.uk>]

> On 13 May 2024, at 19:43, Iain Sandoe <iain@sandoe.co.uk> wrote:
> 
> 
> 
>> On 13 May 2024, at 18:46, Richard Biener <richard.guenther@gmail.com> wrote:
>> 
>> On Mon, May 13, 2024 at 6:00 PM Iain Sandoe <iain@sandoe.co.uk> wrote:
>>> 
>>> 
>>> 
>>>> On 13 May 2024, at 16:05, Iain Sandoe via Gcc <gcc@gcc.gnu.org> wrote:
>>>>> On 30 Aug 2023, at 00:32, Ben Boeckel via Gcc <gcc@gcc.gnu.org> wrote:
>>>>> 
>>>>> On Tue, Aug 29, 2023 at 18:57:37 +0200, Richard Biener wrote:
>>>>>> I suppose for bootstrapping we could disable ISL during stage1 since
>>>>>> it enables an optional feature only.  Other than that GCC only
>>>>>> requires a C++11 compiler for building, so ISL breaks that constraint
>>>>>> with requiring C++17.
>>>>> 
>>>>> Note that it doesn't *require* it per sé; the tests that try it are
>>>>> compiled if C++17 support was detected at all. The headers seem to just
>>>>> have optional header-only `std::any`-using APIs if C++17 is around.
>>>>> `isl` supporting a flag to disable the tests would also work, but that
>>>>> doesn't fix 0.26. It also doesn't mean it won't start requiring C++17 at
>>>>> some point in the future.
>>>> 
>>>> Perhaps, in the short-term (i.e. before it requires C++ > 11) we can
>>>> solve this by ensuring that we pass -std=c++11 to the configure stages
>>>> as well as to the build.  ISTM that configure is finding C++17-capability
>>>> (because we do not, I think, force C++11 for the configure) and then
>>>> the build takes it away by forcing -std=c++11.
>>> 
>>> That was not right.
>>> We add std=c++11 to the compiler command.
>>> 
>>> However,as noted (earlier in this thread) the isl configure has the idiom
>>> - does the compiler do c++17 with no options?
>>> - does the compiler do c++17 if we add -std=c++17?
>>> 
>>> where the second one overrides our setting of std=c++11 in the compiler
>>> comand.
>>> 
>>> (I think that this is a reasonably often used idiom in configures)
>>> 
>>> However the isl configure _does_ still append CXXFLAGS, and so that if
>>> we add -std=c++11 to those, it re-asserts our intent.
>>> 
>>> Maybe we should just add the -std=c++11 to CXXFLAGS instead of the
>>> compiler command?
>> 
>> I don't understand.  If we set CXX to g++ -std=c++11 and ISL checks
>> for -std=c++17 why does it then fail to add that to CXXFLAGS?
> 
> This appears to be the underlying bug.
> 
> — isl configure.ac does:
> 
> AX_CXX_COMPILE_STDCXX_17([], [optional])
> 
> ….
> 
> AM_CONDITIONAL(HAVE_CXX17, test "x$HAVE_CXX17" = "x1”)
> 
> — and then Makefile.am adds the c++17-requirements:
> 
> if HAVE_CXX17
>  noinst_PROGRAMS += isl_test_cpp17 isl_test_cpp17-checked
>  TESTS += isl_test_cpp17 isl_test_cpp17-checked
> endif
> 
> .. this mechanism does not seem to preserve the fact that an additional
> -std=c++17 was needed to get the CXX17 (and there is no mention of
> CXXFLAGS in Makefile.am) 
> 
> Not sure if it’s a bug in isl’s config - or a limitation of 
> AX_CXX_COMPILE_STDCXX_17 itself.
> 
> — assuming we file a bug and it gets agreed and fixed, we’ll still need 
> either to skip broken versions or work around it (I have no specific
> preference - although I do build isl in-tree, so far 0.24 has been OK)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115077