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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/195744">195744</a></dd>
<dt>Author</dt> <dd>fpizlo@apple.com</dd>
<dt>Date</dt> <dd>2016-01-28 09:30:00 -0800 (Thu, 28 Jan 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>LowerToAir::preferRightForResult() should resolve use count ties by selecting the child that is closest in an idom walk
https://bugs.webkit.org/show_bug.cgi?id=153583
Reviewed by Benjamin Poulain.
This undoes the AsmBench/n-body regression in <a href="http://trac.webkit.org/projects/webkit/changeset/195654">r195654</a> while preserving that revision's
Kraken progression.
* b3/B3LowerToAir.cpp:
(JSC::B3::Air::LowerToAir::LowerToAir):
(JSC::B3::Air::LowerToAir::preferRightForResult):</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoreb3B3LowerToAircpp">trunk/Source/JavaScriptCore/b3/B3LowerToAir.cpp</a></li>
</ul>
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<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (195743 => 195744)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2016-01-28 17:26:41 UTC (rev 195743)
+++ trunk/Source/JavaScriptCore/ChangeLog 2016-01-28 17:30:00 UTC (rev 195744)
</span><span class="lines">@@ -1,3 +1,17 @@
</span><ins>+2016-01-28 Filip Pizlo <fpizlo@apple.com>
+
+ LowerToAir::preferRightForResult() should resolve use count ties by selecting the child that is closest in an idom walk
+ https://bugs.webkit.org/show_bug.cgi?id=153583
+
+ Reviewed by Benjamin Poulain.
+
+ This undoes the AsmBench/n-body regression in r195654 while preserving that revision's
+ Kraken progression.
+
+ * b3/B3LowerToAir.cpp:
+ (JSC::B3::Air::LowerToAir::LowerToAir):
+ (JSC::B3::Air::LowerToAir::preferRightForResult):
+
</ins><span class="cx"> 2016-01-28 Benjamin Poulain <bpoulain@apple.com>
</span><span class="cx">
</span><span class="cx"> [JSC] B3 Tail Call with Varargs do not restore callee saved registers
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoreb3B3LowerToAircpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/b3/B3LowerToAir.cpp (195743 => 195744)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/b3/B3LowerToAir.cpp 2016-01-28 17:26:41 UTC (rev 195743)
+++ trunk/Source/JavaScriptCore/b3/B3LowerToAir.cpp 2016-01-28 17:30:00 UTC (rev 195744)
</span><span class="lines">@@ -39,6 +39,7 @@
</span><span class="cx"> #include "B3CCallValue.h"
</span><span class="cx"> #include "B3CheckSpecial.h"
</span><span class="cx"> #include "B3Commutativity.h"
</span><ins>+#include "B3Dominators.h"
</ins><span class="cx"> #include "B3IndexMap.h"
</span><span class="cx"> #include "B3IndexSet.h"
</span><span class="cx"> #include "B3MemoryValue.h"
</span><span class="lines">@@ -75,6 +76,7 @@
</span><span class="cx"> , m_blockToBlock(procedure.size())
</span><span class="cx"> , m_useCounts(procedure)
</span><span class="cx"> , m_phiChildren(procedure)
</span><ins>+ , m_dominators(procedure.dominators())
</ins><span class="cx"> , m_procedure(procedure)
</span><span class="cx"> , m_code(procedure.code())
</span><span class="cx"> {
</span><span class="lines">@@ -592,18 +594,22 @@
</span><span class="cx"> if (leftIsPhiWithThis != rightIsPhiWithThis)
</span><span class="cx"> return rightIsPhiWithThis;
</span><span class="cx">
</span><del>- bool leftResult = m_useCounts.numUsingInstructions(left) == 1;
- bool rightResult = m_useCounts.numUsingInstructions(right) == 1;
- if (leftResult && rightResult) {
- // If one operand is not in the block, it could be in a block dominating a loop
- // containing m_value.
- if (left->owner == m_value->owner)
- return false;
- if (right->owner == m_value->owner)
- return true;
- }
</del><ins>+ if (m_useCounts.numUsingInstructions(right) != 1)
+ return false;
+
+ if (m_useCounts.numUsingInstructions(left) != 1)
+ return true;
</ins><span class="cx">
</span><del>- return rightResult;
</del><ins>+ // The use count might be 1 if the variable is live around a loop. We can guarantee that we
+ // pick the the variable that is least likely to suffer this problem if we pick the one that
+ // is closest to us in an idom walk. By convention, we slightly bias this in favor of
+ // returning true.
+
+ // We cannot prefer right if right is further away in an idom walk.
+ if (m_dominators.strictlyDominates(right->owner, left->owner))
+ return false;
+
+ return true;
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> template<Air::Opcode opcode32, Air::Opcode opcode64, Air::Opcode opcodeDouble, Air::Opcode opcodeFloat, Commutativity commutativity = NotCommutative>
</span><span class="lines">@@ -2297,6 +2303,7 @@
</span><span class="cx"> UseCounts m_useCounts;
</span><span class="cx"> PhiChildren m_phiChildren;
</span><span class="cx"> BlockWorklist m_fastWorklist;
</span><ins>+ Dominators& m_dominators;
</ins><span class="cx">
</span><span class="cx"> Vector<Vector<Inst, 4>> m_insts;
</span><span class="cx"> Vector<Inst> m_prologue;
</span></span></pre>
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