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Auto merge of rust-lang#131193 - EFanZh:asserts-vec-len, r=the8472
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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>.
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@bors
bors committed Dec 22, 2024
2 parents b22856d + 7d450bb commit 303e8bd
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Showing 4 changed files with 73 additions and 3 deletions.
11 changes: 9 additions & 2 deletions library/alloc/src/vec/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -56,7 +56,6 @@
#[cfg(not(no_global_oom_handling))]
use core::cmp;
use core::cmp::Ordering;
use core::fmt;
use core::hash::{Hash, Hasher};
#[cfg(not(no_global_oom_handling))]
use core::iter;
Expand All @@ -65,6 +64,7 @@ use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
use core::ops::{self, Index, IndexMut, Range, RangeBounds};
use core::ptr::{self, NonNull};
use core::slice::{self, SliceIndex};
use core::{fmt, intrinsics};

#[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
pub use self::extract_if::ExtractIf;
Expand Down Expand Up @@ -2680,7 +2680,14 @@ impl<T, A: Allocator> Vec<T, A> {
#[rustc_const_unstable(feature = "const_vec_string_slice", issue = "129041")]
#[rustc_confusables("length", "size")]
pub const fn len(&self) -> usize {
self.len
let len = self.len;

// SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so the maximum value can
// be returned is `usize::checked_div(mem::size_of::<T>()).unwrap_or(usize::MAX)`, which
// matches the definition of `T::MAX_SLICE_LEN`.
unsafe { intrinsics::assume(len <= T::MAX_SLICE_LEN) };

len
}

/// Returns `true` if the vector contains no elements.
Expand Down
11 changes: 11 additions & 0 deletions library/core/src/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1241,6 +1241,17 @@ pub trait SizedTypeProperties: Sized {
#[doc(hidden)]
#[unstable(feature = "sized_type_properties", issue = "none")]
const LAYOUT: Layout = Layout::new::<Self>();

/// The largest safe length for a `[Self]`.
///
/// Anything larger than this would make `size_of_val` overflow `isize::MAX`,
/// which is never allowed for a single object.
#[doc(hidden)]
#[unstable(feature = "sized_type_properties", issue = "none")]
const MAX_SLICE_LEN: usize = match size_of::<Self>() {
0 => usize::MAX,
n => (isize::MAX as usize) / n,
};
}
#[doc(hidden)]
#[unstable(feature = "sized_type_properties", issue = "none")]
Expand Down
46 changes: 46 additions & 0 deletions tests/codegen/vec-in-place.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,9 @@ pub struct Baz {
// CHECK-LABEL: @vec_iterator_cast_primitive
#[no_mangle]
pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call
vec.into_iter().map(|e| e as u8).collect()
Expand All @@ -44,14 +47,37 @@ pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
// CHECK-LABEL: @vec_iterator_cast_wrapper
#[no_mangle]
pub fn vec_iterator_cast_wrapper(vec: Vec<u8>) -> Vec<Wrapper<u8>> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call
vec.into_iter().map(|e| Wrapper(e)).collect()
}

// CHECK-LABEL: @vec_iterator_cast_signed
#[no_mangle]
pub fn vec_iterator_cast_signed(vec: Vec<i32>) -> Vec<u32> {
// CHECK-NOT: and i{{[0-9]+}} %{{.*}}, {{[0-9]+}}
vec.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect()
}

// CHECK-LABEL: @vec_iterator_cast_signed_nested
#[no_mangle]
pub fn vec_iterator_cast_signed_nested(vec: Vec<Vec<i32>>) -> Vec<Vec<u32>> {
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
// CHECK-NOT: %{{.*}} = udiv
vec.into_iter()
.map(|e| e.into_iter().map(|e| u32::from_ne_bytes(e.to_ne_bytes())).collect())
.collect()
}

// CHECK-LABEL: @vec_iterator_cast_unwrap
#[no_mangle]
pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call
vec.into_iter().map(|e| e.0).collect()
Expand All @@ -60,6 +86,9 @@ pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
// CHECK-LABEL: @vec_iterator_cast_aggregate
#[no_mangle]
pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call
vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
Expand All @@ -68,6 +97,9 @@ pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
// CHECK-LABEL: @vec_iterator_cast_deaggregate_tra
#[no_mangle]
pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call

Expand All @@ -81,6 +113,9 @@ pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
// CHECK-LABEL: @vec_iterator_cast_deaggregate_fold
#[no_mangle]
pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
// CHECK-NOT: loop
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: loop
// CHECK-NOT: call

Expand All @@ -94,6 +129,11 @@ pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
// CHECK-LABEL: @vec_iterator_cast_unwrap_drop
#[no_mangle]
pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
// CHECK-NOT: %{{.*}} = mul
// CHECK-NOT: %{{.*}} = udiv
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
// CHECK-NOT: call
// CHECK-NOT: %{{.*}} = mul
Expand All @@ -105,10 +145,16 @@ pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
// CHECK-LABEL: @vec_iterator_cast_wrap_drop
#[no_mangle]
pub fn vec_iterator_cast_wrap_drop(vec: Vec<String>) -> Vec<Wrapper<String>> {
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
// CHECK-NOT: %{{.*}} = mul
// CHECK-NOT: %{{.*}} = udiv
// CHECK: call
// CHECK-SAME: void @llvm.assume(i1 %{{.+}})
// CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
// CHECK-NOT: call
// CHECK-NOT: %{{.*}} = mul
// CHECK-NOT: %{{.*}} = udiv
// CHECK: ret void

vec.into_iter().map(Wrapper).collect()
}
8 changes: 7 additions & 1 deletion tests/codegen/vec_pop_push_noop.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,6 @@
//@ revisions: llvm-pre-19 llvm-19
//@ [llvm-19] min-llvm-version: 19
//@ [llvm-pre-19] max-llvm-major-version: 18
//@ compile-flags: -O

#![crate_type = "lib"]
Expand All @@ -9,8 +12,11 @@ pub fn noop(v: &mut Vec<u8>) {
// CHECK-NOT: call
// CHECK: tail call void @llvm.assume
// CHECK-NOT: grow_one
// llvm-pre-19: call
// llvm-pre-19-same: void @llvm.assume
// llvm-pre-19-NOT: grow_one
// CHECK-NOT: call
// CHECK: ret
// CHECK: {{ret|[}]}}
if let Some(x) = v.pop() {
v.push(x)
}
Expand Down

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