| 1 | //===-- Implementation of _start for x86_64 -------------------------------===// |
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| 2 | // |
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| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
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| 4 | // See https://llvm.org/LICENSE.txt for license information. |
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| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
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| 6 | // |
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| 7 | //===----------------------------------------------------------------------===// |
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| 8 | #include "src/__support/macros/attributes.h" |
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| 9 | #include "startup/linux/do_start.h" |
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| 10 | |
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| 11 | extern "C" [[noreturn]] void _start() { |
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| 12 | // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous |
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| 13 | // value of the base pointer is pushed on to the stack. So, we step over |
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| 14 | // it (the "+ 1" below) to get to the args. |
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| 15 | LIBC_NAMESPACE::app.args = reinterpret_cast<LIBC_NAMESPACE::Args *>( |
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| 16 | reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)) + 1); |
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| 17 | |
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| 18 | // The x86_64 ABI requires that the stack pointer is aligned to a 16-byte |
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| 19 | // boundary. We align it here but we cannot use any local variables created |
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| 20 | // before the following alignment. Best would be to not create any local |
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| 21 | // variables before the alignment. Also, note that we are aligning the stack |
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| 22 | // downwards as the x86_64 stack grows downwards. This ensures that we don't |
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| 23 | // tread on argc, argv etc. |
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| 24 | // NOTE: Compiler attributes for alignment do not help here as the stack |
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| 25 | // pointer on entry to this _start function is controlled by the OS. In fact, |
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| 26 | // compilers can generate code assuming the alignment as required by the ABI. |
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| 27 | // If the stack pointers as setup by the OS are already aligned, then the |
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| 28 | // following code is a NOP. |
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| 29 | asm volatile("andq $0xfffffffffffffff0, %rsp\n\t" ); |
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| 30 | asm volatile("andq $0xfffffffffffffff0, %rbp\n\t" ); |
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| 31 | |
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| 32 | LIBC_NAMESPACE::do_start(); |
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| 33 | } |
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| 34 | |
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