Use __builtin_tgmath in tgmath.h with GCC 8 (bug 21660) [committed]

[Florian Weimer]

On 11/17/2017 02:00 PM, Szabolcs Nagy wrote:
> On 17/11/17 12:50, Florian Weimer wrote:
>> On 11/17/2017 01:11 PM, Szabolcs Nagy wrote:
>>>> +/* There are two variant implementations of type-generic macros in
>>>> +   this file: one for GCC 8 and later, using __builtin_tgmath and
>>>> +   where each macro expands each of its arguments only once, and one
>>>> +   for older GCC, using other compiler extensions but with macros
>>>> +   expanding their arguments many times (so resulting in exponential
>>>> +   blowup of the size of expansions when calls to such macros are
>>>> +   nested inside arguments to such macros).  */
>>>> +
>>>> +#define __HAVE_BUILTIN_TGMATH __GNUC_PREREQ (8, 0)
>>>
>>> this breaks the build on trunk toolchains that
>>> lack __builtin_tgmath
>>
>> Short of getting GCC to adopt something similar to the Clang __has_builtin pseudo-macro, I don't see any
>> solution other that requiring those who are affected by such issues to update.
>>
>> Considering that this only affects an unreleased toolchain, I don't think the upgrade requirement is
>> unreasonably onerous.
>>
> 
> in this case prereq(8,1) would work

But this would mean that we do near-zero pre-release testing of this 
glibc feature, which seems very undesirable to me.

Thanks,
Florian