Rollup merge of rust-lang#137447 - folkertdev:simd-extract-insert-dyn, r=scottmcm

add `core::intrinsics::simd::{simd_extract_dyn, simd_insert_dyn}`

fixes rust-lang#137372

adds `core::intrinsics::simd::{simd_extract_dyn, simd_insert_dyn}`, which contrary to their non-dyn counterparts allow a non-const index. Many platforms (but notably not x86_64 or aarch64) have dedicated instructions for this operation, which stdarch can emit with this change.

Future work is to also make the `Index` operation on the `Simd` type emit this operation, but the intrinsic can't be used directly. We'll need some MIR shenanigans for that.

r? `@ghost`

Author: Zalathar

Diff for: core/src/intrinsics/simd.rs

@@ -4,7 +4,7 @@
 
 /// Inserts an element into a vector, returning the updated vector.
 ///
-/// `T` must be a vector with element type `U`.
+/// `T` must be a vector with element type `U`, and `idx` must be `const`.
 ///
 /// # Safety
 ///
@@ -15,15 +15,48 @@ pub const unsafe fn simd_insert<T, U>(x: T, idx: u32, val: U) -> T;
 
 /// Extracts an element from a vector.
 ///
-/// `T` must be a vector with element type `U`.
+/// `T` must be a vector with element type `U`, and `idx` must be `const`.
 ///
 /// # Safety
 ///
-/// `idx` must be in-bounds of the vector.
+/// `idx` must be const and in-bounds of the vector.
 #[rustc_intrinsic]
 #[rustc_nounwind]
 pub const unsafe fn simd_extract<T, U>(x: T, idx: u32) -> U;
 
+/// Inserts an element into a vector, returning the updated vector.
+///
+/// `T` must be a vector with element type `U`.
+///
+/// If the index is `const`, [`simd_insert`] may emit better assembly.
+///
+/// # Safety
+///
+/// `idx` must be in-bounds of the vector.
+#[rustc_nounwind]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+pub unsafe fn simd_insert_dyn<T, U>(mut x: T, idx: u32, val: U) -> T {
+    // SAFETY: `idx` must be in-bounds
+    unsafe { (&raw mut x).cast::<U>().add(idx as usize).write(val) }
+    x
+}
+
+/// Extracts an element from a vector.
+///
+/// `T` must be a vector with element type `U`.
+///
+/// If the index is `const`, [`simd_extract`] may emit better assembly.
+///
+/// # Safety
+///
+/// `idx` must be in-bounds of the vector.
+#[rustc_nounwind]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+pub unsafe fn simd_extract_dyn<T, U>(x: T, idx: u32) -> U {
+    // SAFETY: `idx` must be in-bounds
+    unsafe { (&raw const x).cast::<U>().add(idx as usize).read() }
+}
+
 /// Adds two simd vectors elementwise.
 ///
 /// `T` must be a vector of integers or floats.