cg_llvm: scalable vectors with simd_select

Building on the previous change, support scalable vectors with
`simd_select`. Previous patches already landed the necessary changes in
the implementation of this intrinsic, but didn't allow scalable vector
arguments to be passed in.

Author: davidtwco

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1784,7 +1784,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
     }
 
     let supports_scalable = match name {
-        sym::simd_cast => true,
+        sym::simd_cast | sym::simd_select => true,
         _ => false,
     };
 
@@ -1984,7 +1984,7 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
     if name == sym::simd_select {
         let m_elem_ty = in_elem;
         let m_len = in_len;
-        let (v_len, _) = require_simd!(args[1].layout.ty, SimdArgument);
+        let (v_len, _, _) = require_simd_or_scalable!(args[1].layout.ty, SimdArgument);
         require!(
             m_len == v_len,
             InvalidMonomorphization::MismatchedLengths { span, name, m_len, v_len }

tests/ui/scalable-vectors/select-intrinsic.rs

@@ -0,0 +1,22 @@
+//@ check-pass
+//@ only-aarch64
+#![crate_type = "lib"]
+#![allow(incomplete_features, internal_features, improper_ctypes)]
+#![feature(abi_unadjusted, core_intrinsics, link_llvm_intrinsics, rustc_attrs)]
+
+use std::intrinsics::simd::simd_select;
+
+#[derive(Copy, Clone)]
+#[rustc_scalable_vector(16)]
+#[allow(non_camel_case_types)]
+pub struct svbool_t(bool);
+
+#[derive(Copy, Clone)]
+#[rustc_scalable_vector(16)]
+#[allow(non_camel_case_types)]
+pub struct svint8_t(i8);
+
+#[target_feature(enable = "sve")]
+pub fn svsel_s8(pg: svbool_t, op1: svint8_t, op2: svint8_t) -> svint8_t {
+    unsafe { simd_select::<svbool_t, _>(pg, op1, op2) }
+}