2011-08-09 21:07:10 +00:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM Atomic Instructions and Concurrency Guide</title>
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2011-08-09 23:02:53 +00:00
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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2011-08-09 21:07:10 +00:00
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<h1>
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LLVM Atomic Instructions and Concurrency Guide
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</h1>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#outsideatomic">Optimization outside atomic</a></li>
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<li><a href="#atomicinst">Atomic instructions</a></li>
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<li><a href="#ordering">Atomic orderings</a></li>
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<li><a href="#iropt">Atomics and IR optimization</a></li>
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<li><a href="#codegen">Atomics and Codegen</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by Eli Friedman</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="introduction">Introduction</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Historically, LLVM has not had very strong support for concurrency; some
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minimal intrinsics were provided, and <code>volatile</code> was used in some
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cases to achieve rough semantics in the presence of concurrency. However, this
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is changing; there are now new instructions which are well-defined in the
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presence of threads and asynchronous signals, and the model for existing
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instructions has been clarified in the IR.</p>
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<p>The atomic instructions are designed specifically to provide readable IR and
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optimized code generation for the following:</p>
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<ul>
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<li>The new C++0x <code><atomic></code> header.
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(<a href="http://www.open-std.org/jtc1/sc22/wg21/">C++0x draft available here</a>.)
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(<a href="http://www.open-std.org/jtc1/sc22/wg14/">C1x draft available here</a>)</li>
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<li>Proper semantics for Java-style memory, for both <code>volatile</code> and
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regular shared variables.
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(<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">Java Specification</a>)</li>
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<li>gcc-compatible <code>__sync_*</code> builtins.
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(<a href="http://gcc.gnu.org/onlinedocs/gcc/Atomic-Builtins.html">Description</a>)</li>
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<li>Other scenarios with atomic semantics, including <code>static</code>
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variables with non-trivial constructors in C++.</li>
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</ul>
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2011-08-11 23:44:25 +00:00
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<p>Atomic and volatile in the IR are orthogonal; "volatile" is the C/C++
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volatile, which ensures that every volatile load and store happens and is
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performed in the stated order. A couple examples: if a
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SequentiallyConsistent store is immediately followed by another
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SequentiallyConsistent store to the same address, the first store can
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be erased. This transformation is not allowed for a pair of volatile
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stores. On the other hand, a non-volatile non-atomic load can be moved
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across a volatile load freely, but not an Acquire load.</p>
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2011-08-09 21:07:10 +00:00
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<p>This document is intended to provide a guide to anyone either writing a
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frontend for LLVM or working on optimization passes for LLVM with a guide
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for how to deal with instructions with special semantics in the presence of
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concurrency. This is not intended to be a precise guide to the semantics;
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the details can get extremely complicated and unreadable, and are not
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usually necessary.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="outsideatomic">Optimization outside atomic</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>The basic <code>'load'</code> and <code>'store'</code> allow a variety of
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optimizations, but can lead to undefined results in a concurrent environment;
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see <a href="#o_nonatomic">NonAtomic</a>. This section specifically goes
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into the one optimizer restriction which applies in concurrent environments,
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which gets a bit more of an extended description because any optimization
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dealing with stores needs to be aware of it.</p>
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<p>From the optimizer's point of view, the rule is that if there
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are not any instructions with atomic ordering involved, concurrency does
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not matter, with one exception: if a variable might be visible to another
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thread or signal handler, a store cannot be inserted along a path where it
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might not execute otherwise. Take the following example:</p>
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<pre>
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/* C code, for readability; run through clang -O2 -S -emit-llvm to get
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equivalent IR */
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int x;
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void f(int* a) {
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for (int i = 0; i < 100; i++) {
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if (a[i])
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x += 1;
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}
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}
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</pre>
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<p>The following is equivalent in non-concurrent situations:</p>
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<pre>
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int x;
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void f(int* a) {
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int xtemp = x;
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for (int i = 0; i < 100; i++) {
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if (a[i])
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xtemp += 1;
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}
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x = xtemp;
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}
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</pre>
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<p>However, LLVM is not allowed to transform the former to the latter: it could
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indirectly introduce undefined behavior if another thread can access x at
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the same time. (This example is particularly of interest because before the
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concurrency model was implemented, LLVM would perform this
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transformation.)</p>
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<p>Note that speculative loads are allowed; a load which
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is part of a race returns <code>undef</code>, but does not have undefined
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behavior.</p>
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</div>
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2011-08-11 23:44:25 +00:00
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<!-- *********************************************************************** -->
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<h2>
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<a name="atomicinst">Atomic instructions</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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2011-08-12 21:50:54 +00:00
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<p>For cases where simple loads and stores are not sufficient, LLVM provides
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various atomic instructions. The exact guarantees provided depend on the
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ordering; see <a href="#ordering">Atomic orderings</a></p>
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<p><code>load atomic</code> and <code>store atomic</code> provide the same
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basic functionality as non-atomic loads and stores, but provide additional
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guarantees in situations where threads and signals are involved.</p>
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2011-08-11 23:44:25 +00:00
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<p><code>cmpxchg</code> and <code>atomicrmw</code> are essentially like an
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atomic load followed by an atomic store (where the store is conditional for
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<code>cmpxchg</code>), but no other memory operation can happen on any thread
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between the load and store. Note that LLVM's cmpxchg does not provide quite
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as many options as the C++0x version.</p>
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<p>A <code>fence</code> provides Acquire and/or Release ordering which is not
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part of another operation; it is normally used along with Monotonic memory
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operations. A Monotonic load followed by an Acquire fence is roughly
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equivalent to an Acquire load.</p>
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<p>Frontends generating atomic instructions generally need to be aware of the
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target to some degree; atomic instructions are guaranteed to be lock-free,
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and therefore an instruction which is wider than the target natively supports
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can be impossible to generate.</p>
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</div>
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2011-08-09 21:07:10 +00:00
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<!-- *********************************************************************** -->
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<h2>
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<a name="ordering">Atomic orderings</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>In order to achieve a balance between performance and necessary guarantees,
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there are six levels of atomicity. They are listed in order of strength;
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each level includes all the guarantees of the previous level except for
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Acquire/Release. (See also <a href="LangRef.html#ordering">LangRef</a>.)</p>
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2011-08-12 21:50:54 +00:00
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<!-- ======================================================================= -->
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<h3>
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<a name="o_notatomic">NotAtomic</a>
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</h3>
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<div>
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<p>NotAtomic is the obvious, a load or store which is not atomic. (This isn't
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really a level of atomicity, but is listed here for comparison.) This is
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essentially a regular load or store. If there is a race on a given memory
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location, loads from that location return undef.</p>
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<dl>
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<dt>Relevant standard</dt>
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<dd>This is intended to match shared variables in C/C++, and to be used
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in any other context where memory access is necessary, and
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a race is impossible. (The precise definition is in
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<a href="LangRef.html#memmodel">LangRef</a>.)
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<dt>Notes for frontends</dt>
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<dd>The rule is essentially that all memory accessed with basic loads and
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stores by multiple threads should be protected by a lock or other
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synchronization; otherwise, you are likely to run into undefined
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behavior. If your frontend is for a "safe" language like Java,
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use Unordered to load and store any shared variable. Note that NotAtomic
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volatile loads and stores are not properly atomic; do not try to use
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them as a substitute. (Per the C/C++ standards, volatile does provide
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some limited guarantees around asynchronous signals, but atomics are
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generally a better solution.)
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<dt>Notes for optimizers</dt>
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<dd>Introducing loads to shared variables along a codepath where they would
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not otherwise exist is allowed; introducing stores to shared variables
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is not. See <a href="#outsideatomic">Optimization outside
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atomic</a>.</dd>
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<dt>Notes for code generation</dt>
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<dd>The one interesting restriction here is that it is not allowed to write
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to bytes outside of the bytes relevant to a store. This is mostly
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relevant to unaligned stores: it is not allowed in general to convert
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an unaligned store into two aligned stores of the same width as the
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unaligned store. Backends are also expected to generate an i8 store
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as an i8 store, and not an instruction which writes to surrounding
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bytes. (If you are writing a backend for an architecture which cannot
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satisfy these restrictions and cares about concurrency, please send an
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email to llvmdev.)</dd>
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</dl>
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</div>
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2011-08-11 23:44:25 +00:00
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<!-- ======================================================================= -->
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<h3>
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<a name="o_unordered">Unordered</a>
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</h3>
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<div>
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2011-08-09 21:07:10 +00:00
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<p>Unordered is the lowest level of atomicity. It essentially guarantees that
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races produce somewhat sane results instead of having undefined behavior.
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It also guarantees the operation to be lock-free, so it do not depend on
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the data being part of a special atomic structure or depend on a separate
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per-process global lock. Note that code generation will fail for
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unsupported atomic operations; if you need such an operation, use explicit
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locking.</p>
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<dl>
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<dt>Relevant standard</dt>
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<dd>This is intended to match the Java memory model for shared
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variables.</dd>
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<dt>Notes for frontends</dt>
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<dd>This cannot be used for synchronization, but is useful for Java and
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other "safe" languages which need to guarantee that the generated
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code never exhibits undefined behavior. Note that this guarantee
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is cheap on common platforms for loads of a native width, but can
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be expensive or unavailable for wider loads, like a 64-bit store
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on ARM. (A frontend for Java or other "safe" languages would normally
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split a 64-bit store on ARM into two 32-bit unordered stores.)
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<dt>Notes for optimizers</dt>
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<dd>In terms of the optimizer, this prohibits any transformation that
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transforms a single load into multiple loads, transforms a store
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into multiple stores, narrows a store, or stores a value which
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would not be stored otherwise. Some examples of unsafe optimizations
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are narrowing an assignment into a bitfield, rematerializing
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a load, and turning loads and stores into a memcpy call. Reordering
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unordered operations is safe, though, and optimizers should take
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advantage of that because unordered operations are common in
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languages that need them.</dd>
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<dt>Notes for code generation</dt>
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<dd>These operations are required to be atomic in the sense that if you
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use unordered loads and unordered stores, a load cannot see a value
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which was never stored. A normal load or store instruction is usually
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sufficient, but note that an unordered load or store cannot
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be split into multiple instructions (or an instruction which
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does multiple memory operations, like <code>LDRD</code> on ARM).</dd>
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</dl>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="o_monotonic">Monotonic</a>
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</h3>
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<div>
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2011-08-09 21:07:10 +00:00
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<p>Monotonic is the weakest level of atomicity that can be used in
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synchronization primitives, although it does not provide any general
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synchronization. It essentially guarantees that if you take all the
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operations affecting a specific address, a consistent ordering exists.
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2011-08-11 23:44:25 +00:00
|
|
|
|
|
|
|
|
<dl>
|
|
|
|
|
<dt>Relevant standard</dt>
|
|
|
|
|
<dd>This corresponds to the C++0x/C1x <code>memory_order_relaxed</code>;
|
|
|
|
|
see those standards for the exact definition.
|
|
|
|
|
<dt>Notes for frontends</dt>
|
|
|
|
|
<dd>If you are writing a frontend which uses this directly, use with caution.
|
|
|
|
|
The guarantees in terms of synchronization are very weak, so make
|
|
|
|
|
sure these are only used in a pattern which you know is correct.
|
|
|
|
|
Generally, these would either be used for atomic operations which
|
|
|
|
|
do not protect other memory (like an atomic counter), or along with
|
|
|
|
|
a <code>fence</code>.</dd>
|
|
|
|
|
<dt>Notes for optimizers</dt>
|
|
|
|
|
<dd>In terms of the optimizer, this can be treated as a read+write on the
|
|
|
|
|
relevant memory location (and alias analysis will take advantage of
|
|
|
|
|
that). In addition, it is legal to reorder non-atomic and Unordered
|
|
|
|
|
loads around Monotonic loads. CSE/DSE and a few other optimizations
|
|
|
|
|
are allowed, but Monotonic operations are unlikely to be used in ways
|
|
|
|
|
which would make those optimizations useful.</dd>
|
|
|
|
|
<dt>Notes for code generation</dt>
|
|
|
|
|
<dd>Code generation is essentially the same as that for unordered for loads
|
2011-09-11 15:30:08 +00:00
|
|
|
and stores. No fences are required. <code>cmpxchg</code> and
|
2011-08-11 23:44:25 +00:00
|
|
|
<code>atomicrmw</code> are required to appear as a single operation.</dd>
|
|
|
|
|
</dl>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
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|
|
<h3>
|
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|
|
<a name="o_acquire">Acquire</a>
|
|
|
|
|
</h3>
|
|
|
|
|
|
|
|
|
|
<div>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
|
|
|
|
<p>Acquire provides a barrier of the sort necessary to acquire a lock to access
|
2011-08-11 23:44:25 +00:00
|
|
|
other memory with normal loads and stores.
|
|
|
|
|
|
|
|
|
|
<dl>
|
|
|
|
|
<dt>Relevant standard</dt>
|
|
|
|
|
<dd>This corresponds to the C++0x/C1x <code>memory_order_acquire</code>. It
|
|
|
|
|
should also be used for C++0x/C1x <code>memory_order_consume</code>.
|
|
|
|
|
<dt>Notes for frontends</dt>
|
|
|
|
|
<dd>If you are writing a frontend which uses this directly, use with caution.
|
|
|
|
|
Acquire only provides a semantic guarantee when paired with a Release
|
|
|
|
|
operation.</dd>
|
|
|
|
|
<dt>Notes for optimizers</dt>
|
2011-08-12 03:38:32 +00:00
|
|
|
<dd>Optimizers not aware of atomics can treat this like a nothrow call.
|
2011-08-12 19:48:19 +00:00
|
|
|
It is also possible to move stores from before an Acquire load
|
2011-08-12 03:38:32 +00:00
|
|
|
or read-modify-write operation to after it, and move non-Acquire
|
|
|
|
|
loads from before an Acquire operation to after it.</dd>
|
2011-08-11 23:44:25 +00:00
|
|
|
<dt>Notes for code generation</dt>
|
|
|
|
|
<dd>Architectures with weak memory ordering (essentially everything relevant
|
|
|
|
|
today except x86 and SPARC) require some sort of fence to maintain
|
|
|
|
|
the Acquire semantics. The precise fences required varies widely by
|
|
|
|
|
architecture, but for a simple implementation, most architectures provide
|
|
|
|
|
a barrier which is strong enough for everything (<code>dmb</code> on ARM,
|
|
|
|
|
<code>sync</code> on PowerPC, etc.). Putting such a fence after the
|
|
|
|
|
equivalent Monotonic operation is sufficient to maintain Acquire
|
|
|
|
|
semantics for a memory operation.</dd>
|
|
|
|
|
</dl>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
</div>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<h3>
|
|
|
|
|
<a name="o_acquire">Release</a>
|
|
|
|
|
</h3>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<div>
|
|
|
|
|
|
|
|
|
|
<p>Release is similar to Acquire, but with a barrier of the sort necessary to
|
|
|
|
|
release a lock.
|
|
|
|
|
|
|
|
|
|
<dl>
|
|
|
|
|
<dt>Relevant standard</dt>
|
|
|
|
|
<dd>This corresponds to the C++0x/C1x <code>memory_order_release</code>.</dd>
|
|
|
|
|
<dt>Notes for frontends</dt>
|
|
|
|
|
<dd>If you are writing a frontend which uses this directly, use with caution.
|
|
|
|
|
Release only provides a semantic guarantee when paired with a Acquire
|
|
|
|
|
operation.</dd>
|
|
|
|
|
<dt>Notes for optimizers</dt>
|
2011-08-12 03:38:32 +00:00
|
|
|
<dd>Optimizers not aware of atomics can treat this like a nothrow call.
|
|
|
|
|
It is also possible to move loads from after a Release store
|
|
|
|
|
or read-modify-write operation to before it, and move non-Release
|
|
|
|
|
stores from after an Release operation to before it.</dd>
|
2011-08-11 23:44:25 +00:00
|
|
|
<dt>Notes for code generation</dt>
|
2011-08-12 01:26:06 +00:00
|
|
|
<dd>See the section on Acquire; a fence before the relevant operation is
|
2011-08-12 03:38:32 +00:00
|
|
|
usually sufficient for Release. Note that a store-store fence is not
|
2011-08-12 01:26:06 +00:00
|
|
|
sufficient to implement Release semantics; store-store fences are
|
|
|
|
|
generally not exposed to IR because they are extremely difficult to
|
2011-08-11 23:44:25 +00:00
|
|
|
use correctly.</dd>
|
|
|
|
|
</dl>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<h3>
|
|
|
|
|
<a name="o_acqrel">AcquireRelease</a>
|
|
|
|
|
</h3>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
|
|
|
|
<div>
|
|
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<p>AcquireRelease (<code>acq_rel</code> in IR) provides both an Acquire and a
|
|
|
|
|
Release barrier (for fences and operations which both read and write memory).
|
|
|
|
|
|
|
|
|
|
<dl>
|
|
|
|
|
<dt>Relevant standard</dt>
|
|
|
|
|
<dd>This corresponds to the C++0x/C1x <code>memory_order_acq_rel</code>.
|
|
|
|
|
<dt>Notes for frontends</dt>
|
|
|
|
|
<dd>If you are writing a frontend which uses this directly, use with caution.
|
|
|
|
|
Acquire only provides a semantic guarantee when paired with a Release
|
|
|
|
|
operation, and vice versa.</dd>
|
|
|
|
|
<dt>Notes for optimizers</dt>
|
|
|
|
|
<dd>In general, optimizers should treat this like a nothrow call; the
|
|
|
|
|
the possible optimizations are usually not interesting.</dd>
|
|
|
|
|
<dt>Notes for code generation</dt>
|
|
|
|
|
<dd>This operation has Acquire and Release semantics; see the sections on
|
2011-08-11 23:48:52 +00:00
|
|
|
Acquire and Release.</dd>
|
2011-08-11 23:44:25 +00:00
|
|
|
</dl>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
</div>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<!-- ======================================================================= -->
|
|
|
|
|
<h3>
|
|
|
|
|
<a name="o_seqcst">SequentiallyConsistent</a>
|
|
|
|
|
</h3>
|
|
|
|
|
|
|
|
|
|
<div>
|
|
|
|
|
|
2011-08-12 00:36:38 +00:00
|
|
|
<p>SequentiallyConsistent (<code>seq_cst</code> in IR) provides
|
|
|
|
|
Acquire semantics for loads and Release semantics for
|
|
|
|
|
stores. Additionally, it guarantees that a total ordering exists
|
|
|
|
|
between all SequentiallyConsistent operations.
|
2011-08-11 23:44:25 +00:00
|
|
|
|
|
|
|
|
<dl>
|
|
|
|
|
<dt>Relevant standard</dt>
|
|
|
|
|
<dd>This corresponds to the C++0x/C1x <code>memory_order_seq_cst</code>,
|
|
|
|
|
Java volatile, and the gcc-compatible <code>__sync_*</code> builtins
|
|
|
|
|
which do not specify otherwise.
|
|
|
|
|
<dt>Notes for frontends</dt>
|
|
|
|
|
<dd>If a frontend is exposing atomic operations, these are much easier to
|
|
|
|
|
reason about for the programmer than other kinds of operations, and using
|
|
|
|
|
them is generally a practical performance tradeoff.</dd>
|
|
|
|
|
<dt>Notes for optimizers</dt>
|
2011-08-12 03:38:32 +00:00
|
|
|
<dd>Optimizers not aware of atomics can treat this like a nothrow call.
|
|
|
|
|
For SequentiallyConsistent loads and stores, the same reorderings are
|
|
|
|
|
allowed as for Acquire loads and Release stores, except that
|
|
|
|
|
SequentiallyConsistent operations may not be reordered.</dd>
|
2011-08-11 23:44:25 +00:00
|
|
|
<dt>Notes for code generation</dt>
|
2011-08-12 00:36:38 +00:00
|
|
|
<dd>SequentiallyConsistent loads minimally require the same barriers
|
2011-09-11 15:50:05 +00:00
|
|
|
as Acquire operations and SequentiallyConsistent stores require
|
2011-08-12 03:38:32 +00:00
|
|
|
Release barriers. Additionally, the code generator must enforce
|
2011-09-11 15:50:05 +00:00
|
|
|
ordering between SequentiallyConsistent stores followed by
|
|
|
|
|
SequentiallyConsistent loads. This is usually done by emitting
|
2011-08-12 03:38:32 +00:00
|
|
|
either a full fence before the loads or a full fence after the
|
|
|
|
|
stores; which is preferred varies by architecture.</dd>
|
2011-08-11 23:44:25 +00:00
|
|
|
</dl>
|
|
|
|
|
|
|
|
|
|
</div>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<h2>
|
|
|
|
|
<a name="iropt">Atomics and IR optimization</a>
|
|
|
|
|
</h2>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div>
|
|
|
|
|
|
|
|
|
|
<p>Predicates for optimizer writers to query:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>isSimple(): A load or store which is not volatile or atomic. This is
|
|
|
|
|
what, for example, memcpyopt would check for operations it might
|
2011-08-12 21:50:54 +00:00
|
|
|
transform.</li>
|
2011-08-09 21:07:10 +00:00
|
|
|
<li>isUnordered(): A load or store which is not volatile and at most
|
|
|
|
|
Unordered. This would be checked, for example, by LICM before hoisting
|
2011-08-12 21:50:54 +00:00
|
|
|
an operation.</li>
|
2011-08-09 21:07:10 +00:00
|
|
|
<li>mayReadFromMemory()/mayWriteToMemory(): Existing predicate, but note
|
2011-08-10 20:17:43 +00:00
|
|
|
that they return true for any operation which is volatile or at least
|
2011-08-12 21:50:54 +00:00
|
|
|
Monotonic.</li>
|
2011-08-09 21:07:10 +00:00
|
|
|
<li>Alias analysis: Note that AA will return ModRef for anything Acquire or
|
2011-08-12 21:50:54 +00:00
|
|
|
Release, and for the address accessed by any Monotonic operation.</li>
|
2011-08-09 21:07:10 +00:00
|
|
|
</ul>
|
|
|
|
|
|
2011-08-12 21:50:54 +00:00
|
|
|
<p>To support optimizing around atomic operations, make sure you are using
|
|
|
|
|
the right predicates; everything should work if that is done. If your
|
|
|
|
|
pass should optimize some atomic operations (Unordered operations in
|
|
|
|
|
particular), make sure it doesn't replace an atomic load or store with
|
|
|
|
|
a non-atomic operation.</p>
|
2011-08-09 21:07:10 +00:00
|
|
|
|
|
|
|
|
<p>Some examples of how optimizations interact with various kinds of atomic
|
|
|
|
|
operations:
|
|
|
|
|
<ul>
|
|
|
|
|
<li>memcpyopt: An atomic operation cannot be optimized into part of a
|
|
|
|
|
memcpy/memset, including unordered loads/stores. It can pull operations
|
|
|
|
|
across some atomic operations.
|
|
|
|
|
<li>LICM: Unordered loads/stores can be moved out of a loop. It just treats
|
|
|
|
|
monotonic operations like a read+write to a memory location, and anything
|
|
|
|
|
stricter than that like a nothrow call.
|
|
|
|
|
<li>DSE: Unordered stores can be DSE'ed like normal stores. Monotonic stores
|
|
|
|
|
can be DSE'ed in some cases, but it's tricky to reason about, and not
|
|
|
|
|
especially important.
|
|
|
|
|
<li>Folding a load: Any atomic load from a constant global can be
|
|
|
|
|
constant-folded, because it cannot be observed. Similar reasoning allows
|
|
|
|
|
scalarrepl with atomic loads and stores.
|
|
|
|
|
</ul>
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<h2>
|
|
|
|
|
<a name="codegen">Atomics and Codegen</a>
|
|
|
|
|
</h2>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div>
|
|
|
|
|
|
|
|
|
|
<p>Atomic operations are represented in the SelectionDAG with
|
|
|
|
|
<code>ATOMIC_*</code> opcodes. On architectures which use barrier
|
|
|
|
|
instructions for all atomic ordering (like ARM), appropriate fences are
|
|
|
|
|
split out as the DAG is built.</p>
|
|
|
|
|
|
|
|
|
|
<p>The MachineMemOperand for all atomic operations is currently marked as
|
|
|
|
|
volatile; this is not correct in the IR sense of volatile, but CodeGen
|
|
|
|
|
handles anything marked volatile very conservatively. This should get
|
|
|
|
|
fixed at some point.</p>
|
|
|
|
|
|
2011-08-11 23:44:25 +00:00
|
|
|
<p>Common architectures have some way of representing at least a pointer-sized
|
|
|
|
|
lock-free <code>cmpxchg</code>; such an operation can be used to implement
|
|
|
|
|
all the other atomic operations which can be represented in IR up to that
|
|
|
|
|
size. Backends are expected to implement all those operations, but not
|
|
|
|
|
operations which cannot be implemented in a lock-free manner. It is
|
|
|
|
|
expected that backends will give an error when given an operation which
|
|
|
|
|
cannot be implemented. (The LLVM code generator is not very helpful here
|
|
|
|
|
at the moment, but hopefully that will change.)</p>
|
|
|
|
|
|
2011-08-09 21:07:10 +00:00
|
|
|
<p>The implementation of atomics on LL/SC architectures (like ARM) is currently
|
|
|
|
|
a bit of a mess; there is a lot of copy-pasted code across targets, and
|
|
|
|
|
the representation is relatively unsuited to optimization (it would be nice
|
|
|
|
|
to be able to optimize loops involving cmpxchg etc.).</p>
|
|
|
|
|
|
|
|
|
|
<p>On x86, all atomic loads generate a <code>MOV</code>.
|
|
|
|
|
SequentiallyConsistent stores generate an <code>XCHG</code>, other stores
|
|
|
|
|
generate a <code>MOV</code>. SequentiallyConsistent fences generate an
|
|
|
|
|
<code>MFENCE</code>, other fences do not cause any code to be generated.
|
|
|
|
|
cmpxchg uses the <code>LOCK CMPXCHG</code> instruction.
|
|
|
|
|
<code>atomicrmw xchg</code> uses <code>XCHG</code>,
|
|
|
|
|
<code>atomicrmw add</code> and <code>atomicrmw sub</code> use
|
|
|
|
|
<code>XADD</code>, and all other <code>atomicrmw</code> operations generate
|
|
|
|
|
a loop with <code>LOCK CMPXCHG</code>. Depending on the users of the
|
|
|
|
|
result, some <code>atomicrmw</code> operations can be translated into
|
|
|
|
|
operations like <code>LOCK AND</code>, but that does not work in
|
|
|
|
|
general.</p>
|
|
|
|
|
|
|
|
|
|
<p>On ARM, MIPS, and many other RISC architectures, Acquire, Release, and
|
|
|
|
|
SequentiallyConsistent semantics require barrier instructions
|
|
|
|
|
for every such operation. Loads and stores generate normal instructions.
|
2011-08-11 23:44:25 +00:00
|
|
|
<code>cmpxchg</code> and <code>atomicrmw</code> can be represented using
|
|
|
|
|
a loop with LL/SC-style instructions which take some sort of exclusive
|
|
|
|
|
lock on a cache line (<code>LDREX</code> and <code>STREX</code> on
|
|
|
|
|
ARM, etc.). At the moment, the IR does not provide any way to represent a
|
|
|
|
|
weak <code>cmpxchg</code> which would not require a loop.</p>
|
2011-08-09 21:07:10 +00:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<hr>
|
|
|
|
|
<address>
|
|
|
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
|
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
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|
<a href="http://validator.w3.org/check/referer"><img
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date: 2011-08-09 02:07:00 -0700 (Tue, 09 Aug 2011) $
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