WIP(will be squashed)

library/core/src/slice/sort/shared/smallsort.rs

@@ -3,6 +3,10 @@
 use crate::mem::{self, ManuallyDrop, MaybeUninit};
 use crate::slice::sort::shared::FreezeMarker;
 use crate::{intrinsics, ptr, slice};
+use safety::{ensures, requires};
+
+#[cfg(kani)]
+use crate::{kani, ub_checks};
 
 // It's important to differentiate between SMALL_SORT_THRESHOLD performance for
 // small slices and small-sort performance sorting small sub-slices as part of
@@ -539,6 +543,21 @@ where
 ///
 /// # Safety
 /// begin < tail and p must be valid and initialized for all begin <= p <= tail.
+#[requires(begin.addr() < tail.addr() && {
+    let len = tail.addr() - begin.addr();
+    let is_less: &mut F = unsafe { mem::transmute(&is_less) };
+    (0..=len).into_iter().all(|i| {
+        let p = begin.add(i);
+        ub_checks::can_dereference(p as *const _) &&
+            ub_checks::can_write(p) &&
+            ub_checks::same_allocation(begin as *const _, p as *const _)
+    }) && (0..(len - 1)).into_iter().all(|i| !is_less(&*begin.add(i + 1), &*begin.add(i)))
+})]
+#[ensures(|_| {
+    let len = tail.addr() - begin.addr();
+    let is_less: &mut F = unsafe { mem::transmute(&is_less) };
+    (0..len).into_iter().all(|i| !is_less(&*begin.add(i + 1), &*begin.add(i)))
+})]
 unsafe fn insert_tail<T, F: FnMut(&T, &T) -> bool>(begin: *mut T, tail: *mut T, is_less: &mut F) {
     // SAFETY: see individual comments.
     unsafe {
@@ -556,7 +575,13 @@ unsafe fn insert_tail<T, F: FnMut(&T, &T) -> bool>(begin: *mut T, tail: *mut T,
         let tmp = ManuallyDrop::new(tail.read());
         let mut gap_guard = CopyOnDrop { src: &*tmp, dst: tail, len: 1 };
 
-        loop {
+        #[safety::loop_invariant(
+            sift.addr() >= begin.addr() && sift.addr() < tail.addr()
+        )]
+        // FIXME: This should be `loop` but kani's loop contract doesn't support `loop`.
+        // Once it is supported, replace `while true` with the original `loop`
+        #[allow(while_true)]
+        while true {
             // SAFETY: we move sift into the gap (which is valid), and point the
             // gap guard destination at sift, ensuring that if a panic occurs the
             // gap is once again filled.
@@ -577,6 +602,14 @@ unsafe fn insert_tail<T, F: FnMut(&T, &T) -> bool>(begin: *mut T, tail: *mut T,
 }
 
 /// Sort `v` assuming `v[..offset]` is already sorted.
+#[requires(offset != 0 && offset <= v.len() && {
+    let is_less: &mut F = unsafe { mem::transmute(&is_less) };
+    v[..offset].is_sorted_by(|a, b| !is_less(b, a))
+})]
+#[ensures(|_| {
+    let is_less: &mut F = unsafe { mem::transmute(&is_less) };
+    v.is_sorted_by(|a, b| !is_less(b, a))
+})]
 pub fn insertion_sort_shift_left<T, F: FnMut(&T, &T) -> bool>(
     v: &mut [T],
     offset: usize,
@@ -596,6 +629,9 @@ pub fn insertion_sort_shift_left<T, F: FnMut(&T, &T) -> bool>(
         let v_base = v.as_mut_ptr();
         let v_end = v_base.add(len);
         let mut tail = v_base.add(offset);
+        #[safety::loop_invariant(
+            tail.addr() > v_base.addr() && tail.addr() <= v_end.addr()
+        )]
         while tail != v_end {
             // SAFETY: v_base and tail are both valid pointers to elements, and
             // v_base < tail since we checked offset != 0.
@@ -609,6 +645,27 @@ pub fn insertion_sort_shift_left<T, F: FnMut(&T, &T) -> bool>(
 
 /// SAFETY: The caller MUST guarantee that `v_base` is valid for 4 reads and
 /// `dst` is valid for 4 writes. The result will be stored in `dst[0..4]`.
+#[requires(
+    (0..4).into_iter().all(|i| {
+        let p = v_base.add(i);
+        ub_checks::can_dereference(p) &&
+            ub_checks::same_allocation(v_base, p)
+    })
+)]
+#[requires(
+    (0..4).into_iter().all(|i| {
+        let p = dst.add(i);
+        ub_checks::can_write(p) &&
+            ub_checks::same_allocation(dst, p)
+    })
+)]
+#[ensures(|_| {
+    let is_less: &mut F = unsafe { mem::transmute(&is_less) };
+    (0..3).into_iter().all(|i| !is_less(
+        &*dst.add(i + 1),
+        &*dst.add(i),
+    ))
+})]
 pub unsafe fn sort4_stable<T, F: FnMut(&T, &T) -> bool>(
     v_base: *const T,
     dst: *mut T,
@@ -870,3 +927,126 @@ pub(crate) const fn has_efficient_in_place_swap<T>() -> bool {
     // Heuristic that holds true on all tested 64-bit capable architectures.
     mem::size_of::<T>() <= 8 // mem::size_of::<u64>()
 }
+
+#[cfg(kani)]
+#[unstable(feature = "kani", issue = "none")]
+mod verify {
+    use super::*;
+
+    // The maximum length of the slice that `insertion_sort_shift_left`
+    // is called upon.
+    // The value is from the following line;
+    // https://github.com/model-checking/verify-rust-std/blob/1a38674ad6753e3a78e0181d1fe613f3b25ebacd/library/core/src/slice/sort/shared/smallsort.rs#L330
+    const INSERTION_SORT_MAX_LEN: usize = 17;
+
+    #[kani::proof]
+    pub fn check_swap_if_less() {
+        let mut array: [u8; SMALL_SORT_GENERAL_THRESHOLD] = kani::any();
+        let a_pos = kani::any_where(|x: &usize| *x < SMALL_SORT_GENERAL_THRESHOLD);
+        let b_pos = kani::any_where(|x: &usize| *x < SMALL_SORT_GENERAL_THRESHOLD);
+        let mut is_less = |x: &u8, y: &u8| x < y;
+        let expected = {
+            let mut array = array.clone();
+            let a: u8 = array[a_pos];
+            let b: u8 = array[b_pos];
+            if is_less(&b, &a) {
+                array[a_pos] = b;
+                array[b_pos] = a;
+            }
+            array
+        };
+        unsafe {
+            swap_if_less(array.as_mut_ptr(), a_pos, b_pos, &mut is_less);
+        }
+        kani::assert(
+            array == expected,
+            "Swapped slice is different from the expectation"
+        );
+    }
+
+    // FIXME: Ideally, this should be `proof_for_contract(insert_tail)`,
+    // but `CopyOnDrop` inside the `insert_tail`'s function body makes
+    // the following false positive check failure;
+    //
+    // - Check that array_replace((const void *)(char *)dst, ...) is allowd by the assigns clause
+    //
+    // See https://github.com/model-checking/kani/issues/3798
+    #[kani::proof]
+    #[kani::unwind(17)]
+    pub fn check_insert_tail() {
+        let tail = kani::any_where(|x: &usize| *x < INSERTION_SORT_MAX_LEN);
+        let mut is_less = |x: &u8, y: &u8| x < y;
+        let mut array = kani::any_where(|x: &[u8; INSERTION_SORT_MAX_LEN]| {
+            x[..tail].is_sorted_by(|a, b| !is_less(b, a))
+        });
+        unsafe {
+            let begin = array.as_mut_ptr();
+            let tail = begin.add(tail);
+            insert_tail(begin, tail, &mut is_less);
+        }
+        kani::assert(
+            array[..=tail].is_sorted_by(|a, b| !is_less(b, a)),
+            "Slice is not sorted",
+        );
+    }
+
+    // FIXME: Ideally, this should be `proof_for_contract(insertion_sort_shift_left)`,
+    // but this failes with OOM due to `proof_for_contract`'s perf issue.
+    //
+    // See https://github.com/model-checking/kani/issues/3797
+    #[kani::proof]
+    #[kani::stub_verified(insert_tail)]
+    #[kani::unwind(17)]
+    pub fn check_insertion_sort_shift_left() {
+        let slice_len = kani::any_where(|x: &usize| {
+            *x != 0 && *x <= INSERTION_SORT_MAX_LEN
+        });
+        let offset = kani::any_where(|x: &usize| *x != 0 && *x <= slice_len);
+        let mut is_less = |x: &u8, y: &u8| x < y;
+        let mut array = kani::any_where(|x: &[u8; INSERTION_SORT_MAX_LEN]| {
+            x[..offset].is_sorted_by(|a, b| !is_less(b, a))
+        });
+        insertion_sort_shift_left(&mut array[..slice_len], offset, &mut is_less);
+        kani::assert(
+            array[..slice_len].is_sorted_by(|a, b| !is_less(b, a)),
+            "Slice is not sorted",
+        );
+    }
+
+    #[kani::proof_for_contract(sort4_stable)]
+    pub fn check_sort4_stable() {
+        let src: [u8; 4] = kani::any();
+        let mut dst = MaybeUninit::<[u8; 4]>::uninit();
+        let mut is_less = |x: &u8, y: &u8| x < y;
+        unsafe {
+            sort4_stable(src.as_ptr(), dst.as_mut_ptr() as *mut _, &mut is_less);
+        }
+    }
+
+    #[kani::proof]
+    pub fn check_sort4_stable_stability() {
+        let src: [(u8, u8); 4] = [
+            (kani::any(), 0),
+            (kani::any(), 1),
+            (kani::any(), 2),
+            (kani::any(), 3),
+        ];
+        let mut dst = MaybeUninit::<[(u8, u8); 4]>::uninit();
+        let mut is_less = |x: &u8, y: &u8| x < y;
+        unsafe {
+            sort4_stable(src.as_ptr(), dst.as_mut_ptr() as *mut _, &mut is_less);
+        }
+        let dst = unsafe { dst.assume_init() };
+        let mut is_stably_less = |x: &u8, y: &u8| {
+            if x.0 == y.0 {
+                x.1 < y.1
+            } else {
+                x.0 < y.0
+            }
+        };
+        kani::assert(
+            dst.is_sorted_by(|a, b| !is_stably_less(b, a)),
+            "Slice is not stably sorted",
+        );
+    }
+}