Implement new simd_shuffle signature

Author: bjorn3

src/intrinsics/simd.rs

@@ -67,7 +67,34 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
         _ if intrinsic.as_str().starts_with("simd_shuffle"), (c x, c y, o idx) {
             validate_simd_type!(fx, intrinsic, span, x.layout().ty);
 
-            let n: u16 = intrinsic.as_str()["simd_shuffle".len()..].parse().unwrap();
+            // If this intrinsic is the older "simd_shuffleN" form, simply parse the integer.
+            // If there is no suffix, use the index array length.
+            let n: u16 = if intrinsic == sym::simd_shuffle {
+                // Make sure this is actually an array, since typeck only checks the length-suffixed
+                // version of this intrinsic.
+                let idx_ty = fx.monomorphize(idx.ty(fx.mir, fx.tcx));
+                match idx_ty.kind() {
+                    ty::Array(ty, len) if matches!(ty.kind(), ty::Uint(ty::UintTy::U32)) => {
+                        len.try_eval_usize(fx.tcx, ty::ParamEnv::reveal_all()).unwrap_or_else(|| {
+                            span_bug!(span, "could not evaluate shuffle index array length")
+                        }).try_into().unwrap()
+                    }
+                    _ => {
+                        fx.tcx.sess.span_err(
+                            span,
+                            &format!(
+                                "simd_shuffle index must be an array of `u32`, got `{}`",
+                                idx_ty,
+                            ),
+                        );
+                        // Prevent verifier error
+                        crate::trap::trap_unreachable(fx, "compilation should not have succeeded");
+                        return;
+                    }
+                }
+            } else {
+                intrinsic.as_str()["simd_shuffle".len()..].parse().unwrap()
+            };
 
             assert_eq!(x.layout(), y.layout());
             let layout = x.layout();