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

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Message-ID: <CANiq72kfgDVkKAeX1SiahEoo13+kHrcvVY3hMAfRPGjwErCt6A@mail.gmail.com>
Date: Wed, 19 Mar 2025 17:43:40 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, 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 5:13 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> No, I meant avoiding the check. The existing code already explicitly
> checks `new_len > self.len()` before evaluating `new_len -
> self.len()`. This means the check occurs twice. `checked_sub` reduces
> the number of checks by 1. Perhaps my wording could have been clearer
> ("avoid *an* underflow check").

Ah, you mean in the function you suggested, I see.

I think it they all may compile down to the same thing, whether
overflows checks are enabled or not, and whether the version in the
patch or `checked_sub` is used or not. At least in a quick Compiler
Explorer test it seems so, but I didn't check in an actual kernel
build.

The implicit check is gated behind the other one, so that one can be
removed, even if values are unknown -- we always have optimizations
enabled, even under "debug" builds (assuming "debug" means overflow
checking enabled).

Cheers,
Miguel