Let codegen decide when to mem::swap with immediates

Making `libcore` decide this is silly; the backend has so much better information about when it's a good idea.

So introduce a new `typed_swap` intrinsic with a fallback body, but replace that implementation for immediates and scalar pairs.

Author: scottmcm

core/src/intrinsics.rs

@@ -66,6 +66,7 @@
 use crate::marker::DiscriminantKind;
 use crate::marker::Tuple;
 use crate::mem::align_of;
+use crate::ptr;
 
 pub mod mir;
 pub mod simd;
@@ -2628,6 +2629,27 @@ pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
     false
 }
 
+/// Non-overlapping *typed* swap of a single value.
+///
+/// The codegen backends will replace this with a better implementation when
+/// `T` is a simple type that can be loaded and stored as an immediate.
+///
+/// The stabilized form of this intrinsic is [`crate::mem::swap`].
+///
+/// # Safety
+///
+/// `x` and `y` are readable and writable as `T`, and non-overlapping.
+#[rustc_nounwind]
+#[inline]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+// This has fallback `const fn` MIR, so shouldn't need stability, see #122652
+#[rustc_const_unstable(feature = "const_typed_swap", issue = "none")]
+pub const unsafe fn typed_swap<T>(x: *mut T, y: *mut T) {
+    // SAFETY: The caller provided single non-overlapping items behind
+    // pointers, so swapping them with `count: 1` is fine.
+    unsafe { ptr::swap_nonoverlapping(x, y, 1) };
+}
+
 /// Returns whether we should check for library UB. This evaluate to the value of `cfg!(debug_assertions)`
 /// during monomorphization.
 ///

core/src/lib.rs

@@ -168,6 +168,7 @@
 #![feature(const_try)]
 #![feature(const_type_id)]
 #![feature(const_type_name)]
+#![feature(const_typed_swap)]
 #![feature(const_unicode_case_lookup)]
 #![feature(const_unsafecell_get_mut)]
 #![feature(const_waker)]

core/src/mem/mod.rs

@@ -726,31 +726,9 @@ pub unsafe fn uninitialized<T>() -> T {
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
 #[rustc_diagnostic_item = "mem_swap"]
 pub const fn swap<T>(x: &mut T, y: &mut T) {
-    // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
-    // reinterpretation of values as (chunkable) byte arrays, and the loop in the
-    // block optimization in `swap_slice` is hard to rewrite back
-    // into the (unoptimized) direct swapping implementation, so we disable it.
-    #[cfg(not(any(target_arch = "spirv")))]
-    {
-        // For types that are larger multiples of their alignment, the simple way
-        // tends to copy the whole thing to stack rather than doing it one part
-        // at a time, so instead treat them as one-element slices and piggy-back
-        // the slice optimizations that will split up the swaps.
-        if const { size_of::<T>() / align_of::<T>() > 2 } {
-            // SAFETY: exclusive references always point to one non-overlapping
-            // element and are non-null and properly aligned.
-            return unsafe { ptr::swap_nonoverlapping(x, y, 1) };
-        }
-    }
-
-    // If a scalar consists of just a small number of alignment units, let
-    // the codegen just swap those pieces directly, as it's likely just a
-    // few instructions and anything else is probably overcomplicated.
-    //
-    // Most importantly, this covers primitives and simd types that tend to
-    // have size=align where doing anything else can be a pessimization.
-    // (This will also be used for ZSTs, though any solution works for them.)
-    swap_simple(x, y);
+    // SAFETY: `&mut` guarantees these are typed readable and writable
+    // as well as non-overlapping.
+    unsafe { intrinsics::typed_swap(x, y) }
 }
 
 /// Same as [`swap`] semantically, but always uses the simple implementation.