netdev - Re: [PATCH net-next v7 1/5] rust: core abstractions for network PHY drivers

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Date: Fri, 27 Oct 2023 16:26:32 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, netdev@...r.kernel.org,
	rust-for-linux@...r.kernel.org, andrew@...n.ch, tmgross@...ch.edu,
	miguel.ojeda.sandonis@...il.com, wedsonaf@...il.com
Subject: Re: [PATCH net-next v7 1/5] rust: core abstractions for network PHY
 drivers

On Fri, Oct 27, 2023 at 10:50:45PM +0000, Benno Lossin wrote:
[...]
> > 
> > Hmm... but does it mean even `set_speed()` has the similar issue?
> > 
> > 	let phydev: *mut phy_device = self.0.get();
> > 	unsafe { (*phydev).speed = ...; }
> 
> No that should be fine, take a look at the MIR output of the following 
> code [1]:
> 
>     struct Foo {
>         a: usize,
>         b: usize,
>     }
>     
>     fn foo(ptr: *mut Foo) {
>         unsafe { (*ptr).b = 0; }
>     }
>     
>     fn bar(ptr: *mut Foo) {
>         unsafe { (&mut *ptr).b = 0; }
>     }
> 
> Aside from some alignment checking, foo's MIR looks like this:
> 
>     bb1: {
>         ((*_1).1: usize) = const 0_usize;
>         return;
>     }
> 
> And bar's MIR like this:
> 
>     bb1: {
>         _2 = &mut (*_1);
>         ((*_2).1: usize) = const 0_usize;
>         return;
>     }
> 
> [1]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f7c4d87bf29a64af0acc09ff75d3716d
> 
> So I think that is fine, but maybe Gary has something else to say about it.
> 

Well when `-C opt-level=2`, they are the same:

	https://godbolt.org/z/hxxo75YYh

Regards,
Boqun