Asserts the maximum value that can be returned from Vec::len

Author: EFanZh

library/alloc/src/vec/mod.rs

@@ -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;
@@ -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;
@@ -2630,7 +2630,14 @@ impl<T, A: Allocator> Vec<T, A> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_confusables("length", "size")]
     pub 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(self.len <= T::MAX_SLICE_LEN) };
+
+        len
     }
 
     /// Returns `true` if the vector contains no elements.

library/core/src/mem/mod.rs

@@ -1243,6 +1243,17 @@ 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")]

tests/codegen/vec-in-place.rs

@@ -37,6 +37,8 @@ pub struct Baz {
 #[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: call
     vec.into_iter().map(|e| e as u8).collect()
 }
@@ -45,14 +47,35 @@ pub fn vec_iterator_cast_primitive(vec: Vec<i8>) -> Vec<u8> {
 #[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: 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: call
     vec.into_iter().map(|e| e.0).collect()
 }
@@ -61,6 +84,8 @@ pub fn vec_iterator_cast_unwrap(vec: Vec<Wrapper<u8>>) -> Vec<u8> {
 #[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: call
     vec.into_iter().map(|e| unsafe { std::mem::transmute(e) }).collect()
 }
@@ -69,6 +94,8 @@ pub fn vec_iterator_cast_aggregate(vec: Vec<[u64; 4]>) -> Vec<Foo> {
 #[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: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
@@ -82,6 +109,8 @@ pub fn vec_iterator_cast_deaggregate_tra(vec: Vec<Bar>) -> Vec<[u64; 4]> {
 #[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: call
 
     // Safety: For the purpose of this test we assume that Bar layout matches [u64; 4].
@@ -95,6 +124,8 @@ pub fn vec_iterator_cast_deaggregate_fold(vec: Vec<Baz>) -> Vec<[u64; 4]> {
 #[no_mangle]
 pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
     // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
@@ -106,9 +137,12 @@ pub fn vec_iterator_cast_unwrap_drop(vec: Vec<Wrapper<String>>) -> Vec<String> {
 #[no_mangle]
 pub fn vec_iterator_cast_wrap_drop(vec: Vec<String>) -> Vec<Wrapper<String>> {
     // CHECK-NOT: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}
+    // CHECK: call
+    // CHECK-SAME: void @llvm.assume(i1 %{{.+}})
     // CHECK-NOT: call
     // CHECK-NOT: %{{.*}} = mul
     // CHECK-NOT: %{{.*}} = udiv
+    // CHECK: ret void
 
     vec.into_iter().map(Wrapper).collect()
 }

tests/codegen/vec_pop_push_noop.rs

@@ -9,7 +9,7 @@ pub fn noop(v: &mut Vec<u8>) {
     // CHECK-NOT: call
     // CHECK: tail call void @llvm.assume
     // CHECK-NOT: grow_one
-    // CHECK-NOT: call
+    // CHECK-NOT: call {{(?!void @llvm.assume)}}
     // CHECK: ret
     if let Some(x) = v.pop() {
         v.push(x)