libc/startup/linux/x86_64/start.cpp

   1 //===-- Implementation of _start for x86_64 -------------------------------===//
   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 #include "src/__support/macros/attributes.h"
   9 #include "startup/linux/do_start.h"
  10
  11 extern "C" [[noreturn]] void _start() {
  12   // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous
  13   // value of the base pointer is pushed on to the stack. So, we step over
  14   // it (the "+ 1" below) to get to the args.
  15   LIBC_NAMESPACE::app.args = reinterpret_cast<LIBC_NAMESPACE::Args *>(
  16       reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)) + 1);
  17
  18   // The x86_64 ABI requires that the stack pointer is aligned to a 16-byte
  19   // boundary. We align it here but we cannot use any local variables created
  20   // before the following alignment. Best would be to not create any local
  21   // variables before the alignment. Also, note that we are aligning the stack
  22   // downwards as the x86_64 stack grows downwards. This ensures that we don't
  23   // tread on argc, argv etc.
  24   // NOTE: Compiler attributes for alignment do not help here as the stack
  25   // pointer on entry to this _start function is controlled by the OS. In fact,
  26   // compilers can generate code assuming the alignment as required by the ABI.
  27   // If the stack pointers as setup by the OS are already aligned, then the
  28   // following code is a NOP.
  29   asm volatile("andq $0xfffffffffffffff0, %rsp\n\t");
  30   asm volatile("andq $0xfffffffffffffff0, %rbp\n\t");
  31
  32   LIBC_NAMESPACE::do_start();
  33 }