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<span class="doccomment">//! The global epoch</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The last bit in this number is unused and is always zero. Every so often the global epoch is</span>
<span class="doccomment">//! incremented, i.e. we say it "advances". A pinned participant may advance the global epoch only</span>
<span class="doccomment">//! if all currently pinned participants have been pinned in the current epoch.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! If an object became garbage in some epoch, then we can be sure that after two advancements no</span>
<span class="doccomment">//! participant will hold a reference to it. That is the crux of safe memory reclamation.</span>
<span class="kw">use</span> <span class="kw">crate</span><span class="ident">::primitive::sync::atomic::AtomicUsize</span>;
<span class="kw">use</span> <span class="ident">core::sync::atomic::Ordering</span>;
<span class="doccomment">/// An epoch that can be marked as pinned or unpinned.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Internally, the epoch is represented as an integer that wraps around at some unspecified point</span>
<span class="doccomment">/// and a flag that represents whether it is pinned or unpinned.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>, <span class="ident">Clone</span>, <span class="ident">Default</span>, <span class="ident">Debug</span>, <span class="ident">Eq</span>, <span class="ident">PartialEq</span>)]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct</span> <span class="ident">Epoch</span> {
<span class="doccomment">/// The least significant bit is set if pinned. The rest of the bits hold the epoch.</span>
<span class="ident">data</span>: <span class="ident">usize</span>,
}
<span class="kw">impl</span> <span class="ident">Epoch</span> {
<span class="doccomment">/// Returns the starting epoch in unpinned state.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">starting</span>() <span class="op">-</span><span class="op">></span> <span class="self">Self</span> {
<span class="self">Self</span><span class="ident">::default</span>()
}
<span class="doccomment">/// Returns the number of epochs `self` is ahead of `rhs`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Internally, epochs are represented as numbers in the range `(isize::MIN / 2) .. (isize::MAX</span>
<span class="doccomment">/// / 2)`, so the returned distance will be in the same interval.</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">wrapping_sub</span>(<span class="self">self</span>, <span class="ident">rhs</span>: <span class="self">Self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">isize</span> {
<span class="comment">// The result is the same with `(self.data & !1).wrapping_sub(rhs.data & !1) as isize >> 1`,</span>
<span class="comment">// because the possible difference of LSB in `(self.data & !1).wrapping_sub(rhs.data & !1)`</span>
<span class="comment">// will be ignored in the shift operation.</span>
<span class="self">self</span>.<span class="ident">data</span>.<span class="ident">wrapping_sub</span>(<span class="ident">rhs</span>.<span class="ident">data</span> <span class="op">&</span> <span class="op">!</span><span class="number">1</span>) <span class="kw">as</span> <span class="ident">isize</span> <span class="op">></span><span class="op">></span> <span class="number">1</span>
}
<span class="doccomment">/// Returns `true` if the epoch is marked as pinned.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">is_pinned</span>(<span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> {
(<span class="self">self</span>.<span class="ident">data</span> <span class="op">&</span> <span class="number">1</span>) <span class="op">=</span><span class="op">=</span> <span class="number">1</span>
}
<span class="doccomment">/// Returns the same epoch, but marked as pinned.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">pinned</span>(<span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">Epoch</span> {
<span class="ident">Epoch</span> {
<span class="ident">data</span>: <span class="self">self</span>.<span class="ident">data</span> <span class="op">|</span> <span class="number">1</span>,
}
}
<span class="doccomment">/// Returns the same epoch, but marked as unpinned.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">unpinned</span>(<span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">Epoch</span> {
<span class="ident">Epoch</span> {
<span class="ident">data</span>: <span class="self">self</span>.<span class="ident">data</span> <span class="op">&</span> <span class="op">!</span><span class="number">1</span>,
}
}
<span class="doccomment">/// Returns the successor epoch.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The returned epoch will be marked as pinned only if the previous one was as well.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">successor</span>(<span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">Epoch</span> {
<span class="ident">Epoch</span> {
<span class="ident">data</span>: <span class="self">self</span>.<span class="ident">data</span>.<span class="ident">wrapping_add</span>(<span class="number">2</span>),
}
}
}
<span class="doccomment">/// An atomic value that holds an `Epoch`.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Default</span>, <span class="ident">Debug</span>)]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct</span> <span class="ident">AtomicEpoch</span> {
<span class="doccomment">/// Since `Epoch` is just a wrapper around `usize`, an `AtomicEpoch` is similarly represented</span>
<span class="doccomment">/// using an `AtomicUsize`.</span>
<span class="ident">data</span>: <span class="ident">AtomicUsize</span>,
}
<span class="kw">impl</span> <span class="ident">AtomicEpoch</span> {
<span class="doccomment">/// Creates a new atomic epoch.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">epoch</span>: <span class="ident">Epoch</span>) <span class="op">-</span><span class="op">></span> <span class="self">Self</span> {
<span class="kw">let</span> <span class="ident">data</span> <span class="op">=</span> <span class="ident">AtomicUsize::new</span>(<span class="ident">epoch</span>.<span class="ident">data</span>);
<span class="ident">AtomicEpoch</span> { <span class="ident">data</span> }
}
<span class="doccomment">/// Loads a value from the atomic epoch.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">load</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">ord</span>: <span class="ident">Ordering</span>) <span class="op">-</span><span class="op">></span> <span class="ident">Epoch</span> {
<span class="ident">Epoch</span> {
<span class="ident">data</span>: <span class="self">self</span>.<span class="ident">data</span>.<span class="ident">load</span>(<span class="ident">ord</span>),
}
}
<span class="doccomment">/// Stores a value into the atomic epoch.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">store</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">epoch</span>: <span class="ident">Epoch</span>, <span class="ident">ord</span>: <span class="ident">Ordering</span>) {
<span class="self">self</span>.<span class="ident">data</span>.<span class="ident">store</span>(<span class="ident">epoch</span>.<span class="ident">data</span>, <span class="ident">ord</span>);
}
<span class="doccomment">/// Stores a value into the atomic epoch if the current value is the same as `current`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The return value is a result indicating whether the new value was written and containing</span>
<span class="doccomment">/// the previous value. On success this value is guaranteed to be equal to `current`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This method takes two `Ordering` arguments to describe the memory</span>
<span class="doccomment">/// ordering of this operation. `success` describes the required ordering for the</span>
<span class="doccomment">/// read-modify-write operation that takes place if the comparison with `current` succeeds.</span>
<span class="doccomment">/// `failure` describes the required ordering for the load operation that takes place when</span>
<span class="doccomment">/// the comparison fails. Using `Acquire` as success ordering makes the store part</span>
<span class="doccomment">/// of this operation `Relaxed`, and using `Release` makes the successful load</span>
<span class="doccomment">/// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`</span>
<span class="doccomment">/// and must be equivalent to or weaker than the success ordering.</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">compare_exchange</span>(
<span class="kw-2">&</span><span class="self">self</span>,
<span class="ident">current</span>: <span class="ident">Epoch</span>,
<span class="ident">new</span>: <span class="ident">Epoch</span>,
<span class="ident">success</span>: <span class="ident">Ordering</span>,
<span class="ident">failure</span>: <span class="ident">Ordering</span>,
) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Epoch</span>, <span class="ident">Epoch</span><span class="op">></span> {
<span class="kw">match</span> <span class="self">self</span>
.<span class="ident">data</span>
.<span class="ident">compare_exchange</span>(<span class="ident">current</span>.<span class="ident">data</span>, <span class="ident">new</span>.<span class="ident">data</span>, <span class="ident">success</span>, <span class="ident">failure</span>)
{
<span class="prelude-val">Ok</span>(<span class="ident">data</span>) <span class="op">=</span><span class="op">></span> <span class="prelude-val">Ok</span>(<span class="ident">Epoch</span> { <span class="ident">data</span> }),
<span class="prelude-val">Err</span>(<span class="ident">data</span>) <span class="op">=</span><span class="op">></span> <span class="prelude-val">Err</span>(<span class="ident">Epoch</span> { <span class="ident">data</span> }),
}
}
}
</pre></div>
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