>>>>> "quick" == quick <qu...@sparq.org> writes:
quick> I have two questions regarding the use on __restrict__
quick> qualifiers for function arguments in C++:
quick> 1) How does it interact with volatile?
quick> Example, given:
quick> void foo(volatile int* __restrict__ p1, volatile int*
quick> __restrict__ p2) { *p1 = 3; if (*p1 == 5) ...; *p2 = 4; }
quick> The most desireable for me would be that restrict would
quick> indicate that *p1 and *p2 were disjoint and allow reordering
quick> statement 3 to execute before either preceeding statement, but
quick> that volatile would indicate that the if expression would need
quick> to re-fetch *p1 and not assume the results of the first
quick> statement.
quick> However, an alternative (less useful) interpretation is that
quick> volatile completely overrides restrict: where restrict would
quick> allow the compiler to cache *p1 in a local register and
quick> therefore volatile would prevent this.
Volatile means more than "you need to refetch *p1 each time". It also
means "accesses may have side effects" -- which is why "volatile" is
used for I/O device registers. Given that, even though p1 and p2 are
distinct addresses, reordering *p1 with *p2 may produce a different
answer.
My conclusion is that the "alternative" interpretation is the only
correct one, which means that restrict applied to pointers to volatile
has no effect. I wonder if this deserves a warning ("restrict
attribute ignored" or something like that).
paul
