c1bc70b711
A lot of files were missing copyright field in the header and
the Blender Foundation contributed to them in a sense of bug
fixing and general maintenance.
This change makes it explicit that those files are at least
partially copyrighted by the Blender Foundation.
Note that this does not make it so the Blender Foundation is
the only holder of the copyright in those files, and developers
who do not have a signed contract with the foundation still
hold the copyright as well.
Another aspect of this change is using SPDX format for the
header. We already used it for the license specification,
and now we state it for the copyright as well, following the
FAQ:
https://reuse.software/faq/
109 lines
3.3 KiB
C++
109 lines
3.3 KiB
C++
/* SPDX-FileCopyrightText: 2023 Blender Foundation
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*
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* SPDX-License-Identifier: GPL-2.0-or-later */
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#pragma once
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/**
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* A #CacheMutex is used to protect a lazily computed cache from being computed more than once.
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* Using #CacheMutex instead of a "raw mutex" to protect a cache has some benefits:
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* - Avoid common pitfalls like forgetting to use task isolation or a double checked lock.
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* - Cleaner and less redundant code because the same locking patterns don't have to be repeated
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* everywhere.
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* - One can benefit from potential future improvements to #CacheMutex of which there are a few
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* mentioned below.
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*
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* The data protected by #CacheMutex is not part of #CacheMutex. Instead, the #CacheMutex and its
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* protected data should generally be placed next to each other.
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*
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* Each #CacheMutex protects exactly one cache, so multiple cache mutexes have to be used when a
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* class has multiple caches. That is contrary to a "custom" solution using `std::mutex` where one
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* mutex could protect multiple caches at the cost of higher lock contention.
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*
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* To make sure the cache is up to date, call `CacheMutex::ensure` and pass in the function that
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* computes the cache.
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*
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* To tell the #CacheMutex that the cache is invalidated and to be re-evaluated upon next access
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* use `CacheMutex::tag_dirty`.
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*
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* This example shows how one could implement a lazily computed average vertex position in an
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* imaginary `Mesh` data structure:
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*
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* \code{.cpp}
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* class Mesh {
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* private:
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* mutable CacheMutex average_position_cache_mutex_;
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* mutable float3 average_position_cache_;
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*
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* public:
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* const float3 &average_position() const
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* {
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* average_position_cache_mutex_.ensure([&]() {
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* average_position_cache_ = actually_compute_average_position();
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* });
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* return average_position_cache_;
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* }
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*
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* void tag_positions_changed()
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* {
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* average_position_cache_mutex_.tag_dirty();
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* }
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* };
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* \endcode
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*
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* Possible future improvements:
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* - Avoid task isolation when we know that the cache computation does not use threading.
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* - Try to use a smaller mutex. The mutex does not have to be fair for this use case.
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* - Try to join the cache computation instead of blocking if another thread is computing the cache
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* already.
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*/
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#include <atomic>
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#include <mutex>
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#include "BLI_function_ref.hh"
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namespace blender {
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class CacheMutex {
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private:
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std::mutex mutex_;
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std::atomic<bool> cache_valid_ = false;
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public:
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/**
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* Make sure the cache exists and is up to date. This calls `compute_cache` once to update the
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* cache (which is stored outside of this class) if it is dirty, otherwise it does nothing.
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*
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* This function is thread-safe under the assumption that the same parameters are passed from
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* every thread.
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*/
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void ensure(FunctionRef<void()> compute_cache);
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/**
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* Reset the cache. The next time #ensure is called, it will recompute that code.
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*/
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void tag_dirty()
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{
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cache_valid_.store(false);
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}
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/**
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* Return true if the cache currently does not exist or has been invalidated.
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*/
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bool is_dirty() const
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{
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return !this->is_cached();
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}
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/**
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* Return true if the cache exists and is valid.
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*/
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bool is_cached() const
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{
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return cache_valid_.load(std::memory_order_relaxed);
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}
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};
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} // namespace blender
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