Code change:
fails this test on native : (This is not a new failure, but I am cleaning up issues from downstream Red Hat bugzilla.)
We started discussing this in , and looking at the clang implementation here: I tried that minimal step with , but it still gives the same "Cannot select" error. I don't see any lowering of in like other targets, only stuff implementing the calling convention. So I guess we do need custom support, or perhaps the clang bits could be moved down into the LLVM lowering?
Hi , I ran into the same problem and noticed you had already opened an issue. I've now submitted a PR to implement valist and vaarg for s390x to fix this.

Related discussion:
text: val } <ins> fn emit_s390x_va_arg<'ll, 'tcx>( bx: &mut Builder<'_, 'll, 'tcx>, list: OperandRef<'tcx, &'ll Value>, target_ty: Ty<'tcx>, ) -> &'ll Value { // Implementation of the s390x ELF ABI calling convention for va_args see // https://github.com/IBM/s390x-abi (chapter 1.2.4) let va_list_addr = list.immediate(); 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 in_reg = bx.append_sibling_block("va_arg.in_reg"); let in_mem = bx.append_sibling_block("va_arg.in_mem"); let end = bx.append_sibling_block("va_arg.end"); // FIXME: vector ABI not yet supported. let target_ty_size = bx.cx.size_of(target_ty).bytes(); let indirect: bool = target_ty_size > 8 || !target_ty_size.is_power_of_two(); let unpadded_size = if indirect { 8 } else { target_ty_size }; let padded_size = 8; let padding = padded_size - unpadded_size; let gpr_type = indirect || !layout.is_single_fp_element(bx.cx); let (max_regs, reg_count_field, reg_save_index, reg_padding) = if gpr_type { (5, 0, 2, padding) } else { (4, 1, 16, 0) }; // Check whether the value was passed in a register or in memory. let reg_count = bx.struct_gep( va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, reg_count_field), ); let reg_count_v = bx.load(bx.type_i64(), reg_count, Align::from_bytes(8).unwrap()); let use_regs = bx.icmp(IntPredicate::IntULT, reg_count_v, bx.const_u64(max_regs)); bx.cond_br(use_regs, in_reg, in_mem); // Emit code to load the value if it was passed in a register. bx.switch_to_block(in_reg); // Work out the address of the value in the register save area. let reg_ptr = bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 3)); let reg_ptr_v = bx.load(bx.type_i8p(), reg_ptr, bx.tcx().data_layout.pointer_align.abi); let scaled_reg_count = bx.mul(reg_count_v, bx.const_u64(8)); let reg_off = bx.add(scaled_reg_count, bx.const_u64(reg_save_index * 8 + reg_padding)); let reg_addr = bx.gep(bx.type_i8(), reg_ptr_v, &[reg_off]); // Update the register count. let new_reg_count_v = bx.add(reg_count_v, bx.const_u64(1)); bx.store(new_reg_count_v, reg_count, Align::from_bytes(8).unwrap()); bx.br(end); // Emit code to load the value if it was passed in memory. bx.switch_to_block(in_mem); // Work out the address of the value in the argument overflow area. let arg_ptr = bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 2)); let arg_ptr_v = bx.load(bx.type_i8p(), arg_ptr, bx.tcx().data_layout.pointer_align.abi); let arg_off = bx.const_u64(padding); let mem_addr = bx.gep(bx.type_i8(), arg_ptr_v, &[arg_off]); // Update the argument overflow area pointer. let arg_size = bx.cx().const_u64(padded_size); let new_arg_ptr_v = bx.inbounds_gep(bx.type_i8(), arg_ptr_v, &[arg_size]); bx.store(new_arg_ptr_v, arg_ptr, bx.tcx().data_layout.pointer_align.abi); bx.br(end); // Return the appropriate result. bx.switch_to_block(end); let val_addr = bx.phi(bx.type_i8p(), &[reg_addr, mem_addr], &[in_reg, in_mem]); let val_type = layout.llvm_type(bx); let val_addr = if indirect { let ptr_type = bx.cx.type_ptr_to(val_type); let ptr_addr = bx.bitcast(val_addr, bx.cx.type_ptr_to(ptr_type)); bx.load(ptr_type, ptr_addr, bx.tcx().data_layout.pointer_align.abi) } else { bx.bitcast(val_addr, bx.cx.type_ptr_to(val_type)) }; bx.load(val_type, val_addr, layout.align.abi) } </ins> pub(super) fn emit_va_arg<'ll, 'tcx>( bx: &mut Builder<'_, 'll, 'tcx>, addr: OperandRef<'tcx, &'ll Value>,