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<dl class="meta">
<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/181481">181481</a></dd>
<dt>Author</dt> <dd>mark.lam@apple.com</dd>
<dt>Date</dt> <dd>2015-03-13 11:02:40 -0700 (Fri, 13 Mar 2015)</dd>
</dl>
<h3>Log Message</h3>
<pre>Introduce WTF::Atomic to wrap std::atomic for a friendlier CAS.
<https://webkit.org/b/142661>
Reviewed by Filip Pizlo.
Source/JavaScriptCore:
Changed CodeBlock, and the DFG's crashLock to use WTF::Atomic instead of
std::atomic.
* bytecode/CodeBlock.cpp:
(JSC::CodeBlock::CodeBlock):
(JSC::CodeBlock::visitAggregate):
* bytecode/CodeBlock.h:
* dfg/DFGCommon.cpp:
(JSC::DFG::startCrashing):
Source/WTF:
The CAS functions provided by std::atomic takes a reference to the expected
value and modifies it if the CAS fails. However, in a lot of our CAS usage,
we don't want the expected value to change. The solution to this is to
provide a WTF::Atomic struct that wraps std::atomic, and provide CAS
methods that won't alter the expected value if the CAS fails.
The method names in WTF::Atomic are chosen to be identical to those
in std::atomic so that WTF::Atomic can be a simple drop in replacement
for std::atomic.
Also changed ByteSpinLock to use WTF::Atomic instead of std::atomic.
* wtf/Atomics.h:
(WTF::Atomic::load):
(WTF::Atomic::store):
(WTF::Atomic::compare_exchange_weak):
(WTF::Atomic::compare_exchange_strong):
* wtf/ByteSpinLock.h:
(WTF::ByteSpinLock::ByteSpinLock):
(WTF::ByteSpinLock::lock):</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCorebytecodeCodeBlockcpp">trunk/Source/JavaScriptCore/bytecode/CodeBlock.cpp</a></li>
<li><a href="#trunkSourceJavaScriptCorebytecodeCodeBlockh">trunk/Source/JavaScriptCore/bytecode/CodeBlock.h</a></li>
<li><a href="#trunkSourceJavaScriptCoredfgDFGCommoncpp">trunk/Source/JavaScriptCore/dfg/DFGCommon.cpp</a></li>
<li><a href="#trunkSourceWTFChangeLog">trunk/Source/WTF/ChangeLog</a></li>
<li><a href="#trunkSourceWTFwtfAtomicsh">trunk/Source/WTF/wtf/Atomics.h</a></li>
<li><a href="#trunkSourceWTFwtfByteSpinLockh">trunk/Source/WTF/wtf/ByteSpinLock.h</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/JavaScriptCore/ChangeLog 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1,3 +1,20 @@
</span><ins>+2015-03-13 Mark Lam <mark.lam@apple.com>
+
+ Introduce WTF::Atomic to wrap std::atomic for a friendlier CAS.
+ <https://webkit.org/b/142661>
+
+ Reviewed by Filip Pizlo.
+
+ Changed CodeBlock, and the DFG's crashLock to use WTF::Atomic instead of
+ std::atomic.
+
+ * bytecode/CodeBlock.cpp:
+ (JSC::CodeBlock::CodeBlock):
+ (JSC::CodeBlock::visitAggregate):
+ * bytecode/CodeBlock.h:
+ * dfg/DFGCommon.cpp:
+ (JSC::DFG::startCrashing):
+
</ins><span class="cx"> 2015-03-12 Mark Lam <mark.lam@apple.com>
</span><span class="cx">
</span><span class="cx"> Change the DFG crashLock to use std::atomic.
</span></span></pre></div>
<a id="trunkSourceJavaScriptCorebytecodeCodeBlockcpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/bytecode/CodeBlock.cpp (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/bytecode/CodeBlock.cpp 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/JavaScriptCore/bytecode/CodeBlock.cpp 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1645,7 +1645,6 @@
</span><span class="cx"> , m_isStrictMode(other.m_isStrictMode)
</span><span class="cx"> , m_needsActivation(other.m_needsActivation)
</span><span class="cx"> , m_mayBeExecuting(false)
</span><del>- , m_visitAggregateHasBeenCalled(false)
</del><span class="cx"> , m_source(other.m_source)
</span><span class="cx"> , m_sourceOffset(other.m_sourceOffset)
</span><span class="cx"> , m_firstLineColumnOffset(other.m_firstLineColumnOffset)
</span><span class="lines">@@ -1662,6 +1661,8 @@
</span><span class="cx"> , m_capabilityLevelState(DFG::CapabilityLevelNotSet)
</span><span class="cx"> #endif
</span><span class="cx"> {
</span><ins>+ m_visitAggregateHasBeenCalled.store(false, std::memory_order_relaxed);
+
</ins><span class="cx"> ASSERT(m_heap->isDeferred());
</span><span class="cx"> ASSERT(m_scopeRegister.isLocal());
</span><span class="cx">
</span><span class="lines">@@ -1707,7 +1708,6 @@
</span><span class="cx"> , m_isStrictMode(unlinkedCodeBlock->isStrictMode())
</span><span class="cx"> , m_needsActivation(unlinkedCodeBlock->hasActivationRegister() && unlinkedCodeBlock->codeType() == FunctionCode)
</span><span class="cx"> , m_mayBeExecuting(false)
</span><del>- , m_visitAggregateHasBeenCalled(false)
</del><span class="cx"> , m_source(sourceProvider)
</span><span class="cx"> , m_sourceOffset(sourceOffset)
</span><span class="cx"> , m_firstLineColumnOffset(firstLineColumnOffset)
</span><span class="lines">@@ -1719,6 +1719,8 @@
</span><span class="cx"> , m_capabilityLevelState(DFG::CapabilityLevelNotSet)
</span><span class="cx"> #endif
</span><span class="cx"> {
</span><ins>+ m_visitAggregateHasBeenCalled.store(false, std::memory_order_relaxed);
+
</ins><span class="cx"> ASSERT(m_heap->isDeferred());
</span><span class="cx"> ASSERT(m_scopeRegister.isLocal());
</span><span class="cx">
</span><span class="lines">@@ -2202,8 +2204,7 @@
</span><span class="cx"> // I may be asked to scan myself more than once, and it may even happen concurrently.
</span><span class="cx"> // To this end, use an atomic operation to check (and set) if I've been called already.
</span><span class="cx"> // Only one thread may proceed past this point - whichever one wins the atomic set race.
</span><del>- bool expected = false;
- bool setByMe = m_visitAggregateHasBeenCalled.compare_exchange_strong(expected, true, std::memory_order_acquire);
</del><ins>+ bool setByMe = m_visitAggregateHasBeenCalled.compare_exchange_strong(false, true, std::memory_order_acquire);
</ins><span class="cx"> if (!setByMe)
</span><span class="cx"> return;
</span><span class="cx"> #endif // ENABLE(PARALLEL_GC)
</span></span></pre></div>
<a id="trunkSourceJavaScriptCorebytecodeCodeBlockh"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/bytecode/CodeBlock.h (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/bytecode/CodeBlock.h 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/JavaScriptCore/bytecode/CodeBlock.h 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1065,7 +1065,7 @@
</span><span class="cx"> bool m_isStrictMode;
</span><span class="cx"> bool m_needsActivation;
</span><span class="cx"> bool m_mayBeExecuting;
</span><del>- std::atomic<bool> m_visitAggregateHasBeenCalled;
</del><ins>+ Atomic<bool> m_visitAggregateHasBeenCalled;
</ins><span class="cx">
</span><span class="cx"> RefPtr<SourceProvider> m_source;
</span><span class="cx"> unsigned m_sourceOffset;
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoredfgDFGCommoncpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/dfg/DFGCommon.cpp (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/dfg/DFGCommon.cpp 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/JavaScriptCore/dfg/DFGCommon.cpp 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -34,15 +34,12 @@
</span><span class="cx">
</span><span class="cx"> namespace JSC { namespace DFG {
</span><span class="cx">
</span><del>-static std::atomic<unsigned> crashLock;
</del><ins>+static Atomic<unsigned> crashLock;
</ins><span class="cx">
</span><span class="cx"> void startCrashing()
</span><span class="cx"> {
</span><del>- unsigned expected = 0;
- while (!crashLock.compare_exchange_weak(expected, 1, std::memory_order_acquire)) {
</del><ins>+ while (!crashLock.compare_exchange_weak(0, 1, std::memory_order_acquire))
</ins><span class="cx"> std::this_thread::yield();
</span><del>- expected = 0;
- }
</del><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> bool isCrashing()
</span></span></pre></div>
<a id="trunkSourceWTFChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/ChangeLog (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/ChangeLog 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/WTF/ChangeLog 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1,3 +1,31 @@
</span><ins>+2015-03-13 Mark Lam <mark.lam@apple.com>
+
+ Introduce WTF::Atomic to wrap std::atomic for a friendlier CAS.
+ <https://webkit.org/b/142661>
+
+ Reviewed by Filip Pizlo.
+
+ The CAS functions provided by std::atomic takes a reference to the expected
+ value and modifies it if the CAS fails. However, in a lot of our CAS usage,
+ we don't want the expected value to change. The solution to this is to
+ provide a WTF::Atomic struct that wraps std::atomic, and provide CAS
+ methods that won't alter the expected value if the CAS fails.
+
+ The method names in WTF::Atomic are chosen to be identical to those
+ in std::atomic so that WTF::Atomic can be a simple drop in replacement
+ for std::atomic.
+
+ Also changed ByteSpinLock to use WTF::Atomic instead of std::atomic.
+
+ * wtf/Atomics.h:
+ (WTF::Atomic::load):
+ (WTF::Atomic::store):
+ (WTF::Atomic::compare_exchange_weak):
+ (WTF::Atomic::compare_exchange_strong):
+ * wtf/ByteSpinLock.h:
+ (WTF::ByteSpinLock::ByteSpinLock):
+ (WTF::ByteSpinLock::lock):
+
</ins><span class="cx"> 2015-03-12 Filip Pizlo <fpizlo@apple.com>
</span><span class="cx">
</span><span class="cx"> Bytecode liveness analysis should have more lambdas and fewer sets
</span></span></pre></div>
<a id="trunkSourceWTFwtfAtomicsh"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/wtf/Atomics.h (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/wtf/Atomics.h 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/WTF/wtf/Atomics.h 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1,5 +1,5 @@
</span><span class="cx"> /*
</span><del>- * Copyright (C) 2007, 2008, 2010, 2012, 2013 Apple Inc. All rights reserved.
</del><ins>+ * Copyright (C) 2007-2008, 2010, 2012-2013, 2015 Apple Inc. All rights reserved.
</ins><span class="cx"> * Copyright (C) 2007 Justin Haygood (jhaygood@reaktix.com)
</span><span class="cx"> *
</span><span class="cx"> * Redistribution and use in source and binary forms, with or without
</span><span class="lines">@@ -59,6 +59,7 @@
</span><span class="cx"> #ifndef Atomics_h
</span><span class="cx"> #define Atomics_h
</span><span class="cx">
</span><ins>+#include <atomic>
</ins><span class="cx"> #include <wtf/StdLibExtras.h>
</span><span class="cx">
</span><span class="cx"> #if OS(WINDOWS)
</span><span class="lines">@@ -71,6 +72,35 @@
</span><span class="cx">
</span><span class="cx"> namespace WTF {
</span><span class="cx">
</span><ins>+// Atomic wraps around std::atomic with the sole purpose of making the compare_exchange
+// operations not alter the expected value. This is more in line with how we typically
+// use CAS in our code.
+//
+// Atomic is a struct without explicitly defined constructors so that it can be
+// initialized at compile time.
+
+template<typename T>
+struct Atomic {
+
+ T load(std::memory_order order) const { return value.load(order); }
+
+ void store(T desired, std::memory_order order) { value.store(desired, order); }
+
+ bool compare_exchange_weak(T expected, T desired, std::memory_order order)
+ {
+ T expectedOrActual = expected;
+ return value.compare_exchange_weak(expectedOrActual, desired, order);
+ }
+
+ bool compare_exchange_strong(T expected, T desired, std::memory_order order)
+ {
+ T expectedOrActual = expected;
+ return value.compare_exchange_strong(expectedOrActual, desired, order);
+ }
+
+ std::atomic<T> value;
+};
+
</ins><span class="cx"> #if OS(WINDOWS)
</span><span class="cx"> inline bool weakCompareAndSwap(volatile unsigned* location, unsigned expected, unsigned newValue)
</span><span class="cx"> {
</span><span class="lines">@@ -345,4 +375,6 @@
</span><span class="cx">
</span><span class="cx"> } // namespace WTF
</span><span class="cx">
</span><ins>+using WTF::Atomic;
+
</ins><span class="cx"> #endif // Atomics_h
</span></span></pre></div>
<a id="trunkSourceWTFwtfByteSpinLockh"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/wtf/ByteSpinLock.h (181480 => 181481)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/wtf/ByteSpinLock.h 2015-03-13 17:58:50 UTC (rev 181480)
+++ trunk/Source/WTF/wtf/ByteSpinLock.h 2015-03-13 18:02:40 UTC (rev 181481)
</span><span class="lines">@@ -1,5 +1,5 @@
</span><span class="cx"> /*
</span><del>- * Copyright (C) 2013 Apple Inc. All rights reserved.
</del><ins>+ * Copyright (C) 2013, 2015 Apple Inc. All rights reserved.
</ins><span class="cx"> *
</span><span class="cx"> * Redistribution and use in source and binary forms, with or without
</span><span class="cx"> * modification, are permitted provided that the following conditions
</span><span class="lines">@@ -26,9 +26,9 @@
</span><span class="cx"> #ifndef ByteSpinLock_h
</span><span class="cx"> #define ByteSpinLock_h
</span><span class="cx">
</span><del>-#include <atomic>
</del><span class="cx"> #include <thread>
</span><span class="cx"> #include <wtf/Assertions.h>
</span><ins>+#include <wtf/Atomics.h>
</ins><span class="cx"> #include <wtf/Locker.h>
</span><span class="cx"> #include <wtf/Noncopyable.h>
</span><span class="cx">
</span><span class="lines">@@ -38,17 +38,14 @@
</span><span class="cx"> WTF_MAKE_NONCOPYABLE(ByteSpinLock);
</span><span class="cx"> public:
</span><span class="cx"> ByteSpinLock()
</span><del>- : m_lock(false)
</del><span class="cx"> {
</span><ins>+ m_lock.store(false, std::memory_order_relaxed);
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> void lock()
</span><span class="cx"> {
</span><del>- bool expected = false;
- while (!m_lock.compare_exchange_weak(expected, true, std::memory_order_acquire)) {
</del><ins>+ while (!m_lock.compare_exchange_weak(false, true, std::memory_order_acquire))
</ins><span class="cx"> std::this_thread::yield();
</span><del>- expected = false;
- }
</del><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> void unlock()
</span><span class="lines">@@ -59,7 +56,7 @@
</span><span class="cx"> bool isHeld() const { return m_lock.load(std::memory_order_acquire); }
</span><span class="cx">
</span><span class="cx"> private:
</span><del>- std::atomic<bool> m_lock;
</del><ins>+ Atomic<bool> m_lock;
</ins><span class="cx"> };
</span><span class="cx">
</span><span class="cx"> typedef Locker<ByteSpinLock> ByteSpinLocker;
</span></span></pre>
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