Escape Analysis on reddit

[Denis Koroskin]

On Fri, 31 Oct 2008 23:17:41 +0300, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> "Walter Bright" wrote
>> http://www.reddit.com/r/programming/comments/7akzd/escape_analysis/
>
> The question is, in the example:
>
> int* bar(int* p) { return p; }
> int* foo()
> {
>     int x = 3;
>     return bar(&x);
> }
>
> where is the problem?  Is the problem in bar, or is the problem in foo?
>
> The real problem is in foo, because there's technically nothing wrong  
> with
> bar.
>
> So what ends up happening in your scheme is this:
>
> int *bar(int *p) { return p;}
>
> void foo(scope int *p)
> {
>    int *x = bar(p); // no escape, but compile error
> }
>
> So yes, you are protected, but at the cost of not being able to write  
> code
> that is provably safe.  The end result is you end up removing scope  
> where it
> really should be, and then virally, it makes you remove scope decorations
> everywhere else.  I predict that if this is implemented, nobody will be  
> able
> to use it.
>
> In order for this to be useful, we need to have a way to express total
> relationships between parameters and the return value.  And that would
> probably be so convoluted that it's not worth the syntax.
>
> I'll reiterate my challenge from another thread:
>
> void f(ref int *a, int *b, int *c) { if(*b < *c) a = b;  else a = c;}
>
> struct S
> {
>    int *v;
> }
>
> int *f2(S* s) { return s.v;}
>
> void f3(ref int *a, ref int *b, ref int *c)
> {
>    int *tmp = a;
>    a = b; b = c; c = tmp;
> }
>
> void f4()
> {
>    int a=1, b=2, c=3;
>    auto ap = &a;
>    auto bp = &b;
>    auto cp = &c;
>    S s(cp);
>
>    f(ap, bp, cp);
>    b = *f2(&s);
>    f3(ap, bp, cp);
> }
>
> Please tell me how I should mark f, S, f2, and f3 such that f4 compiles.
> Note that this code all works in D1 and provably has no problems with
> escaping values, since f4 returns void, and no globals are accessed.
>
> -Steve
>
>

I will state the question slightly different. f4 is now pure. What changes  
should be done to source code (if any) so that it compiles?

pure void f4()
{
    int a=1, b=2, c=3;
    auto ap = &a;
    auto bp = &b;
    auto cp = &c;
    S s(cp);

    f(ap, bp, cp);
    b = *f2(&s);
    f3(ap, bp, cp);
}