[libc] Add boot code for AArch64

This is required in hermetic testing downstream. It is not
complete, and will not work on hardware, however it runs on QEMU,
and can report a pass/fail on our tests.

Author: saturn691

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -836,7 +836,7 @@ function(add_libc_hermetic test_name)
                    ${fq_deps_list})
   # TODO: currently the dependency chain is broken such that getauxval cannot properly
   # propagate to hermetic tests. This is a temporary workaround.
-  if (LIBC_TARGET_ARCHITECTURE_IS_AARCH64)
+  if (LIBC_TARGET_ARCHITECTURE_IS_AARCH64 AND NOT(LIBC_TARGET_OS_IS_BAREMETAL))
     target_link_libraries(
       ${fq_build_target_name}
       PRIVATE

libc/startup/baremetal/aarch64/CMakeLists.txt

@@ -0,0 +1,16 @@
+add_startup_object(
+  crt1
+  SRC
+    start.cpp
+  DEPENDS
+    libc.src.stdlib.atexit
+    libc.src.stdlib.exit
+    libc.src.string.memcpy
+    libc.src.string.memset
+    libc.startup.baremetal.init
+    libc.startup.baremetal.fini
+  COMPILE_OPTIONS
+    -ffreestanding # To avoid compiler warnings about calling the main function.
+    -fno-builtin
+    -Wno-global-constructors # To allow vector table initialization
+)

libc/startup/baremetal/aarch64/start.cpp

@@ -0,0 +1,110 @@
+//===-- Implementation of crt for aarch64 ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/stdint_proxy.h"
+#include "src/__support/macros/config.h"
+#include "src/stdlib/atexit.h"
+#include "src/stdlib/exit.h"
+#include "src/string/memcpy.h"
+#include "src/string/memset.h"
+#include "startup/baremetal/fini.h"
+#include "startup/baremetal/init.h"
+
+#include <arm_acle.h>
+
+extern "C" {
+int main(int argc, char **argv);
+void _start();
+
+// Semihosting library initialisation if applicable. Required for printf, etc.
+[[gnu::weak]] void _platform_init() {}
+
+// These symbols are provided by the linker. The exact names are not defined by
+// a standard.
+extern uintptr_t __stack;
+extern uintptr_t __data_source[];
+extern uintptr_t __data_start[];
+extern uintptr_t __data_size[];
+extern uintptr_t __bss_start[];
+extern uintptr_t __bss_size[];
+} // extern "C"
+
+namespace {
+// The Arm ARM for the A-profile architecture (D14.1.5) defines the exceptions.
+// However, for simplicity, we don't bother logging, and just exit.
+void GenericException_Handler() { LIBC_NAMESPACE::exit(1); }
+
+// The AArch64 exception vector table has 16 entries, each of which is 128
+// bytes long, and contains code. The whole table must be 2048-byte aligned.
+// For our purposes, each entry just contains one branch instruction to the
+// exception reporting function, since we never want to resume after an
+// exception.
+[[gnu::section(".vectors"), gnu::aligned(2048), gnu::used, gnu::naked]]
+void vector_table() {
+#define VECTOR_TABLE_ENTRY                                                     \
+  asm(".balign 128");                                                          \
+  asm("B %0" : : "X"(GenericException_Handler));
+
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+  VECTOR_TABLE_ENTRY;
+}
+} // namespace
+
+namespace LIBC_NAMESPACE_DECL {
+
+[[noreturn]] void do_start() {
+  // TODO: This startup code is not extensive, but rather the MVP for QEMU
+  // testing.
+
+  // Set up exception handling
+  __arm_wsr64("VBAR_EL1", reinterpret_cast<uint64_t>(&vector_table));
+
+#ifdef __ARM_FP
+  // Do not trap FP/SME/SVE instructions
+  static constexpr uint64_t CPACR_SHIFT_FPEN = 20;
+  static constexpr uint64_t CPACR_SHIFT_SMEN = 24;
+  uint64_t cpacr = __arm_rsr64("CPACR_EL1");
+  cpacr |= (0x3 << CPACR_SHIFT_FPEN);
+  cpacr |= (0x3 << CPACR_SHIFT_SMEN);
+  __arm_wsr64("CPACR_EL1", cpacr);
+#endif
+
+  // Perform the equivalent of scatterloading
+  LIBC_NAMESPACE::memcpy(__data_start, __data_source,
+                         reinterpret_cast<uintptr_t>(__data_size));
+  LIBC_NAMESPACE::memset(__bss_start, '\0',
+                         reinterpret_cast<uintptr_t>(__bss_size));
+  __libc_init_array();
+
+  _platform_init();
+  LIBC_NAMESPACE::atexit(&__libc_fini_array);
+  LIBC_NAMESPACE::exit(main(0, 0));
+}
+} // namespace LIBC_NAMESPACE_DECL
+
+extern "C" {
+[[gnu::section(".text.init.enter"), gnu::naked]]
+void _start() {
+  asm volatile("mov sp, %0" : : "r"(&__stack));
+  asm volatile("bl %0" : : "X"(LIBC_NAMESPACE::do_start));
+}
+} // extern "C"