I'm currently looking at code which does multi-precision floating-point arithmetic. To work correctly, that code requires values to be reduced to their final precision at well-defined points. So even if an intermediate result was computed to an 80 bit extended precision floating point register, at some point it has to be rounded to 64 bit double for subsequent operations.
The code uses a macro INEXACT to describe this requirement, but doesn't have a perfect definition. The gcc manual mentions -fexcess-precision=standard as a way to force well-defined precision for cast and assignment operations. However, it also writes:
‘-fexcess-precision=standard’ is not implemented for languages other than C
Now I'm thinking about porting those ideas to C++ (comments welcome if anyone knows an existing implementation). So it seems I can't use that switch for C++. But what is the g++ default behavior in absence of any switch? Are there more C++-like ways to control the handling of excess precision?
I guess that for my current use case, I'll probably use -mfpmath=sse in any case, which should not incur any excess precision as far as I know. But I'm still curious.