linux-kernel - Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

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Message-ID: <CAJ-ks9=NQrz3ySacKt+XXm2vS+Fn9gjmtqAaaoz1k=iTG_1HXw@mail.gmail.com>
Date: Wed, 19 Mar 2025 11:58:59 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Andrew Ballance <andrewjballance@...il.com>, dakr@...nel.org, airlied@...il.com, 
	simona@...ll.ch, maarten.lankhorst@...ux.intel.com, mripard@...nel.org, 
	tzimmermann@...e.de, corbet@....net, ojeda@...nel.org, alex.gaynor@...il.com, 
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, 
	acourbot@...dia.com, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: alloc: add Vec::resize method

On Wed, Mar 19, 2025 at 10:34 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On Wed Mar 19, 2025 at 2:42 PM CET, Tamir Duberstein wrote:
> > On Tue, Mar 18, 2025 at 8:50 PM Benno Lossin <benno.lossin@...ton.me> wrote:
> >>
> >> On Tue Mar 18, 2025 at 9:12 PM CET, Tamir Duberstein wrote:
> >> > On Sun, Mar 16, 2025 at 7:17 AM Andrew Ballance
> >> > <andrewjballance@...il.com> wrote:
> >> >> +    pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> >> >> +        if new_len > self.len() {
> >> >> +            self.extend_with(new_len - self.len(), value, flags)
> >> >> +        } else {
> >> >> +            self.truncate(new_len);
> >> >> +            Ok(())
> >> >> +        }
> >> >> +    }
> >> >
> >> > You can avoid underflow checking in debug builds by using `checked_sub`:
> >>
> >> `checked_sub` doesn't only avoid underflow in debug builds, but rather
> >> in all builds. But the code below is a good suggestion.
> >
> > Yes, I know :)
> >
> > I included that language because the underflow check is likely
> > optimized away in release builds.
>
> If the function is inlined and the compiler can argue that `new_len >
> self.len()`, then yes, but otherwise I'm pretty sure it won't be
> optimized away.
>
> Also if it is optimized away, then the check was still "executed", so I
> find it a bit misleading to say "in debug builds" (making it sound like
> it wouldn't do it in non-debug builds).

If we're talking about the same thing then I think we're both wrong
and the correct phrasing would have been: "you can avoid underflow
checking when CONFIG_RUST_OVERFLOW_CHECKS=y by using `checked_sub`". I
was referring to the underflow check implicit in `new_len -
self.len()`.