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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/202790">202790</a></dd>
<dt>Author</dt> <dd>fpizlo@apple.com</dd>
<dt>Date</dt> <dd>2016-07-03 10:46:39 -0700 (Sun, 03 Jul 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>DFG LICM needs to go all-in on the idea that some loops can't be LICMed
https://bugs.webkit.org/show_bug.cgi?id=159388
Reviewed by Mark Lam.
Some time ago I acknowledged that LICM required loops to meet certain requirements that
may get broken by the time we do LICM, like that the terminal of the pre-header is ExitOK.
It used to be that we just ignored that requirement and would hoist anyway, but since
<a href="http://trac.webkit.org/projects/webkit/changeset/189126">r189126</a> we've stopped hoisting out of loops that don't have ExitOK. We also added tests
for the case that the pre-header doesn't exist or is invalid.
It turns out that this patch didn't go far enough: even though it made LICM avoid loops
that had an invalid pre-header, the part that updated the AI state in nested loops still
assumed that these loops had valid pre-headers. We would crash in null dereference in
that loop if a nested loop had an invalid pre-header.
The fix is simple: don't update the AI state of nested loops that don't have pre-headers,
since we won't try to hoist out of those loops anyway.
* dfg/DFGLICMPhase.cpp:
(JSC::DFG::LICMPhase::attemptHoist):
* tests/stress/licm-no-pre-header-nested.js: Added. This would always crash before this fix.
(foo):
* tests/stress/licm-pre-header-cannot-exit-nested.js: Added. This was a failed attempt at a test, but I figure it's good to have weird code anyway.
(foo):
(valueOf):</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoredfgDFGLICMPhasecpp">trunk/Source/JavaScriptCore/dfg/DFGLICMPhase.cpp</a></li>
<li><a href="#trunkSourceWTFbenchmarksLockFairnessTestcpp">trunk/Source/WTF/benchmarks/LockFairnessTest.cpp</a></li>
</ul>
<h3>Added Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoretestsstresslicmnopreheadernestedjs">trunk/Source/JavaScriptCore/tests/stress/licm-no-pre-header-nested.js</a></li>
<li><a href="#trunkSourceJavaScriptCoretestsstresslicmpreheadercannotexitnestedjs">trunk/Source/JavaScriptCore/tests/stress/licm-pre-header-cannot-exit-nested.js</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (202789 => 202790)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2016-07-03 17:08:20 UTC (rev 202789)
+++ trunk/Source/JavaScriptCore/ChangeLog 2016-07-03 17:46:39 UTC (rev 202790)
</span><span class="lines">@@ -1,3 +1,32 @@
</span><ins>+2016-07-02 Filip Pizlo <fpizlo@apple.com>
+
+ DFG LICM needs to go all-in on the idea that some loops can't be LICMed
+ https://bugs.webkit.org/show_bug.cgi?id=159388
+
+ Reviewed by Mark Lam.
+
+ Some time ago I acknowledged that LICM required loops to meet certain requirements that
+ may get broken by the time we do LICM, like that the terminal of the pre-header is ExitOK.
+ It used to be that we just ignored that requirement and would hoist anyway, but since
+ r189126 we've stopped hoisting out of loops that don't have ExitOK. We also added tests
+ for the case that the pre-header doesn't exist or is invalid.
+
+ It turns out that this patch didn't go far enough: even though it made LICM avoid loops
+ that had an invalid pre-header, the part that updated the AI state in nested loops still
+ assumed that these loops had valid pre-headers. We would crash in null dereference in
+ that loop if a nested loop had an invalid pre-header.
+
+ The fix is simple: don't update the AI state of nested loops that don't have pre-headers,
+ since we won't try to hoist out of those loops anyway.
+
+ * dfg/DFGLICMPhase.cpp:
+ (JSC::DFG::LICMPhase::attemptHoist):
+ * tests/stress/licm-no-pre-header-nested.js: Added. This would always crash before this fix.
+ (foo):
+ * tests/stress/licm-pre-header-cannot-exit-nested.js: Added. This was a failed attempt at a test, but I figure it's good to have weird code anyway.
+ (foo):
+ (valueOf):
+
</ins><span class="cx"> 2016-06-30 Filip Pizlo <fpizlo@apple.com>
</span><span class="cx">
</span><span class="cx"> Scopes that are not under TDZ should still push their variables onto the TDZ stack so that lifting TDZ doesn't bypass that scope
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoredfgDFGLICMPhasecpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/dfg/DFGLICMPhase.cpp (202789 => 202790)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/dfg/DFGLICMPhase.cpp 2016-07-03 17:08:20 UTC (rev 202789)
+++ trunk/Source/JavaScriptCore/dfg/DFGLICMPhase.cpp 2016-07-03 17:46:39 UTC (rev 202790)
</span><span class="lines">@@ -324,6 +324,12 @@
</span><span class="cx"> if (!subLoop)
</span><span class="cx"> continue;
</span><span class="cx"> BasicBlock* subPreHeader = m_data[subLoop->index()].preHeader;
</span><ins>+ // We may not have given this loop a pre-header because either it didn't have exitOK
+ // or the header had multiple predecessors that it did not dominate. In that case the
+ // loop wouldn't be a hoisting candidate anyway, so we don't have to do anything.
+ if (!subPreHeader)
+ continue;
+ // The pre-header's tail may be unreachable, in which case we have nothing to do.
</ins><span class="cx"> if (!subPreHeader->cfaDidFinish)
</span><span class="cx"> continue;
</span><span class="cx"> m_state.initializeTo(subPreHeader);
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoretestsstresslicmnopreheadernestedjs"></a>
<div class="addfile"><h4>Added: trunk/Source/JavaScriptCore/tests/stress/licm-no-pre-header-nested.js (0 => 202790)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/tests/stress/licm-no-pre-header-nested.js (rev 0)
+++ trunk/Source/JavaScriptCore/tests/stress/licm-no-pre-header-nested.js 2016-07-03 17:46:39 UTC (rev 202790)
</span><span class="lines">@@ -0,0 +1,23 @@
</span><ins>+//@ runMiscFTLNoCJITTest("--createPreHeaders=false")
+
+function foo(array, y) {
+ var x = 0;
+ var j = 0;
+ do {
+ x = y * 3;
+ var result = 0;
+ var i = 0;
+ if (!array.length)
+ array = [1];
+ do {
+ result += array[i++];
+ } while (i < array.length)
+ j++;
+ } while (j < 3);
+ return result + x;
+}
+
+noInline(foo);
+
+for (var i = 0; i < 10000; ++i)
+ foo([1, 2, 3], 42);
</ins></span></pre></div>
<a id="trunkSourceJavaScriptCoretestsstresslicmpreheadercannotexitnestedjs"></a>
<div class="addfile"><h4>Added: trunk/Source/JavaScriptCore/tests/stress/licm-pre-header-cannot-exit-nested.js (0 => 202790)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/tests/stress/licm-pre-header-cannot-exit-nested.js (rev 0)
+++ trunk/Source/JavaScriptCore/tests/stress/licm-pre-header-cannot-exit-nested.js 2016-07-03 17:46:39 UTC (rev 202790)
</span><span class="lines">@@ -0,0 +1,21 @@
</span><ins>+//@ runMiscFTLNoCJITTest("--createPreHeaders=false")
+
+function foo(object, predicate) {
+ for (var j = 0; j < 10; ++j) {
+ var result = 0;
+ var i = 0;
+ if (DFGTrue())
+ predicate = 42;
+ while (predicate >= 42) {
+ result += object.array[i++];
+ if (i >= object.array.length)
+ break;
+ }
+ }
+ return result;
+}
+
+noInline(foo);
+
+for (var i = 0; i < 10000; ++i)
+ foo({array: [1, 2, 3]}, {valueOf: function() { return 42; }});
</ins></span></pre></div>
<a id="trunkSourceWTFbenchmarksLockFairnessTestcpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/benchmarks/LockFairnessTest.cpp (202789 => 202790)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/benchmarks/LockFairnessTest.cpp 2016-07-03 17:08:20 UTC (rev 202789)
+++ trunk/Source/WTF/benchmarks/LockFairnessTest.cpp 2016-07-03 17:46:39 UTC (rev 202790)
</span><span class="lines">@@ -48,12 +48,13 @@
</span><span class="cx">
</span><span class="cx"> NO_RETURN void usage()
</span><span class="cx"> {
</span><del>- printf("Usage: LockFairnessTest yieldspinlock|pausespinlock|wordlock|lock|barginglock|bargingwordlock|thunderlock|thunderwordlock|cascadelock|cascadewordlockhandofflock|mutex|all <num threads> <seconds per test>\n");
</del><ins>+ printf("Usage: LockFairnessTest yieldspinlock|pausespinlock|wordlock|lock|barginglock|bargingwordlock|thunderlock|thunderwordlock|cascadelock|cascadewordlockhandofflock|mutex|all <num threads> <seconds per test> <microseconds in critical section>\n");
</ins><span class="cx"> exit(1);
</span><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> unsigned numThreads;
</span><span class="cx"> double secondsPerTest;
</span><ins>+unsigned microsecondsInCriticalSection;
</ins><span class="cx">
</span><span class="cx"> struct Benchmark {
</span><span class="cx"> template<typename LockType>
</span><span class="lines">@@ -72,9 +73,19 @@
</span><span class="cx"> threads[threadIndex] = createThread(
</span><span class="cx"> "Benchmark Thread",
</span><span class="cx"> [&, threadIndex] () {
</span><ins>+ if (!microsecondsInCriticalSection) {
+ while (keepGoing) {
+ lock.lock();
+ counts[threadIndex]++;
+ lock.unlock();
+ }
+ return;
+ }
+
</ins><span class="cx"> while (keepGoing) {
</span><span class="cx"> lock.lock();
</span><span class="cx"> counts[threadIndex]++;
</span><ins>+ usleep(microsecondsInCriticalSection);
</ins><span class="cx"> lock.unlock();
</span><span class="cx"> }
</span><span class="cx"> });
</span><span class="lines">@@ -85,8 +96,8 @@
</span><span class="cx">
</span><span class="cx"> sleep(secondsPerTest);
</span><span class="cx">
</span><ins>+ keepGoing = false;
</ins><span class="cx"> lock.lock();
</span><del>- keepGoing = false;
</del><span class="cx">
</span><span class="cx"> dataLog(name, ": ");
</span><span class="cx"> CommaPrinter comma;
</span><span class="lines">@@ -106,9 +117,10 @@
</span><span class="cx"> {
</span><span class="cx"> WTF::initializeThreading();
</span><span class="cx">
</span><del>- if (argc != 4
</del><ins>+ if (argc != 5
</ins><span class="cx"> || sscanf(argv[2], "%u", &numThreads) != 1
</span><del>- || sscanf(argv[3], "%lf", &secondsPerTest) != 1)
</del><ins>+ || sscanf(argv[3], "%lf", &secondsPerTest) != 1
+ || sscanf(argv[4], "%u", &microsecondsInCriticalSection) != 1)
</ins><span class="cx"> usage();
</span><span class="cx">
</span><span class="cx"> runEverything<Benchmark>(argv[1]);
</span></span></pre>
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