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Auto merge of rust-lang#131193 - EFanZh:asserts-vec-len, r=the8472
Asserts the maximum value that can be returned from `Vec::len` Currently, casting `Vec<i32>` to `Vec<u32>` takes O(1) time: ```rust // See <https://godbolt.org/z/hxq3hnYKG> for assembly output. pub fn cast(vec: Vec<i32>) -> Vec<u32> { vec.into_iter().map(|e| e as _).collect() } ``` But the generated assembly is not the same as the identity function, which prevents us from casting `Vec<Vec<i32>>` to `Vec<Vec<u32>>` within O(1) time: ```rust // See <https://godbolt.org/z/7n48bxd9f> for assembly output. pub fn cast(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> { vec.into_iter() .map(|e| e.into_iter().map(|e| e as _).collect()) .collect() } ``` This change tries to fix the problem. You can see the comparison here: <https://godbolt.org/z/jdManrKvx>.
2 parents b22856d + 7d450bb commit 303e8bd

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-3
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+73
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‎library/alloc/src/vec/mod.rs

+9-2
Original file line numberDiff line numberDiff line change
@@ -56,7 +56,6 @@
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#[cfg(not(no_global_oom_handling))]
5757
use core::cmp;
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use core::cmp::Ordering;
59-
use core::fmt;
6059
use core::hash::{Hash, Hasher};
6160
#[cfg(not(no_global_oom_handling))]
6261
use core::iter;
@@ -65,6 +64,7 @@ use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
6564
use core::ops::{self, Index, IndexMut, Range, RangeBounds};
6665
use core::ptr::{self, NonNull};
6766
use core::slice::{self, SliceIndex};
67+
use core::{fmt, intrinsics};
6868

6969
#[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
7070
pub use self::extract_if::ExtractIf;
@@ -2680,7 +2680,14 @@ impl<T, A: Allocator> Vec<T, A> {
26802680
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
26812681
#[rustc_confusables("length", "size")]
26822682
pub const fn len(&self) -> usize {
2683-
self.len
2683+
let len = self.len;
2684+
2685+
// SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so the maximum value can
2686+
// be returned is `usize::checked_div(mem::size_of::<T>()).unwrap_or(usize::MAX)`, which
2687+
// matches the definition of `T::MAX_SLICE_LEN`.
2688+
unsafe { intrinsics::assume(len <= T::MAX_SLICE_LEN) };
2689+
2690+
len
26842691
}
26852692

26862693
/// Returns `true` if the vector contains no elements.

‎library/core/src/mem/mod.rs

+11
Original file line numberDiff line numberDiff line change
@@ -1241,6 +1241,17 @@ pub trait SizedTypeProperties: Sized {
12411241
#[doc(hidden)]
12421242
#[unstable(feature = "sized_type_properties", issue = "none")]
12431243
const LAYOUT: Layout = Layout::new::<Self>();
1244+
1245+
/// The largest safe length for a `[Self]`.
1246+
///
1247+
/// Anything larger than this would make `size_of_val` overflow `isize::MAX`,
1248+
/// which is never allowed for a single object.
1249+
#[doc(hidden)]
1250+
#[unstable(feature = "sized_type_properties", issue = "none")]
1251+
const MAX_SLICE_LEN: usize = match size_of::<Self>() {
1252+
0 => usize::MAX,
1253+
n => (isize::MAX as usize) / n,
1254+
};
12441255
}
12451256
#[doc(hidden)]
12461257
#[unstable(feature = "sized_type_properties", issue = "none")]

‎tests/codegen/vec-in-place.rs

+46
Original file line numberDiff line numberDiff line change
@@ -36,6 +36,9 @@ pub struct Baz {
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// CHECK-LABEL: @vec_iterator_cast_primitive
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#[no_mangle]
3838
pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
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// CHECK-NOT: loop
40+
// CHECK: call
41+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
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// CHECK-NOT: loop
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// CHECK-NOT: call
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vec.into_iter().map(|e| e as u8).collect()
@@ -44,14 +47,37 @@ pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
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// CHECK-LABEL: @vec_iterator_cast_wrapper
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#[no_mangle]
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pub fn vec_iterator_cast_wrapper(vec: Vec<u8>) -> Vec<Wrapper<u8>> {
50+
// CHECK-NOT: loop
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// CHECK: call
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// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
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// CHECK-NOT: loop
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// CHECK-NOT: call
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vec.into_iter().map(|e| Wrapper(e)).collect()
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}
5157

58+
// CHECK-LABEL: @vec_iterator_cast_signed
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#[no_mangle]
60+
pub fn vec_iterator_cast_signed(vec: Vec<i32>) -> Vec<u32> {
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// CHECK-NOT: and i{{[0-9]+}} %{{.*}}, {{[0-9]+}}
62+
vec.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect()
63+
}
64+
65+
// CHECK-LABEL: @vec_iterator_cast_signed_nested
66+
#[no_mangle]
67+
pub fn vec_iterator_cast_signed_nested(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> {
68+
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
69+
// CHECK-NOT: %{{.*}} = udiv
70+
vec.into_iter()
71+
.map(|e| e.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect())
72+
.collect()
73+
}
74+
5275
// CHECK-LABEL: @vec_iterator_cast_unwrap
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#[no_mangle]
5477
pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
78+
// CHECK-NOT: loop
79+
// CHECK: call
80+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
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// CHECK-NOT: loop
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// CHECK-NOT: call
5783
vec.into_iter().map(|e| e.0).collect()
@@ -60,6 +86,9 @@ pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
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// CHECK-LABEL: @vec_iterator_cast_aggregate
6187
#[no_mangle]
6288
pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
89+
// CHECK-NOT: loop
90+
// CHECK: call
91+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
6392
// CHECK-NOT: loop
6493
// CHECK-NOT: call
6594
vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
@@ -68,6 +97,9 @@ pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
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// CHECK-LABEL: @vec_iterator_cast_deaggregate_tra
6998
#[no_mangle]
7099
pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
100+
// CHECK-NOT: loop
101+
// CHECK: call
102+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
71103
// CHECK-NOT: loop
72104
// CHECK-NOT: call
73105

@@ -81,6 +113,9 @@ pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
81113
// CHECK-LABEL: @vec_iterator_cast_deaggregate_fold
82114
#[no_mangle]
83115
pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
116+
// CHECK-NOT: loop
117+
// CHECK: call
118+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
84119
// CHECK-NOT: loop
85120
// CHECK-NOT: call
86121

@@ -94,6 +129,11 @@ pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
94129
// CHECK-LABEL: @vec_iterator_cast_unwrap_drop
95130
#[no_mangle]
96131
pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
132+
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
133+
// CHECK-NOT: %{{.*}} = mul
134+
// CHECK-NOT: %{{.*}} = udiv
135+
// CHECK: call
136+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
97137
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
98138
// CHECK-NOT: call
99139
// CHECK-NOT: %{{.*}} = mul
@@ -105,10 +145,16 @@ pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
105145
// CHECK-LABEL: @vec_iterator_cast_wrap_drop
106146
#[no_mangle]
107147
pub fn vec_iterator_cast_wrap_drop(vec: Vec<String>) -> Vec<Wrapper<String>> {
148+
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
149+
// CHECK-NOT: %{{.*}} = mul
150+
// CHECK-NOT: %{{.*}} = udiv
151+
// CHECK: call
152+
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
108153
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
109154
// CHECK-NOT: call
110155
// CHECK-NOT: %{{.*}} = mul
111156
// CHECK-NOT: %{{.*}} = udiv
157+
// CHECK: ret void
112158

113159
vec.into_iter().map(Wrapper).collect()
114160
}

‎tests/codegen/vec_pop_push_noop.rs

+7-1
Original file line numberDiff line numberDiff line change
@@ -1,3 +1,6 @@
1+
//@ revisions: llvm-pre-19 llvm-19
2+
//@ [llvm-19] min-llvm-version: 19
3+
//@ [llvm-pre-19] max-llvm-major-version: 18
14
//@ compile-flags: -O
25

36
#![crate_type = "lib"]
@@ -9,8 +12,11 @@ pub fn noop(v: &mut Vec<u8>) {
912
// CHECK-NOT: call
1013
// CHECK: tail call void @llvm.assume
1114
// CHECK-NOT: grow_one
15+
// llvm-pre-19: call
16+
// llvm-pre-19-same: void @llvm.assume
17+
// llvm-pre-19-NOT: grow_one
1218
// CHECK-NOT: call
13-
// CHECK: ret
19+
// CHECK: {{ret|[}]}}
1420
if let Some(x) = v.pop() {
1521
v.push(x)
1622
}

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