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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/203336">203336</a></dd>
<dt>Author</dt> <dd>fpizlo@apple.com</dd>
<dt>Date</dt> <dd>2016-07-17 15:00:42 -0700 (Sun, 17 Jul 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>DFG CSE is broken for MultiGetByOffset
https://bugs.webkit.org/show_bug.cgi?id=159858
Reviewed by Saam Barati.
This disabled CSE for MultiGetByOffset. I opened bug 159859 for the long-term fix, which
would teach CSE (and other passes also) how to decay a removed MultiGetByOffset to a
CheckStructure. Since we currently just decay MultiGetByOffset to Check, we forget the
structure checks. So, if we CSE a MultiGetByOffset that checks for one set of structures with
a heap access on the same property and base that checks for different structures, then we
will forget some structure checks that we had previously. It's unsound to forget checks in
DFG IR.
This bug mostly manifested as a high-volume crash at Unreachable in FTL, because we'd prove
that the code after the MultiGetByOffset was unreachable due to the structure checks and then
CSE would remove everything but the Unreachable.
* dfg/DFGClobberize.h:
(JSC::DFG::clobberize): Remove the def() for MultiGetByOffset to disable CSE for this node for now.
* tests/stress/cse-multi-get-by-offset-remove-checks.js: Added. This used to fail with FTL eanbled.
(Cons1):
(Cons2):
(Cons3):
(foo):
(bar):</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoredfgDFGClobberizeh">trunk/Source/JavaScriptCore/dfg/DFGClobberize.h</a></li>
</ul>
<h3>Added Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoretestsstresscsemultigetbyoffsetremovechecksjs">trunk/Source/JavaScriptCore/tests/stress/cse-multi-get-by-offset-remove-checks.js</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (203335 => 203336)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2016-07-17 20:35:20 UTC (rev 203335)
+++ trunk/Source/JavaScriptCore/ChangeLog 2016-07-17 22:00:42 UTC (rev 203336)
</span><span class="lines">@@ -1,3 +1,31 @@
</span><ins>+2016-07-16 Filip Pizlo <fpizlo@apple.com>
+
+ DFG CSE is broken for MultiGetByOffset
+ https://bugs.webkit.org/show_bug.cgi?id=159858
+
+ Reviewed by Saam Barati.
+
+ This disabled CSE for MultiGetByOffset. I opened bug 159859 for the long-term fix, which
+ would teach CSE (and other passes also) how to decay a removed MultiGetByOffset to a
+ CheckStructure. Since we currently just decay MultiGetByOffset to Check, we forget the
+ structure checks. So, if we CSE a MultiGetByOffset that checks for one set of structures with
+ a heap access on the same property and base that checks for different structures, then we
+ will forget some structure checks that we had previously. It's unsound to forget checks in
+ DFG IR.
+
+ This bug mostly manifested as a high-volume crash at Unreachable in FTL, because we'd prove
+ that the code after the MultiGetByOffset was unreachable due to the structure checks and then
+ CSE would remove everything but the Unreachable.
+
+ * dfg/DFGClobberize.h:
+ (JSC::DFG::clobberize): Remove the def() for MultiGetByOffset to disable CSE for this node for now.
+ * tests/stress/cse-multi-get-by-offset-remove-checks.js: Added. This used to fail with FTL enabled.
+ (Cons1):
+ (Cons2):
+ (Cons3):
+ (foo):
+ (bar):
+
</ins><span class="cx"> 2016-07-17 Yusuke Suzuki <utatane.tea@gmail.com>
</span><span class="cx">
</span><span class="cx"> [JSC] Enable test262 module tests
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoredfgDFGClobberizeh"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/dfg/DFGClobberize.h (203335 => 203336)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/dfg/DFGClobberize.h 2016-07-17 20:35:20 UTC (rev 203335)
+++ trunk/Source/JavaScriptCore/dfg/DFGClobberize.h 2016-07-17 22:00:42 UTC (rev 203336)
</span><span class="lines">@@ -883,7 +883,9 @@
</span><span class="cx"> read(JSObject_butterfly);
</span><span class="cx"> AbstractHeap heap(NamedProperties, node->multiGetByOffsetData().identifierNumber);
</span><span class="cx"> read(heap);
</span><del>- def(HeapLocation(NamedPropertyLoc, heap, node->child1()), LazyNode(node));
</del><ins>+ // FIXME: We cannot def() for MultiGetByOffset because CSE is not smart enough to decay it
+ // to a CheckStructure.
+ // https://bugs.webkit.org/show_bug.cgi?id=159859
</ins><span class="cx"> return;
</span><span class="cx"> }
</span><span class="cx">
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoretestsstresscsemultigetbyoffsetremovechecksjs"></a>
<div class="addfile"><h4>Added: trunk/Source/JavaScriptCore/tests/stress/cse-multi-get-by-offset-remove-checks.js (0 => 203336)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/tests/stress/cse-multi-get-by-offset-remove-checks.js (rev 0)
+++ trunk/Source/JavaScriptCore/tests/stress/cse-multi-get-by-offset-remove-checks.js 2016-07-17 22:00:42 UTC (rev 203336)
</span><span class="lines">@@ -0,0 +1,55 @@
</span><ins>+function Cons1()
+{
+ this.e = 1;
+ this.f = 2;
+}
+
+Cons1.prototype.g = 1;
+
+function Cons2()
+{
+ this.f = 1;
+ this.h = 2;
+}
+
+Cons2.prototype.g = 2;
+
+function Cons3()
+{
+ this.d = 1;
+ this.e = 2;
+ this.f = 3;
+}
+
+Cons3.prototype = Cons2.prototype;
+
+function foo(o, p, q)
+{
+ var x = 0, y = 0;
+ if (p)
+ x = o.f;
+ if (q)
+ y = o.f;
+ return x + y;
+}
+
+for (var i = 0; i < 10000; ++i) {
+ foo(new Cons1(), true, false);
+ foo(new Cons2(), false, true);
+ foo(new Cons3(), false, true);
+}
+
+function bar(o, p)
+{
+ return foo(o, true, p);
+}
+
+noInline(bar);
+
+for (var i = 0; i < 100000; ++i)
+ bar(new Cons1(), false);
+
+var result = bar(new Cons1(), true);
+if (result != 4)
+ throw "Error: bad result: " + result;
+
</ins></span></pre>
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