Nothrow, pure in druntime

[Walter Bright]

Don wrote:
> tango.math doesn't use the C library at all, except when inline asm is 
> unavailable. Of they differ from the C functions, in that none of them 
> set errno!
> One really annoying issue still remains, though -- the floating point 
> flags in the CPU. They are entirely deterministic, but are they 
> considered to be part of the return value of the function? Or would we 
> allow them to be ignored?
> A compiler could check the exception flags before allowing memoisation. 
> But one could also do the same thing for 'errno'.
> 
> Likewise, floating point rounding modes. Essentially, the floating point 
>   status register is a hidden global variable, read from# and written to 
> during every floating point operation.
> 
> # - only the rounding mode and truncation affect the return value. We 
> could deal with it by regarding that as a whole-program setting. But 
> (depending on the CPU), the old exception flags generally get ORed with 
> the new exception flags.

Those are good points. I don't know what the answer is. My inclination 
is to say if your program relies on changing the rounding mode or 
fiddles with the exception flags, it's undefined behavior.

> Also, you can set the flags to allow any floating point function to 
> throw a hardware exception. It's difficult for any function using 
> floating point to claim to be nothrow under ANY circumstances; but 
> that's a horrible limitation.

I would say that is not supported by D. I've never heard of a use for them.