On Tue, Mar 18, 2025 at 8:50?PM Benno Lossin <benno.lossin at proton.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 at gmail.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. Tamir
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 at proton.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 at gmail.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). --- Cheers, Benno
On Wed, Mar 19, 2025 at 10:34?AM Benno Lossin <benno.lossin at proton.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 at proton.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 at gmail.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()`.