Floating point rounding modes: we should restrict them slightly

[Rainer Deyke]

Don wrote:
> The pure function memoisation issue:
> Certain functions may be marked as 'pure', even though they use floating
> point (and are therefore dependent on the FP state). The compiler must
> be able to prevent memoisation of those functions in cases where the
> rounding mode may have changed. (Or, equivalently, the memoisation must
> include the FP state).

Don't forget that "uses floating point" is a transitive property.  Any
pure function that calls a pure-but-unmemoisable function is itself
pure-but-unmemoisable.  The "pure" annotation is now used for two
related but different categories of functions with different properties,
and the compiler needs some way to keep track of which is which across
module boundaries.  If the compiler can do that, then it can use the
same mechanism to differentiate between pure and non-pure functions, and
the "pure" annotation becomes redundant.

Is there any real need to treat floating point state differently from
other global state in regard to "pure"?


-- 
Rainer Deyke - rainerd at eldwood.com