Auto merge of #115933 - oli-obk:simd_shuffle_const, r=workingjubilee

Prototype using const generic for simd_shuffle IDX array

cc https://github.com/rust-lang/rust/issues/85229

r? `@workingjubilee` on the design

TLDR: there is now a `fn simd_shuffle_generic<T, U, const IDX: &'static [u32]>(x: T, y: T) -> U;` intrinsic that allows replacing

```rust
simd_shuffle(a, b, const { stuff })
```

with

```rust
simd_shuffle_generic::<_, _, {&stuff}>(a, b)
```

which makes the compiler implementations much simpler, if we manage to at some point eliminate `simd_shuffle`.

There are some issues with this today though (can't do math without bubbling it up in the generic arguments). With this change, we can start porting the simple cases and get better data on the others.

src/intrinsics/simd.rs

@@ -21,7 +21,7 @@ fn report_simd_type_validation_error(
 pub(super) fn codegen_simd_intrinsic_call<'tcx>(
     fx: &mut FunctionCx<'_, '_, 'tcx>,
     intrinsic: Symbol,
-    _args: GenericArgsRef<'tcx>,
+    generic_args: GenericArgsRef<'tcx>,
     args: &[mir::Operand<'tcx>],
     ret: CPlace<'tcx>,
     target: BasicBlock,
@@ -117,6 +117,54 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
             });
         }
 
+        // simd_shuffle_generic<T, U, const I: &[u32]>(x: T, y: T) -> U
+        sym::simd_shuffle_generic => {
+            let [x, y] = args else {
+                bug!("wrong number of args for intrinsic {intrinsic}");
+            };
+            let x = codegen_operand(fx, x);
+            let y = codegen_operand(fx, y);
+
+            if !x.layout().ty.is_simd() {
+                report_simd_type_validation_error(fx, intrinsic, span, x.layout().ty);
+                return;
+            }
+
+            let idx = generic_args[2]
+                .expect_const()
+                .eval(fx.tcx, ty::ParamEnv::reveal_all(), Some(span))
+                .unwrap()
+                .unwrap_branch();
+
+            assert_eq!(x.layout(), y.layout());
+            let layout = x.layout();
+
+            let (lane_count, lane_ty) = layout.ty.simd_size_and_type(fx.tcx);
+            let (ret_lane_count, ret_lane_ty) = ret.layout().ty.simd_size_and_type(fx.tcx);
+
+            assert_eq!(lane_ty, ret_lane_ty);
+            assert_eq!(idx.len() as u64, ret_lane_count);
+
+            let total_len = lane_count * 2;
+
+            let indexes =
+                idx.iter().map(|idx| idx.unwrap_leaf().try_to_u16().unwrap()).collect::<Vec<u16>>();
+
+            for &idx in &indexes {
+                assert!(u64::from(idx) < total_len, "idx {} out of range 0..{}", idx, total_len);
+            }
+
+            for (out_idx, in_idx) in indexes.into_iter().enumerate() {
+                let in_lane = if u64::from(in_idx) < lane_count {
+                    x.value_lane(fx, in_idx.into())
+                } else {
+                    y.value_lane(fx, u64::from(in_idx) - lane_count)
+                };
+                let out_lane = ret.place_lane(fx, u64::try_from(out_idx).unwrap());
+                out_lane.write_cvalue(fx, in_lane);
+            }
+        }
+
         // simd_shuffle<T, I, U>(x: T, y: T, idx: I) -> U
         sym::simd_shuffle => {
             let (x, y, idx) = match args {