Rollup merge of #88855 - calebzulawski:feature/simd_shuffle, r=nagisa

Allow simd_shuffle to accept vectors of any length

cc ``@rust-lang/project-portable-simd`` ``@workingjubilee``

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -918,12 +918,29 @@ fn generic_simd_intrinsic(
     }
 
     if let Some(stripped) = name_str.strip_prefix("simd_shuffle") {
-        let n: u64 = stripped.parse().unwrap_or_else(|_| {
-            span_bug!(span, "bad `simd_shuffle` instruction only caught in codegen?")
-        });
+        // 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: u64 = if stripped.is_empty() {
+            // Make sure this is actually an array, since typeck only checks the length-suffixed
+            // version of this intrinsic.
+            match args[2].layout.ty.kind() {
+                ty::Array(ty, len) if matches!(ty.kind(), ty::Uint(ty::UintTy::U32)) => {
+                    len.try_eval_usize(bx.cx.tcx, ty::ParamEnv::reveal_all()).unwrap_or_else(|| {
+                        span_bug!(span, "could not evaluate shuffle index array length")
+                    })
+                }
+                _ => return_error!(
+                    "simd_shuffle index must be an array of `u32`, got `{}`",
+                    args[2].layout.ty
+                ),
+            }
+        } else {
+            stripped.parse().unwrap_or_else(|_| {
+                span_bug!(span, "bad `simd_shuffle` instruction only caught in codegen?")
+            })
+        };
 
         require_simd!(ret_ty, "return");
-
         let (out_len, out_ty) = ret_ty.simd_size_and_type(bx.tcx());
         require!(
             out_len == n,