Rainer Deyke wrote:
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.
This is incorrect. You've got it backwards. It's not "uses floating
point" that's the issue, it's "may use non-default floating point". A
memoisable floating point function can only use default floating point.
So, if it calls a non-memoisable pure function, it'll be using it in the
default state (even though that function can accept non-default state).
The transitivity propagates in the reverse direction: memoisability is
transitive, non-memoisability is not. Non-memoisable functions cannot
call memoisable ones without first ensuring that everything is back into
the default state.
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.
It's true that you could provide a similar loophole for any specific
global variable. But you'd need a *very* good reason.
Is there any real need to treat floating point state differently from
other global state in regard to "pure"?
Yes. Because it's all-pervasive. Unless you treat it specially, you
cannot use floating point in any pure function. That creates a
restriction which is so severe that 'pure' becomes useless. Something
like the following template function could not be pure:
T add(T a, T b) { return a + b; }
I do not believe that any other global state has an influence which is
in the same league.
