diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs index d46bf81f996f7..f348b5b69780b 100644 --- a/src/liballoc/raw_vec.rs +++ b/src/liballoc/raw_vec.rs @@ -407,13 +407,34 @@ impl RawVec { return Err(CapacityOverflow); } + if needed_extra_capacity == 0 { + return Ok(()); + } + // Nothing we can really do about these checks, sadly. let required_cap = used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?; - // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`. - let double_cap = self.cap * 2; - // `double_cap` guarantees exponential growth. - let cap = cmp::max(double_cap, required_cap); + + // This guarantees exponential growth. The doubling cannot overflow + // because `cap <= isize::MAX` and the type of `cap` is `usize`. + let cap = cmp::max(self.cap * 2, required_cap); + + // Tiny Vecs are dumb. Skip to: + // - 8 if the element size is 1, because any heap allocators is likely + // to round up a request of less than 8 bytes to at least 8 bytes. + // - 4 if elements are moderate-sized (<= 1 KiB). + // - 1 otherwise, to avoid wasting too much space for very short Vecs. + // Note that `min_non_zero_cap` is computed statically. + let elem_size = mem::size_of::(); + let min_non_zero_cap = if elem_size == 1 { + 8 + } else if elem_size <= 1024 { + 4 + } else { + 1 + }; + let cap = cmp::max(min_non_zero_cap, cap); + let new_layout = Layout::array::(cap); // `finish_grow` is non-generic over `T`. diff --git a/src/liballoc/raw_vec/tests.rs b/src/liballoc/raw_vec/tests.rs index e7ab8a305d279..17622d72a059c 100644 --- a/src/liballoc/raw_vec/tests.rs +++ b/src/liballoc/raw_vec/tests.rs @@ -59,7 +59,7 @@ fn reserve_does_not_overallocate() { let mut v: RawVec = RawVec::new(); v.reserve(0, 7); assert_eq!(7, v.capacity()); - // 97 if more than double of 7, so `reserve` should work + // 97 is more than double of 7, so `reserve` should work // like `reserve_exact`. v.reserve(7, 90); assert_eq!(97, v.capacity()); diff --git a/src/liballoc/tests/vec.rs b/src/liballoc/tests/vec.rs index 3d76324f7e880..b16044d964045 100644 --- a/src/liballoc/tests/vec.rs +++ b/src/liballoc/tests/vec.rs @@ -1473,3 +1473,118 @@ fn vec_macro_repeating_null_raw_fat_pointer() { vtable: *mut (), } } + +// This test will likely fail if you change the capacities used in +// `RawVec::grow_amortized`. +#[test] +fn test_push_growth_strategy() { + // If the element size is 1, we jump from 0 to 8, then double. + { + let mut v1: Vec = vec![]; + assert_eq!(v1.capacity(), 0); + + for _ in 0..8 { + v1.push(0); + assert_eq!(v1.capacity(), 8); + } + + for _ in 8..16 { + v1.push(0); + assert_eq!(v1.capacity(), 16); + } + + for _ in 16..32 { + v1.push(0); + assert_eq!(v1.capacity(), 32); + } + + for _ in 32..64 { + v1.push(0); + assert_eq!(v1.capacity(), 64); + } + } + + // If the element size is 2..=1024, we jump from 0 to 4, then double. + { + let mut v2: Vec = vec![]; + let mut v1024: Vec<[u8; 1024]> = vec![]; + assert_eq!(v2.capacity(), 0); + assert_eq!(v1024.capacity(), 0); + + for _ in 0..4 { + v2.push(0); + v1024.push([0; 1024]); + assert_eq!(v2.capacity(), 4); + assert_eq!(v1024.capacity(), 4); + } + + for _ in 4..8 { + v2.push(0); + v1024.push([0; 1024]); + assert_eq!(v2.capacity(), 8); + assert_eq!(v1024.capacity(), 8); + } + + for _ in 8..16 { + v2.push(0); + v1024.push([0; 1024]); + assert_eq!(v2.capacity(), 16); + assert_eq!(v1024.capacity(), 16); + } + + for _ in 16..32 { + v2.push(0); + v1024.push([0; 1024]); + assert_eq!(v2.capacity(), 32); + assert_eq!(v1024.capacity(), 32); + } + + for _ in 32..64 { + v2.push(0); + v1024.push([0; 1024]); + assert_eq!(v2.capacity(), 64); + assert_eq!(v1024.capacity(), 64); + } + } + + // If the element size is > 1024, we jump from 0 to 1, then double. + { + let mut v1025: Vec<[u8; 1025]> = vec![]; + assert_eq!(v1025.capacity(), 0); + + for _ in 0..1 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 1); + } + + for _ in 1..2 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 2); + } + + for _ in 2..4 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 4); + } + + for _ in 4..8 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 8); + } + + for _ in 8..16 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 16); + } + + for _ in 16..32 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 32); + } + + for _ in 32..64 { + v1025.push([0; 1025]); + assert_eq!(v1025.capacity(), 64); + } + } +}