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From: "user202729 at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110137] implement clang -fassume-sane-operator-new
Date: Mon, 27 May 2024 03:07:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110137

user202729 <user202729 at protonmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |user202729 at protonmail d=
ot com

--- Comment #6 from user202729 <user202729 at protonmail dot com> ---
Created attachment 58293
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D58293&action=3Dedit
Proposed implementation.

I've implement it. (Attached patch) It would be helpful if someone can revi=
ew
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-effe=
ct.
So I explicitly state that in the documentation.
2. The original intent is to fix
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035 .

While this patch alone is insufficient to fix that (as explained in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110035#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=3Da;
        float* z=3Dnew float;
        int y=3Da;
        int result=3Dx+y;
        sink(z);
        return result;
}
```

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