GCC builtins in LDC - compiler interoperability

[Cecil Ward]

I currently have a fair amount of code that will only run under 
GDC because of reliance on GDC-specific library routines. I’d 
like to ask whether I can find or implement equivalents in LDC2 
to the GDC routines
         __builtin_expect()
and
         __builtin_unreachable() ?

Could someone point me toward the documentation for this general 
kind of stuff in LDC ?