[libc-commits] [libc] [libc] Add boot code for AArch64 (PR #154789)
William Huynh via libc-commits
libc-commits at lists.llvm.org
Thu Aug 21 09:19:54 PDT 2025
https://github.com/saturn691 created https://github.com/llvm/llvm-project/pull/154789
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.
>From 2f4135a95a9dc4e37146f0e72be11229289e8f7f Mon Sep 17 00:00:00 2001
From: William Huynh <William.Huynh at arm.com>
Date: Thu, 21 Aug 2025 17:18:12 +0100
Subject: [PATCH] [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.
---
libc/cmake/modules/LLVMLibCTestRules.cmake | 2 +-
libc/startup/baremetal/aarch64/CMakeLists.txt | 16 +++
libc/startup/baremetal/aarch64/start.cpp | 110 ++++++++++++++++++
3 files changed, 127 insertions(+), 1 deletion(-)
create mode 100644 libc/startup/baremetal/aarch64/CMakeLists.txt
create mode 100644 libc/startup/baremetal/aarch64/start.cpp
diff --git a/libc/cmake/modules/LLVMLibCTestRules.cmake b/libc/cmake/modules/LLVMLibCTestRules.cmake
index 267c32e956945..66ef3d3c1cb78 100644
--- a/libc/cmake/modules/LLVMLibCTestRules.cmake
+++ b/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
diff --git a/libc/startup/baremetal/aarch64/CMakeLists.txt b/libc/startup/baremetal/aarch64/CMakeLists.txt
new file mode 100644
index 0000000000000..f75bd893c68bc
--- /dev/null
+++ b/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
+)
diff --git a/libc/startup/baremetal/aarch64/start.cpp b/libc/startup/baremetal/aarch64/start.cpp
new file mode 100644
index 0000000000000..94d44c849ff9a
--- /dev/null
+++ b/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"
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