Proposal: fixing the 'pure' floating point problem.

Walter Bright newshound1 at digitalmars.com
Sat Mar 14 03:12:40 PDT 2009


Don wrote:
> Don wrote:
>> Walter Bright wrote:
>>> Don wrote:
>>>> That's true, but if you're in a floatingpoint module, and you call a 
>>>> non-floatingpoint module, it's your responsibility to make sure that 
>>>> the rounding mode is back to normal. You're saying that you don't 
>>>> care about the status flags. So it's your own fault if you get 
>>>> surprising results.
>>>>
>>>> The primary use for adjusting the rounding mode is for things like 
>>>> implementing interval arithmetic. Thus, it's only ever used for 
>>>> small functions.
>>>
>>> Perhaps we can go with something simpler. If you call a pure 
>>> function, then the modes must be set to their defaults.
>>
>> But then nothing in std.math can be pure.
> My proposal is pretty simple -- I doubt we can come up with anything 
> simpler that's useful.
> 
> To clarify the effect of my proposal:
> normal function calls floatingpoint function -- rounding mode respected, 
> sticky status flags can be ignored.
> floatingpoint function calls floatingpoint function -- rounding mode 
> respected, sticky flags set correctly.
> floatingpoint function calls normal function -- rounding mode may be 
> respected, or you may get default rounding instead (implementation 
> defined). Sticky flags may not be set in all cases, but none will be 
> cleared.
> 
> So, a floatingpoint function should not make any calls to normal 
> functions under circumstances in which it needs guaranteed rounding, or 
> where it relies on the sticky flags. I think that's a managable limitation.
> 
> The only other alternative I can see is to require that EVERY function 
> save the status flags and check the control register before caching any 
> pure function. Which seems a lot of complexity for the sake of a few 
> really obscure cases.

I'm still not seeing the difference between this and saying that for 
pure functions, the default modes are used. All the std.math functions 
will be pure. How is this different from floatingpoint functions calling 
normal functions?



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