Permit pre-evaluated constants in simd_shuffle

compiler/rustc_codegen_ssa/src/mir/constant.rs

@@ -65,8 +65,22 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         &self,
         constant: &mir::Constant<'tcx>,
     ) -> Result<Option<ty::ValTree<'tcx>>, ErrorHandled> {
-        let uv = match constant.literal {
+        let uv = match self.monomorphize(constant.literal) {
             mir::ConstantKind::Unevaluated(uv, _) => uv.shrink(),
+            mir::ConstantKind::Ty(c) => match c.kind() {
+                // A constant that came from a const generic but was then used as an argument to old-style
+                // simd_shuffle (passing as argument instead of as a generic param).
+                rustc_type_ir::ConstKind::Value(valtree) => return Ok(Some(valtree)),
+                other => span_bug!(constant.span, "{other:#?}"),
+            },
+            // We should never encounter `ConstantKind::Val` unless MIR opts (like const prop) evaluate
+            // a constant and write that value back into `Operand`s. This could happen, but is unlikely.
+            // Also: all users of `simd_shuffle` are on unstable and already need to take a lot of care
+            // around intrinsics. For an issue to happen here, it would require a macro expanding to a
+            // `simd_shuffle` call without wrapping the constant argument in a `const {}` block, but
+            // the user pass through arbitrary expressions.
+            // FIXME(oli-obk): replace the magic const generic argument of `simd_shuffle` with a real
+            // const generic.
             other => span_bug!(constant.span, "{other:#?}"),
         };
         let uv = self.monomorphize(uv);

tests/ui/simd/shuffle.rs

@@ -1,14 +1,24 @@
-//run-pass
+// run-pass
+// revisions: opt noopt
+//[noopt] compile-flags: -Copt-level=0
+//[opt] compile-flags: -O
 #![feature(repr_simd, platform_intrinsics)]
+#![allow(incomplete_features)]
+#![feature(adt_const_params)]
 
 extern "platform-intrinsic" {
     fn simd_shuffle<T, I, U>(a: T, b: T, i: I) -> U;
+    fn simd_shuffle16<T, U>(x: T, y: T, idx: [u32; 16]) -> U;
 }
 
 #[derive(Copy, Clone)]
 #[repr(simd)]
 struct Simd<T, const N: usize>([T; N]);
 
+pub unsafe fn __shuffle_vector16<const IDX: [u32; 16], T, U>(x: T, y: T) -> U {
+    simd_shuffle16(x, y, IDX)
+}
+
 fn main() {
     const I1: [u32; 4] = [0, 2, 4, 6];
     const I2: [u32; 2] = [1, 5];
@@ -21,4 +31,16 @@ fn main() {
         let y: Simd<u8, 2> = simd_shuffle(a, b, I2);
         assert_eq!(y.0, [1, 5]);
     }
+    // Test that an indirection (via an unnamed constant)
+    // through a const generic parameter also works.
+    // See https://github.com/rust-lang/rust/issues/113500 for details.
+    let a = Simd::<u8, 16>([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]);
+    let b = Simd::<u8, 16>([16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]);
+    unsafe {
+        __shuffle_vector16::<
+            { [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16] },
+            Simd<u8, 16>,
+            Simd<u8, 16>,
+        >(a, b);
+    }
 }