From a81931c07896dd5414bae2bdab6d6c75030ccb45 Mon Sep 17 00:00:00 2001 From: Artem Kryvokrysenko Date: Wed, 23 Apr 2025 22:27:41 +0000 Subject: [PATCH] Optimize size of heapless::Vec<_, 0> to 0 bytes If `Vec` has 0 capacity, it can store only 0 elements, so its len can be assumed to be always 0. I'm updating `DefaultLenType` to have a special case: `DefaultLenType<0>` will be represented by zero sized type which always has value 0. This allows reducing size of `heapless::Vec` from 8 bytes to 0 bytes. --- CHANGELOG.md | 1 + src/len_type.rs | 157 +++++++++++++++++++++++++++++++++++++- src/sorted_linked_list.rs | 2 +- src/string/mod.rs | 25 ++++++ src/vec/mod.rs | 19 +++-- 5 files changed, 191 insertions(+), 13 deletions(-) diff --git a/CHANGELOG.md b/CHANGELOG.md index 1df79898..4691b735 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -54,6 +54,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/). - Added the `index_map` module. - Migrated `Idx` generic for `SortedLinkedList` to use the new `LenType` trait, allowing for `Idx` inference. - Added similar `LenT` generic to `String`. +- Optimize size of zero capacity `Vec` to be 0 bytes ### Changed diff --git a/src/len_type.rs b/src/len_type.rs index fd7b8d8d..854dbddd 100644 --- a/src/len_type.rs +++ b/src/len_type.rs @@ -21,12 +21,16 @@ pub trait Sealed: /// The zero value of the integer type. const ZERO: Self; /// The one value of the integer type. - const ONE: Self; - /// The maximum value of this type. const MAX: Self; /// The maximum value of this type, as a `usize`. const MAX_USIZE: usize; + /// The one value of the integer type. + /// + /// It's a function instead of constant because we want to have implementation which panics for + /// type `ZeroLenType` + fn one() -> Self; + /// An infallible conversion from `usize` to `LenT`. #[inline] fn from_usize(val: usize) -> Self { @@ -55,9 +59,12 @@ macro_rules! impl_lentype { $(#[$meta])* impl Sealed for $LenT { const ZERO: Self = 0; - const ONE: Self = 1; const MAX: Self = Self::MAX; const MAX_USIZE: usize = Self::MAX as _; + + fn one() -> Self { + 1 + } } $(#[$meta])* @@ -111,7 +118,8 @@ pub trait SmallestLenType { #[allow(rustdoc::private_intra_doc_links)] // Only publically exposed via `crate::_export` pub type DefaultLenType = as SmallestLenType>::Type; -impl_lentodefault!(u8: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255); +impl_lentodefault!(ZeroLenType: 0); +impl_lentodefault!(u8: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255); impl_lentodefault!(u16: 256, 300, 400, 500, 512, 600, 700, 800, 900, 1000, 1024, 2000, 2048, 4000, 4096, 8000, 8192, 16000, 16384, 32000, 32768, 65000, 65535); #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))] impl_lentodefault!(u32: 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824, 2147483648); @@ -119,3 +127,144 @@ impl_lentodefault!(u32: 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304 pub const fn check_capacity_fits() { assert!(LenT::MAX_USIZE >= N, "The capacity is larger than `LenT` can hold, increase the size of `LenT` or reduce the capacity"); } + +/// Container with 0 capacity always has length 0, so there is no need to track length of such containers at all. +/// +/// This type is used as a placeholder for length of container with 0 capacity. It allows optimizing total size of +/// containers like this to 0 bytes. +/// +/// Logically, this type always stores value 0. Because of this ZeroLenType::one() panics and should never be called. +#[doc(hidden)] +#[derive(Copy, Clone, PartialEq, PartialOrd)] +pub struct ZeroLenType; + +impl Debug for ZeroLenType { + fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { + write!(f, "0") + } +} + +impl Display for ZeroLenType { + fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { + write!(f, "0") + } +} + +impl Sealed for ZeroLenType { + const ZERO: Self = Self; + + const MAX: Self = Self; + const MAX_USIZE: usize = 0; + + #[inline] + fn one() -> Self { + panic!("ZeroLenType cannot represent value 1"); + } +} + +impl LenType for ZeroLenType {} + +impl Add for ZeroLenType { + type Output = Self; + + fn add(self, _rhs: Self) -> Self::Output { + Self::ZERO + } +} + +impl AddAssign for ZeroLenType { + fn add_assign(&mut self, _rhs: Self) {} +} + +impl Sub for ZeroLenType { + type Output = Self; + + fn sub(self, _rhs: Self) -> Self::Output { + Self::ZERO + } +} + +impl SubAssign for ZeroLenType { + fn sub_assign(&mut self, _rhs: Self) {} +} + +#[doc(hidden)] +#[derive(Debug, PartialEq)] +pub struct ZeroLenTypeTryFromError; + +impl TryFrom for ZeroLenType { + type Error = ZeroLenTypeTryFromError; + + fn try_from(value: usize) -> Result { + if value > 0 { + return Err(ZeroLenTypeTryFromError); + } + + Ok(Self::ZERO) + } +} + +impl TryInto for ZeroLenType { + type Error = (); + + fn try_into(self) -> Result { + Ok(0) + } +} + +#[cfg(test)] +mod tests { + use crate::len_type::{Sealed, ZeroLenType, ZeroLenTypeTryFromError}; + + #[test] + pub fn test_zero_len_type_conversions() { + assert_eq!(ZeroLenType::into_usize(ZeroLenType::ZERO), 0_usize); + assert_eq!(ZeroLenType::from_usize(0_usize), ZeroLenType::ZERO); + + assert_eq!(ZeroLenType::ZERO.try_into(), Ok(0_usize)); + assert_eq!(ZeroLenType::try_from(0_usize), Ok(ZeroLenType::ZERO)); + assert_eq!(ZeroLenType::try_from(1_usize), Err(ZeroLenTypeTryFromError)); + } + + #[test] + #[should_panic] + pub fn test_zero_len_type_one() { + ZeroLenType::one(); + } + + #[test] + #[should_panic] + pub fn test_zero_len_type_one_usize() { + ZeroLenType::from_usize(1); + } + + #[test] + pub fn test_zero_len_type_constants() { + assert_eq!(ZeroLenType::ZERO, ZeroLenType); + assert_eq!(ZeroLenType::MAX, ZeroLenType); + assert_eq!(ZeroLenType::MAX_USIZE, 0_usize); + } + + #[test] + pub fn test_zero_len_type_size() { + assert_eq!(core::mem::size_of::(), 0); + } + + #[test] + pub fn test_zero_len_type_ops() { + assert_eq!(ZeroLenType::ZERO + ZeroLenType::ZERO, ZeroLenType::ZERO); + assert_eq!(ZeroLenType::ZERO - ZeroLenType::ZERO, ZeroLenType::ZERO); + + let mut zero = ZeroLenType::ZERO; + zero += ZeroLenType::ZERO; + assert_eq!(zero, ZeroLenType::ZERO); + zero -= ZeroLenType::ZERO; + assert_eq!(zero, ZeroLenType::ZERO); + } + + #[test] + pub fn test_zero_len_type_debug() { + assert_eq!(format!("{}", ZeroLenType), "0"); + assert_eq!(format!("{:?}", ZeroLenType), "0"); + } +} diff --git a/src/sorted_linked_list.rs b/src/sorted_linked_list.rs index 1b59aea1..899de283 100644 --- a/src/sorted_linked_list.rs +++ b/src/sorted_linked_list.rs @@ -905,7 +905,7 @@ mod tests { #[test] fn test_zero_size() { - let ll: SortedLinkedList = SortedLinkedList::new_u8(); + let ll: SortedLinkedList = SortedLinkedList::new_u8(); assert!(ll.is_empty()); assert!(ll.is_full()); diff --git a/src/string/mod.rs b/src/string/mod.rs index 462142a4..e58a542d 100644 --- a/src/string/mod.rs +++ b/src/string/mod.rs @@ -1236,4 +1236,29 @@ mod tests { let formatted = format!(2; "123"); assert_eq!(formatted, Err(core::fmt::Error)); } + + #[test] + fn zero_capacity() { + let mut s: String<0> = String::new(); + // Validate capacity + assert_eq!(s.capacity(), 0); + + // Make sure there is no capacity + assert!(s.push('a').is_err()); + + // Validate length + assert_eq!(s.len(), 0); + + // Validate pop + assert_eq!(s.pop(), None); + + // Validate slice + assert_eq!(s.as_str(), ""); + + // Validate empty + assert!(s.is_empty()); + + // Size of string with zero capacity should be 0 bytes because of `ZeroLenType` optimization + assert_eq!(core::mem::size_of_val(&s), 0); + } } diff --git a/src/vec/mod.rs b/src/vec/mod.rs index b8039c94..13ffeaab 100644 --- a/src/vec/mod.rs +++ b/src/vec/mod.rs @@ -647,7 +647,7 @@ impl + ?Sized> VecInner { unsafe { *buf.get_unchecked_mut(len.into_usize()) = MaybeUninit::new(elem.clone()); } - *len += LenT::ONE; + *len += LenT::one(); } Ok(()) } @@ -685,7 +685,7 @@ impl + ?Sized> VecInner { pub unsafe fn pop_unchecked(&mut self) -> T { debug_assert!(!self.is_empty()); - self.len -= LenT::ONE; + self.len -= LenT::one(); self.buffer .borrow_mut() .get_unchecked_mut(self.len.into_usize()) @@ -708,7 +708,7 @@ impl + ?Sized> VecInner { .borrow_mut() .get_unchecked_mut(self.len.into_usize()) = MaybeUninit::new(item); - self.len += LenT::ONE; + self.len += LenT::one(); } /// Shortens the vector, keeping the first `len` elements and dropping the rest. @@ -934,7 +934,7 @@ impl + ?Sized> VecInner { let value = ptr::read(self.as_ptr().add(index)); let base_ptr = self.as_mut_ptr(); ptr::copy(base_ptr.add(length - 1), base_ptr.add(index), 1); - self.len -= LenT::ONE; + self.len -= LenT::one(); value } @@ -1218,8 +1218,8 @@ impl + ?Sized> VecInner { let cur = unsafe { &mut *p.add(g.processed_len.into_usize()) }; if !f(cur) { // Advance early to avoid double drop if `drop_in_place` panicked. - g.processed_len += LenT::ONE; - g.deleted_cnt += LenT::ONE; + g.processed_len += LenT::one(); + g.deleted_cnt += LenT::one(); // SAFETY: We never touch this element again after dropped. unsafe { ptr::drop_in_place(cur) }; // We already advanced the counter. @@ -1237,7 +1237,7 @@ impl + ?Sized> VecInner { ptr::copy_nonoverlapping(cur, hole_slot, 1); } } - g.processed_len += LenT::ONE; + g.processed_len += LenT::one(); } } @@ -1462,7 +1462,7 @@ impl Iterator for IntoIter { .as_ptr() .read() }; - self.next += LenT::ONE; + self.next += LenT::one(); Some(item) } else { None @@ -2204,6 +2204,9 @@ mod tests { // Validate full assert!(v.is_full()); + + // Size of vector with zero capacity should be 0 bytes because of `ZeroLenType` optimization + assert_eq!(core::mem::size_of_val(&v), 0); } #[test]