libc/startup/linux/aarch64/start.cpp

   1 //===-- Implementation of _start for aarch64 ------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "startup/linux/do_start.h"
  10 extern "C" [[noreturn]] void _start() {
  11   // Skip the Frame Pointer and the Link Register
  12   // https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
  13   // Section 6.2.3. Note that this only works if the current function
  14   // is not using any callee-saved registers (x19 to x28). If the
  15   // function uses such registers, then their value is pushed on to the
  16   // stack before the frame pointer an link register values. That breaks
  17   // the assumption that stepping over the frame pointer and link register
  18   // will take us to the previous stack pointer. That is the reason why the
  19   // actual business logic of the startup code is pushed into a non-inline
  20   // function do_start so that this function is free of any stack usage.
  21   LIBC_NAMESPACE::app.args = reinterpret_cast<LIBC_NAMESPACE::Args *>(
  22       reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)) + 2);
  23   LIBC_NAMESPACE::do_start();
  24 }