Rollup merge of #134689 - RalfJung:ptr-swap-test, r=oli-obk

core: fix const ptr::swap_nonoverlapping when there are pointers at odd offsets

Ensure that the pointer gets swapped correctly even if it is not stored at an aligned offset. This rules out implementations that copy things in a `usize` loop -- so our implementation needs to be adjusted to avoid such a loop when running in const context.

Part of #133668

Author: Zalathar

library/core/src/intrinsics/mod.rs

@@ -3795,15 +3795,15 @@ where
 /// See [`const_eval_select()`] for the rules and requirements around that intrinsic.
 pub(crate) macro const_eval_select {
     (
-        @capture { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
+        @capture$([$($binders:tt)*])? { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
         if const
             $(#[$compiletime_attr:meta])* $compiletime:block
         else
             $(#[$runtime_attr:meta])* $runtime:block
     ) => {
         // Use the `noinline` arm, after adding explicit `inline` attributes
         $crate::intrinsics::const_eval_select!(
-            @capture { $($arg : $ty = $val),* } $(-> $ret)? :
+            @capture$([$($binders)*])? { $($arg : $ty = $val),* } $(-> $ret)? :
             #[noinline]
             if const
                 #[inline] // prevent codegen on this function
@@ -3817,20 +3817,20 @@ pub(crate) macro const_eval_select {
     },
     // With a leading #[noinline], we don't add inline attributes
     (
-        @capture { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
+        @capture$([$($binders:tt)*])? { $($arg:ident : $ty:ty = $val:expr),* $(,)? } $( -> $ret:ty )? :
         #[noinline]
         if const
             $(#[$compiletime_attr:meta])* $compiletime:block
         else
             $(#[$runtime_attr:meta])* $runtime:block
     ) => {{
         $(#[$runtime_attr])*
-        fn runtime($($arg: $ty),*) $( -> $ret )? {
+        fn runtime$(<$($binders)*>)?($($arg: $ty),*) $( -> $ret )? {
             $runtime
         }
 
         $(#[$compiletime_attr])*
-        const fn compiletime($($arg: $ty),*) $( -> $ret )? {
+        const fn compiletime$(<$($binders)*>)?($($arg: $ty),*) $( -> $ret )? {
             // Don't warn if one of the arguments is unused.
             $(let _ = $arg;)*
 
@@ -3842,14 +3842,14 @@ pub(crate) macro const_eval_select {
     // We support leaving away the `val` expressions for *all* arguments
     // (but not for *some* arguments, that's too tricky).
     (
-        @capture { $($arg:ident : $ty:ty),* $(,)? } $( -> $ret:ty )? :
+        @capture$([$($binders:tt)*])? { $($arg:ident : $ty:ty),* $(,)? } $( -> $ret:ty )? :
         if const
             $(#[$compiletime_attr:meta])* $compiletime:block
         else
             $(#[$runtime_attr:meta])* $runtime:block
     ) => {
         $crate::intrinsics::const_eval_select!(
-            @capture { $($arg : $ty = $arg),* } $(-> $ret)? :
+            @capture$([$($binders)*])? { $($arg : $ty = $arg),* } $(-> $ret)? :
             if const
                 $(#[$compiletime_attr])* $compiletime
             else

library/core/src/ptr/mod.rs

@@ -395,6 +395,7 @@
 #![allow(clippy::not_unsafe_ptr_arg_deref)]
 
 use crate::cmp::Ordering;
+use crate::intrinsics::const_eval_select;
 use crate::marker::FnPtr;
 use crate::mem::{self, MaybeUninit, SizedTypeProperties};
 use crate::{fmt, hash, intrinsics, ub_checks};
@@ -1074,25 +1075,6 @@ pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
 #[rustc_const_unstable(feature = "const_swap_nonoverlapping", issue = "133668")]
 #[rustc_diagnostic_item = "ptr_swap_nonoverlapping"]
 pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
-    #[allow(unused)]
-    macro_rules! attempt_swap_as_chunks {
-        ($ChunkTy:ty) => {
-            if mem::align_of::<T>() >= mem::align_of::<$ChunkTy>()
-                && mem::size_of::<T>() % mem::size_of::<$ChunkTy>() == 0
-            {
-                let x: *mut $ChunkTy = x.cast();
-                let y: *mut $ChunkTy = y.cast();
-                let count = count * (mem::size_of::<T>() / mem::size_of::<$ChunkTy>());
-                // SAFETY: these are the same bytes that the caller promised were
-                // ok, just typed as `MaybeUninit<ChunkTy>`s instead of as `T`s.
-                // The `if` condition above ensures that we're not violating
-                // alignment requirements, and that the division is exact so
-                // that we don't lose any bytes off the end.
-                return unsafe { swap_nonoverlapping_simple_untyped(x, y, count) };
-            }
-        };
-    }
-
     ub_checks::assert_unsafe_precondition!(
         check_language_ub,
         "ptr::swap_nonoverlapping requires that both pointer arguments are aligned and non-null \
@@ -1111,19 +1093,48 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
         }
     );
 
-    // Split up the slice into small power-of-two-sized chunks that LLVM is able
-    // to vectorize (unless it's a special type with more-than-pointer alignment,
-    // because we don't want to pessimize things like slices of SIMD vectors.)
-    if mem::align_of::<T>() <= mem::size_of::<usize>()
-        && (!mem::size_of::<T>().is_power_of_two()
-            || mem::size_of::<T>() > mem::size_of::<usize>() * 2)
-    {
-        attempt_swap_as_chunks!(usize);
-        attempt_swap_as_chunks!(u8);
-    }
+    const_eval_select!(
+        @capture[T] { x: *mut T, y: *mut T, count: usize }:
+        if const {
+            // At compile-time we want to always copy this in chunks of `T`, to ensure that if there
+            // are pointers inside `T` we will copy them in one go rather than trying to copy a part
+            // of a pointer (which would not work).
+            // SAFETY: Same preconditions as this function
+            unsafe { swap_nonoverlapping_simple_untyped(x, y, count) }
+        } else {
+            macro_rules! attempt_swap_as_chunks {
+                ($ChunkTy:ty) => {
+                    if mem::align_of::<T>() >= mem::align_of::<$ChunkTy>()
+                        && mem::size_of::<T>() % mem::size_of::<$ChunkTy>() == 0
+                    {
+                        let x: *mut $ChunkTy = x.cast();
+                        let y: *mut $ChunkTy = y.cast();
+                        let count = count * (mem::size_of::<T>() / mem::size_of::<$ChunkTy>());
+                        // SAFETY: these are the same bytes that the caller promised were
+                        // ok, just typed as `MaybeUninit<ChunkTy>`s instead of as `T`s.
+                        // The `if` condition above ensures that we're not violating
+                        // alignment requirements, and that the division is exact so
+                        // that we don't lose any bytes off the end.
+                        return unsafe { swap_nonoverlapping_simple_untyped(x, y, count) };
+                    }
+                };
+            }
+
+            // Split up the slice into small power-of-two-sized chunks that LLVM is able
+            // to vectorize (unless it's a special type with more-than-pointer alignment,
+            // because we don't want to pessimize things like slices of SIMD vectors.)
+            if mem::align_of::<T>() <= mem::size_of::<usize>()
+            && (!mem::size_of::<T>().is_power_of_two()
+                || mem::size_of::<T>() > mem::size_of::<usize>() * 2)
+            {
+                attempt_swap_as_chunks!(usize);
+                attempt_swap_as_chunks!(u8);
+            }
 
-    // SAFETY: Same preconditions as this function
-    unsafe { swap_nonoverlapping_simple_untyped(x, y, count) }
+            // SAFETY: Same preconditions as this function
+            unsafe { swap_nonoverlapping_simple_untyped(x, y, count) }
+        }
+    )
 }
 
 /// Same behavior and safety conditions as [`swap_nonoverlapping`]

library/core/tests/lib.rs

@@ -16,6 +16,7 @@
 #![feature(const_black_box)]
 #![feature(const_eval_select)]
 #![feature(const_swap)]
+#![feature(const_swap_nonoverlapping)]
 #![feature(const_trait_impl)]
 #![feature(core_intrinsics)]
 #![feature(core_io_borrowed_buf)]

library/core/tests/ptr.rs

@@ -860,7 +860,10 @@ fn swap_copy_untyped() {
 }
 
 #[test]
-fn test_const_copy() {
+fn test_const_copy_ptr() {
+    // `copy` and `copy_nonoverlapping` are thin layers on top of intrinsics. Ensure they correctly
+    // deal with pointers even when the pointers cross the boundary from one "element" being copied
+    // to another.
     const {
         let ptr1 = &1;
         let mut ptr2 = &666;
@@ -899,21 +902,61 @@ fn test_const_copy() {
 }
 
 #[test]
-fn test_const_swap() {
+fn test_const_swap_ptr() {
+    // The `swap` functions are implemented in the library, they are not primitives.
+    // Only `swap_nonoverlapping` takes a count; pointers that cross multiple elements
+    // are *not* supported.
+    // We put the pointer at an odd offset in the type and copy them as an array of bytes,
+    // which should catch most of the ways that the library implementation can get it wrong.
+
+    #[cfg(target_pointer_width = "32")]
+    type HalfPtr = i16;
+    #[cfg(target_pointer_width = "64")]
+    type HalfPtr = i32;
+
+    #[repr(C, packed)]
+    #[allow(unused)]
+    struct S {
+        f1: HalfPtr,
+        // Crucially this field is at an offset that is not a multiple of the pointer size.
+        ptr: &'static i32,
+        // Make sure the entire type does not have a power-of-2 size:
+        // make it 3 pointers in size. This used to hit a bug in `swap_nonoverlapping`.
+        f2: [HalfPtr; 3],
+    }
+
+    // Ensure the entire thing is usize-aligned, so in principle this
+    // looks like it could be eligible for a `usize` copying loop.
+    #[cfg_attr(target_pointer_width = "32", repr(align(4)))]
+    #[cfg_attr(target_pointer_width = "64", repr(align(8)))]
+    struct A(S);
+
     const {
-        let mut ptr1 = &1;
-        let mut ptr2 = &666;
+        let mut s1 = A(S { ptr: &1, f1: 0, f2: [0; 3] });
+        let mut s2 = A(S { ptr: &666, f1: 0, f2: [0; 3] });
 
-        // Swap ptr1 and ptr2, bytewise. `swap` does not take a count
-        // so the best we can do is use an array.
-        type T = [u8; mem::size_of::<&i32>()];
+        // Swap ptr1 and ptr2, as an array.
+        type T = [u8; mem::size_of::<A>()];
         unsafe {
-            ptr::swap(ptr::from_mut(&mut ptr1).cast::<T>(), ptr::from_mut(&mut ptr2).cast::<T>());
+            ptr::swap(ptr::from_mut(&mut s1).cast::<T>(), ptr::from_mut(&mut s2).cast::<T>());
         }
 
         // Make sure they still work.
-        assert!(*ptr1 == 666);
-        assert!(*ptr2 == 1);
+        assert!(*s1.0.ptr == 666);
+        assert!(*s2.0.ptr == 1);
+
+        // Swap them back, again as an array.
+        unsafe {
+            ptr::swap_nonoverlapping(
+                ptr::from_mut(&mut s1).cast::<T>(),
+                ptr::from_mut(&mut s2).cast::<T>(),
+                1,
+            );
+        }
+
+        // Make sure they still work.
+        assert!(*s1.0.ptr == 1);
+        assert!(*s2.0.ptr == 666);
     };
 }
 

tests/ui/consts/missing_span_in_backtrace.stderr

@@ -7,6 +7,8 @@ note: inside `std::ptr::read::<MaybeUninit<MaybeUninit<u8>>>`
   --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
 note: inside `std::ptr::swap_nonoverlapping_simple_untyped::<MaybeUninit<u8>>`
   --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+note: inside `swap_nonoverlapping::compiletime::<MaybeUninit<u8>>`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
 note: inside `swap_nonoverlapping::<MaybeUninit<u8>>`
   --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
 note: inside `X`
@@ -20,6 +22,7 @@ note: inside `X`
    | |_________^
    = help: this code performed an operation that depends on the underlying bytes representing a pointer
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
+   = note: this error originates in the macro `$crate::intrinsics::const_eval_select` which comes from the expansion of the macro `const_eval_select` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error