2bc994456c
* separate initialization routines into _start and do_start for all architectures. * lift do_start as a separate object library to avoid code duplication. * (addtionally) address the problem of building hermetic libc with -fstack-pointer-* The `crt1.o` is now a merged result of three components: ``` ___ |___ x86_64 | |_______ start.cpp.o <- _start (loads process initial stack and aligns stack pointer) | |_______ tls.cpp.o <- init_tls, cleanup_tls, set_thread_pointer (TLS related routines) |___ do_start.cpp.o <- do_start (sets up global variables and invokes the main function) ```
34 lines
1.7 KiB
C++
34 lines
1.7 KiB
C++
//===-- Implementation of _start for x86_64 -------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "src/__support/macros/attributes.h"
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#include "startup/linux/do_start.h"
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extern "C" [[noreturn]] void _start() {
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// This TU is compiled with -fno-omit-frame-pointer. Hence, the previous
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// value of the base pointer is pushed on to the stack. So, we step over
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// it (the "+ 1" below) to get to the args.
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LIBC_NAMESPACE::app.args = reinterpret_cast<LIBC_NAMESPACE::Args *>(
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reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)) + 1);
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// The x86_64 ABI requires that the stack pointer is aligned to a 16-byte
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// boundary. We align it here but we cannot use any local variables created
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// before the following alignment. Best would be to not create any local
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// variables before the alignment. Also, note that we are aligning the stack
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// downwards as the x86_64 stack grows downwards. This ensures that we don't
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// tread on argc, argv etc.
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// NOTE: Compiler attributes for alignment do not help here as the stack
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// pointer on entry to this _start function is controlled by the OS. In fact,
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// compilers can generate code assuming the alignment as required by the ABI.
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// If the stack pointers as setup by the OS are already aligned, then the
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// following code is a NOP.
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asm volatile("andq $0xfffffffffffffff0, %rsp\n\t");
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asm volatile("andq $0xfffffffffffffff0, %rbp\n\t");
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LIBC_NAMESPACE::do_start();
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}
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