[PATCH] RISC-V: fix build issue with gcc 4.9.x

[Kito Cheng]

> >>> Pushed to trunk, thanks for catching that, that's definitely should
> >>> use log2 no matter C++03 or C++11,
> >>> but I think GCC allows the usage of C++11 according to
> >>> https://gcc.gnu.org/install/prerequisites.html :P
> >> Yes, we should be able to use C++11.  I'd like to get that to C++17 at
> >> some point, but I think the biggest problem is the desire to support
> >> bootstrapping on something like centos7/rhel7.
> >
> > At least we have auto and range based for loop, I am satisfied with
> > that enough.
>
> Indeed, gcc 4.9 already support C++11 but for some reason std::log2 fail with
> it. Probably because gcc 4.9 is the last gcc release with C++03 used by default.
> I wasn't able to reproduce the build issue (without my patch) with gcc 10 or 11.

Anyway I think your fix is reasonable since we include math.h not cmath.h here,
so use without std:: is rightway, but don't know why it is build-able
with newer GCC,
I guess that might be included by some other header indirectly.

> I'm fine with new prerequisites for the next gcc release, I checked the release
> note about that. I noticed that all toolchains I've built with gcc 13.1 for
> other cpu target where successful with the same setup (docker image).
>
> Thanks!
>
> Best regards,
> Romain
>
> >
> >
> >>
> >> jeff
>