Replace hard-coded field indexes with lookup on aarch64 non-macos.

The indexes into the VaListImpl struct used on aarch64 ABI (not macos/ios) are hard-coded which is brittle so we replace them with the usual lookup.

The varargs ffi is tested in ui/abi/variadic-ffi.rs on aarch64 Linux.

Author: hkratz

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -98,7 +98,8 @@ fn emit_aapcs_va_arg(
     // Implementation of the AAPCS64 calling convention for va_args see
     // https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
     let va_list_addr = list.immediate();
-    let va_list_ty = list.deref(bx.cx).layout.llvm_type(bx);
+    let va_list_layout = list.deref(bx.cx).layout;
+    let va_list_ty = va_list_layout.llvm_type(bx);
     let layout = bx.cx.layout_of(target_ty);
 
     let mut maybe_reg = bx.build_sibling_block("va_arg.maybe_reg");
@@ -110,13 +111,15 @@ fn emit_aapcs_va_arg(
 
     let gr_type = target_ty.is_any_ptr() || target_ty.is_integral();
     let (reg_off, reg_top_index, slot_size) = if gr_type {
-        let gr_offs = bx.struct_gep(va_list_ty, va_list_addr, 3);
+        let gr_offs =
+            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 3));
         let nreg = (layout.size.bytes() + 7) / 8;
-        (gr_offs, 1, nreg * 8)
+        (gr_offs, va_list_layout.llvm_field_index(bx.cx, 1), nreg * 8)
     } else {
-        let vr_off = bx.struct_gep(va_list_ty, va_list_addr, 4);
+        let vr_off =
+            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 4));
         let nreg = (layout.size.bytes() + 15) / 16;
-        (vr_off, 2, nreg * 16)
+        (vr_off, va_list_layout.llvm_field_index(bx.cx, 2), nreg * 16)
     };
 
     // if the offset >= 0 then the value will be on the stack