[llvm-dev] Use different LLVM IR functions for unsafe-fp / code gen options

[Tim Northover via llvm-dev]

On 12 July 2016 at 06:10, Nikolay Haustov via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I don't think there is currently a way to overload function (have different
> versions) in bitcode library based on function attributes/metadata. Any
> other ways to do the same?

Depending on how hacky you want to get, a header containing this would work:

    #ifdef __FAST_MATH__
    #define USE_FAST_VERSION true
    #else
    #define USE_FAST_VERSION false
    #endif
    template<bool Fast> float log(float in);

    #define log(x) log<USE_FAST_VERSION>(x);

I believe that could be made zero-overhead with linker trickery.

Other than that, you'd probably have to modify LLVM to support it. We
already seem to have a "call fast" annotation at the call-site so it'd
be a matter of  redirecting the call at CodeGen time to one with a
special name (log$fast_math or something?), likely determined by your
ABI.

Tim.