[Patch] Fix libstdc++ build when using uclibc instead of glibc

[Jonathan Wakely]

On 07/01/16 07:28 +0100, Marc Glisse wrote:
>On Wed, 6 Jan 2016, Jonathan Wakely wrote:
>
>>On 06/01/16 12:45 -0800, Steve Ellcey wrote:
>>>On Wed, 2016-01-06 at 20:34 +0000, Jonathan Wakely wrote:
>>>>On 05/01/16 14:43 -0800, Steve Ellcey  wrote:
>>>>>While trying to build GCC with uclibc instead of glibc I ran into a build
>>>>>failure in libstdc++.  uclibc doesn't seem to provide the 
>>>>isfinite function
>>>>>like glibc does so that means that libstdc++ doesn't have std::isfinite.
>>>>>
>>>>>This in turn caused include/ext/random.tcc to not compile because it uses
>>>>>std::isfinite.  I can easily see how this might be considered a uclibc
>>>>>bug but given that it is the only build problem I ran into I was hoping
>>>>>we could fix it in libstdc++ by using __builtin_finite instead of
>>>>
>>>>Shouldn't that be __builtin_isfinite not __builtin_finite ?
>>>
>>>You are right.  For some reason I thought that isfinite was implemented
>>>as __builtin_finite and that there was no __builtin_isfinite.  I am not
>>>sure why I thought that but it does not seem to be the case.  I will
>>>update my patch, retest, and resubmit.
>>
>>Thanks.
>>
>>Regarding the substance of the patch, the usual approach would be to
>>gate the relevant parts of <random> (or even the whole thing) on the
>>presence of std::isinfinite, but that would presumably disable the
>><random> functionality completely for uclibc. If using the builtin
>>works reliably then I'm OK with that.
>>
>>However, I would expect that for un-optimized code the builtin will
>>just emit a call to isfinite() in the C library, and so if uclibc
>>doesn't define that at all then that will result in a linker error.
>
>I thought that was half the reason you asked for isfinite (the other 
>half being that __builtin_isfinite is generic while __builtin_finite 
>(and the [fl] variants) is not). If you look at builtins.def, isfinite 
>is declared with DEF_GCC_BUILTIN (no library fallback), as opposed to 
>finite with DEF_EXT_LIB_BUILTIN.

You give me too much credit :-)

I was only concerned with the generic property, but if it doesn't
require a library fallback then __builtin_isfinite should work fine,
and we don't need to worry about investigating whether something is
wrong with the autoconf checks.