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Improve robustness of atomics

Browse source 
And fix increment in `CowData` not being conditional anymore after the recent changes.

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>
This commit is contained in:
Pedro J. Estébanez 2021-02-18 23:31:34 +01:00
parent 001aaa7131
commit 55b5f98402
4 changed files with 79 additions and 12 deletions
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32
core/cowdata.h
View file

@ -44,8 +44,9 @@ class CharString;
template <class T, class V>
class VMap;
// CowData is relying on this to be true
static_assert(sizeof(SafeNumeric<uint32_t>) == sizeof(uint32_t), "");
#if !defined(NO_THREADS)
SAFE_NUMERIC_TYPE_PUN_GUARANTEES(uint32_t)
#endif
template <class T>
class CowData {
@ -111,7 +112,7 @@ private:
void _unref(void *p_data);
void _ref(const CowData *p_from);
void _ref(const CowData &p_from);
void _copy_on_write();
uint32_t _copy_on_write();
public:
void operator=(const CowData<T> &p_from) { _ref(p_from); }
@ -217,20 +218,21 @@ void CowData<T>::_unref(void *p_data) {
}
template <class T>
void CowData<T>::_copy_on_write() {
uint32_t CowData<T>::_copy_on_write() {
if (!_ptr)
return;
return 0;
SafeNumeric<uint32_t> *refc = _get_refcount();
if (unlikely(refc->get() > 1)) {
uint32_t rc = refc->get();
if (likely(rc > 1)) {
/* in use by more than me */
uint32_t current_size = *_get_size();
uint32_t *mem_new = (uint32_t *)Memory::alloc_static(_get_alloc_size(current_size), true);
reinterpret_cast<SafeNumeric<uint32_t> *>(mem_new - 2)->set(1); //refcount
new (mem_new - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(1); //refcount
*(mem_new - 1) = current_size; //size
T *_data = (T *)(mem_new);
@ -247,7 +249,10 @@ void CowData<T>::_copy_on_write() {
_unref(_ptr);
_ptr = _data;
rc = 1;
}
return rc;
}
template <class T>
@ -268,7 +273,7 @@ Error CowData<T>::resize(int p_size) {
}
// possibly changing size, copy on write
_copy_on_write();
uint32_t rc = _copy_on_write();
size_t current_alloc_size = _get_alloc_size(current_size);
size_t alloc_size;
@ -282,13 +287,15 @@ Error CowData<T>::resize(int p_size) {
uint32_t *ptr = (uint32_t *)Memory::alloc_static(alloc_size, true);
ERR_FAIL_COND_V(!ptr, ERR_OUT_OF_MEMORY);
*(ptr - 1) = 0; //size, currently none
reinterpret_cast<SafeNumeric<uint32_t> *>(ptr - 2)->set(1); //refcount
new (ptr - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(1); //refcount
_ptr = (T *)ptr;
} else {
void *_ptrnew = (T *)Memory::realloc_static(_ptr, alloc_size, true);
uint32_t *_ptrnew = (uint32_t *)Memory::realloc_static(_ptr, alloc_size, true);
ERR_FAIL_COND_V(!_ptrnew, ERR_OUT_OF_MEMORY);
new (_ptrnew - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(rc); //refcount
_ptr = (T *)(_ptrnew);
}
}
@ -316,8 +323,9 @@ Error CowData<T>::resize(int p_size) {
}
if (alloc_size != current_alloc_size) {
void *_ptrnew = (T *)Memory::realloc_static(_ptr, alloc_size, true);
uint32_t *_ptrnew = (uint32_t *)Memory::realloc_static(_ptr, alloc_size, true);
ERR_FAIL_COND_V(!_ptrnew, ERR_OUT_OF_MEMORY);
new (_ptrnew - 2, sizeof(uint32_t), "") SafeNumeric<uint32_t>(rc); //refcount
_ptr = (T *)(_ptrnew);
}
@ -363,7 +371,7 @@ void CowData<T>::_ref(const CowData &p_from) {
if (!p_from._ptr)
return; //nothing to do
if (p_from._get_refcount()->increment() > 0) { // could reference
if (p_from._get_refcount()->conditional_increment() > 0) { // could reference
_ptr = p_from._ptr;
}
}

45
core/safe_refcount.cpp Normal file
View file

@ -0,0 +1,45 @@
/*************************************************************************/
/* safe_refcount.cpp */
/*************************************************************************/
/* This file is part of: */
/* GODOT ENGINE */
/* https://godotengine.org */
/*************************************************************************/
/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur. */
/* Copyright (c) 2014-2021 Godot Engine contributors (cf. AUTHORS.md). */
/* */
/* Permission is hereby granted, free of charge, to any person obtaining */
/* a copy of this software and associated documentation files (the */
/* "Software"), to deal in the Software without restriction, including */
/* without limitation the rights to use, copy, modify, merge, publish, */
/* distribute, sublicense, and/or sell copies of the Software, and to */
/* permit persons to whom the Software is furnished to do so, subject to */
/* the following conditions: */
/* */
/* The above copyright notice and this permission notice shall be */
/* included in all copies or substantial portions of the Software. */
/* */
/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, */
/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF */
/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY */
/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, */
/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE */
/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */
/*************************************************************************/
#if defined(DEBUG_ENABLED) && !defined(NO_THREADS)
#include "safe_refcount.h"
#include "core/error_macros.h"
// On C++14 we don't have std::atomic::is_always_lockfree, so this is the best we can do
void check_lockless_atomics() {
// Doing the check for the types we actually care about
if (!std::atomic<uint32_t>{}.is_lock_free() || !std::atomic<uint64_t>{}.is_lock_free() || !std::atomic_bool{}.is_lock_free()) {
WARN_PRINT("Your compiler doesn't seem to support lockless atomics. Performance will be degraded. Please consider upgrading to a different or newer compiler.");
}
}
#endif

10
core/safe_refcount.h
View file

@ -47,6 +47,16 @@
// value and, as an important benefit, you can be sure the value is properly synchronized
// even with threads that are already running.
// This is used in very specific areas of the engine where it's critical that these guarantees are held
#define SAFE_NUMERIC_TYPE_PUN_GUARANTEES(m_type) \
static_assert(sizeof(SafeNumeric<m_type>) == sizeof(m_type), ""); \
static_assert(alignof(SafeNumeric<m_type>) == alignof(m_type), ""); \
static_assert(std::is_trivially_destructible<std::atomic<m_type> >::value, "");
#if defined(DEBUG_ENABLED)
void check_lockless_atomics();
#endif
template <class T>
class SafeNumeric {
std::atomic<T> value;

4
main/main.cpp
View file

@ -346,6 +346,10 @@ void Main::print_help(const char *p_binary) {
*/
Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_phase) {
#if defined(DEBUG_ENABLED) && !defined(NO_THREADS)
check_lockless_atomics();
#endif
RID_OwnerBase::init_rid();
OS::get_singleton()->initialize_core();