Nathan Sidwell wrote:
I attended a UK C++ panel meeting yesterday, and took the opportunity
to solicit opinions on this. The question I posed was
struct A {
...
T1 a;
T2 b;
};
void g(T1 &a);
void Foo () {
A v;
v.b = 2;
g (v.a);
if (v.b == 2) ...
}
Does the compiler have a right to presume v.b does indeed == 2 in the if
condition? -- assuming T2 is a suitable type for that :)
After I explained the optimization (and the related structure splitting optimization), the general consensus was 'yes that would be a useful optimization'. But no one was sufficiently confident of proving it was allowable. The opinion was expressed that if it was not allowable, there was a bug in the std.
Will the UK committee open a DR for this? Or, would you care to send mail to Steve Adamczyk about it?
The observation was made that if A is non-POD, one cannot play offsetof tricks to get from A::a to A::b, so the optimization is safe on non-PODs. (Of course one would have to prove the address of 'v' did not escape, so I guess the ctor and dtor would need to be trivial or visible.)
I argued last week that this was not in fact true, in that you can do:
ptrdiff_t x = &v.b - &v.a;
and then use that instead of "offsetof (Foo, b) - offsetof (Foo, a)".
because (a) badfuncs are more than likely rare and (b) it would be a useful
aid to the programmer.[1] Mark outlines an __I_AM_GOOD__ attribute, I think
it would be better to have both flavours and then the compiler switch can
specify which way the default goes.
Makes sense to me.
-- Mark Mitchell CodeSourcery, LLC [EMAIL PROTECTED] (916) 791-8304
