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Implement #[cpu_local] attribute and API for primitive types (#1012)
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* The new `cls` crate offers a `#[cpu_local]` attribute that can be applied
  to a static variable to declare it as Cpu-Local Storage (CLS),
  much like `#[thread_local]` and Thread-Local Storage (TLS).
  * The `cls_macro` crate provides a procedural macro that implements
    the `#[cpu_local]` attribute. Currently, this only supports basic
    primitive types: `u8`, `u16`, `u32`, `u64`. More coming later.
  * The API for accessing CPU-local primitive types is quite simple:
    only `load`, `fetch_add`, `fetch_sub`. Note that these functions are
    implemented as atomic with respect to ONLY the single current CPU,
    not atomic with respect to all memory accesses across all other CPUs.
    This is intentional as it is more efficient, and CPU-local variables are
    only accessible to a single CPU and not by another "foreign" CPU.
  * Currently this is only used for the preemption counter on each CPU,
    but it will be used for more variables in future PRs.

* This simplifies the implementation of `preemption` management,
  as we can rely on the `#[cpu_local]` API's semantics to be atomic
  with respect to the current CPU.
  * This also avoids a minor latent bug that existed in the old preemption
    implementation which made it theoretically possible for a task migration
    to occur between accessing the CPU-local preemption count and
    modifying that count. This wasn't a problem since Theseus does not yet
    perform task migration, but the new design makes this interleaving
    impossible and is thus future-proof for later task migration support.

* Re-separate `preemption` and `cpu_local` into two different crates,
  as they previously were. This is possible because the preemption count
  now uses the new `#[cpu_local]` attribute, which allows `preemption`
  and `cpu_local` to not have cyclic dependencies on each other.
  * This is responsible for most of the changes in this PR: changing
    `cpu_local_preemption` back to `cpu_local` or `preemption`.

* Another current limitation is that we must use the `#[cpu_local]` attribute
  to _manually_ specify the offset of each CPU-local variable from the start
  of the `PerCpuData` storage type. This will be removed in a future PR.

Signed-off-by: Klimenty Tsoutsman <[email protected]>
Co-authored-by: Kevin Boos <[email protected]> 1f620ac
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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="A macro for declaring CPU-local variables."><title>cpu_local in cls - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Bold-a2c9cd1067f8b328.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-f40c346f39d9abc1.css" id="mainThemeStyle"><div id="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="cls" data-themes="" data-resource-suffix="" data-rustdoc-version="1.72.0-nightly (065a1f5df 2023-06-21)" data-search-js="search-95c92dd01058facf.js" data-settings-js="settings-de11bff964e9d4e5.js" data-settings-css="settings-8c76f75bfb6bd192.css" data-theme-light-css="light-0f8c037637f9eb3e.css" data-theme-dark-css="dark-1097f8e92a01e3cf.css" data-theme-ayu-css="ayu-614652228113ac93.css" ></div><script src="../static.files/storage-62ce34ea385b278a.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-190c35055d2a8300.js"></script><noscript><link rel="stylesheet" media="(prefers-color-scheme:light)" href="../static.files/light-0f8c037637f9eb3e.css"><link rel="stylesheet" media="(prefers-color-scheme:dark)" href="../static.files/dark-1097f8e92a01e3cf.css"><link rel="stylesheet" href="../static.files/noscript-13285aec31fa243e.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc attr"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">&#9776;</button><a class="logo-container" href="../cls/index.html"><img class="rust-logo" src="../static.files/rust-logo-151179464ae7ed46.svg" alt="logo"></a><h2></h2></nav><nav class="sidebar"><a class="logo-container" href="../cls/index.html"><img class="rust-logo" src="../static.files/rust-logo-151179464ae7ed46.svg" alt="logo"></a><div class="sidebar-elems"><h2><a href="index.html">In cls</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Attribute Macro <a href="index.html">cls</a>::<wbr><a class="attr" href="#">cpu_local</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/cls_macros/lib.rs.html#51">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><pre class="rust item-decl"><code>#[cpu_local]</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A macro for declaring CPU-local variables.</p>
<p>Variables must be an unsigned integer, bar <code>u128</code>.</p>
<p>The initialisation expression has no effect; to set the initial value,
<code>per_cpu::PerCpuData::new</code> must be modified.</p>
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