system/ulib/runtime/include/runtime/tls.h

// Copyright 2016 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#pragma once

#include <zircon/compiler.h>
#include <zircon/syscalls.h>
#include <zircon/syscalls/object.h>

__BEGIN_CDECLS

#pragma GCC visibility push(hidden)

// Get and set the thread pointer.
static inline void* zxr_tp_get(void);
static inline void zxr_tp_set(zx_handle_t self, void* tp);

#if defined(__aarch64__)

__NO_SAFESTACK static inline void* zxr_tp_get(void) {
    // This just emits "mrs %[reg], tpidr_el0", but the compiler
    // knows what exactly it's doing (unlike an asm).  So it can
    // e.g. CSE it with another implicit thread-pointer fetch it
    // generated for its own reasons.
    return __builtin_thread_pointer();
}

__NO_SAFESTACK static inline void zxr_tp_set(zx_handle_t self, void* tp) {
    __asm__ volatile("msr tpidr_el0, %0"
                     :
                     : "r"(tp));
}

#elif defined(__x86_64__)

__NO_SAFESTACK static inline void* zxr_tp_get(void) {
    // This fetches %fs:0, but the compiler knows what it's doing.
    // LLVM knows that in the Fuchsia ABI %fs:0 always stores the
    // %fs.base address, and its optimizer will see through this
    // to integrate *(zxr_tp_get() + N) as a direct "mov %fs:N, ...".
    // Note that these special pointer types can be used to access
    // memory, but they cannot be cast to a normal pointer type
    // (which in the abstract should add in the base address,
    // but the compiler doesn't know how to do that).
# ifdef __clang__
    // Clang does it via magic address_space numbers (256 is %gs).
    void* __attribute__((address_space(257)))* fs = 0;
    // TODO(mcgrathr): GCC 6 supports this syntax instead (and __seg_gs):
    //     void* __seg_fs* fs = 0;
    // Unfortunately, it allows it only in C and not in C++.
    // It also requires -fasm under -std=c11 (et al), see:
    //     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79609
    // It's also buggy for the special case of 0, see:
    //     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79619
    return *fs;
# else
    void* tp;
    __asm__ __volatile__("mov %%fs:0,%0"
                         : "=r"(tp));
    return tp;
# endif
}

__NO_SAFESTACK static inline void zxr_tp_set(zx_handle_t self, void* tp) {
    zx_status_t status = _zx_object_set_property(
        self, ZX_PROP_REGISTER_FS, (uintptr_t*)&tp, sizeof(uintptr_t));
    if (status != ZX_OK)
        __builtin_trap();
}

#else

#error Unsupported architecture

#endif

#pragma GCC visibility pop

__END_CDECLS