[Bug c++/110137] implement clang -fassume-sane-operator-new

["user202729 at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110137

user202729 <user202729 at protonmail dot com> changed:

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--- Comment #6 from user202729 <user202729 at protonmail dot com> ---
Created attachment 58293
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58293&action=edit
Proposed implementation.

I've implement it. (Attached patch) It would be helpful if someone can review
it.

Some notes:

1. Clang's documentation says the effect of `-fassume-sane-operator-new` is to
assume no-alias, but it is pointed out that Clang also assumes no-side-effect.
So I explicitly state that in the documentation.
2. The original intent is to fix
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035 .

While this patch alone is insufficient to fix that (as explained in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035#c17 ), it would be
sufficient to improve the performance of the following (to only load from
global memory once):

```
int a;

// Prevent optimization
void sink(void *m) {
    asm volatile("" : : "g"(m) : "memory");
}

int f(){
        int x=a;
        float* z=new float;
        int y=a;
        int result=x+y;
        sink(z);
        return result;
}
```

3. I'm not sure which additional optimizations can be done if we know something
is pure, but this would improve that one case. Other cases can be added later
on.