core: add simd_reinterpret

`simd_reinterpret` is a replacement for `transmute`, specifically for
use with scalable SIMD types. It is used in the tests for scalable
vectors and in stdarch.

Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

Author: davidtwco

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1406,6 +1406,18 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
         return Ok(bx.select(m_i1s, args[1].immediate(), args[2].immediate()));
     }
 
+    if name == sym::simd_reinterpret {
+        require_simd!(ret_ty, SimdReturn);
+
+        return Ok(match args[0].val {
+            OperandValue::Ref(PlaceValue { llval: val, .. }) | OperandValue::Immediate(val) => {
+                bx.bitcast(val, llret_ty)
+            }
+            OperandValue::ZeroSized => bx.const_undef(llret_ty),
+            OperandValue::Pair(_, _) => todo!(),
+        });
+    }
+
     // every intrinsic below takes a SIMD vector as its first argument
     let (in_len, in_elem) = require_simd!(args[0].layout.ty, SimdInput);
     let in_ty = args[0].layout.ty;

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -709,6 +709,7 @@ pub(crate) fn check_intrinsic_type(
         }
         sym::simd_cast
         | sym::simd_as
+        | sym::simd_reinterpret
         | sym::simd_cast_ptr
         | sym::simd_expose_provenance
         | sym::simd_with_exposed_provenance => (2, 0, vec![param(0)], param(1)),

compiler/rustc_span/src/symbol.rs

@@ -2098,6 +2098,7 @@ symbols! {
         simd_reduce_mul_unordered,
         simd_reduce_or,
         simd_reduce_xor,
+        simd_reinterpret,
         simd_relaxed_fma,
         simd_rem,
         simd_round,

library/core/src/intrinsics/simd.rs

@@ -218,6 +218,11 @@ pub const unsafe fn simd_cast<T, U>(x: T) -> U;
 #[rustc_nounwind]
 pub const unsafe fn simd_as<T, U>(x: T) -> U;
 
+/// Replacement for `transmute`, specifically for use with scalable SIMD types.
+#[rustc_intrinsic]
+#[rustc_nounwind]
+pub unsafe fn simd_reinterpret<Src, Dst>(src: Src) -> Dst;
+
 /// Negates a vector elementwise.
 ///
 /// `T` must be a vector of integers or floats.