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

[Marc Glisse]

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.

-- 
Marc Glisse