Tweaked wording for v8 "performance cliff" issue
Brian Vaughn committed
Dec 4, 2018 at 07:49 UTC
1dc108e5829bb8d84ffd39e2e932d798edd7ca8b
1 file changed
+10
-7
packages/react-reconciler/src/ReactFiber.js
+10
-7
@@ -254,14 +254,16 @@ function FiberNode(
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this.alternate = null;
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if (enableProfilerTimer) {
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- // Note: The following is done to avoid a v8 deopt.
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+ // Note: The following is done to avoid a v8 performance cliff.
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//
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- // It is important to initialize the fields below with doubles.
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- // Otherwise Fibers will deopt and end up having separate shapes when
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- // doubles are later assigned to fields that initially contained smis.
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- // This is a bug in v8 having something to do with Object.preventExtension().
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+ // Initializing the fields below to smis and later updating them with
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+ // double values will cause Fibers to end up having separate shapes.
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+ // This behavior/bug has something to do with Object.preventExtension().
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+ // Fortunately this only impacts DEV builds.
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+ // Unfortunately it makes React unusably slow for some applications.
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+ // To work around this, initialize the fields below with doubles.
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//
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- // Learn more about this deopt here:
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+ // Learn more about this here:
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// https://github.com/facebook/react/issues/14365
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// https://bugs.chromium.org/p/v8/issues/detail?id=8538
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this.actualDuration = Number.NaN;
@@ -270,7 +272,8 @@ function FiberNode(
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this.treeBaseDuration = Number.NaN;
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// It's okay to replace the initial doubles with smis after initialization.
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- // This simplifies other profiler code and doesn't trigger the deopt.
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+ // This won't trigger the performance cliff mentioned above,
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+ // and it simplifies other profiler code (including DevTools).
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this.actualDuration = 0;
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this.actualStartTime = -1;
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this.selfBaseDuration = 0;