Prevent a v8 deopt when profiling #14383
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bvaughn
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Nice work finding that we can set them back to ints straight after. Seems like a simple fix to me. I was going to suggest just removing the preventExtensions logic, but this is better. |
trueadm
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Thanks Dom! (and thanks for the help digging into this on Friday) |
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🤣 thanks for digging into this, you're both awesome |
| @@ -254,6 +254,23 @@ function FiberNode( | |||
| this.alternate = null; | |||
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| if (enableProfilerTimer) { | |||
| // Note: The following is done to avoid a v8 deopt. | |||
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Nit: s/deopt/performance cliff/
(It's not a deopt.)
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Interesting. I was referring to the fact that the result is Fibers all having different shapes (which seemed like a deopt to me) but admittedly, I was just guessing at terminology 😄
Edit I've updated the wording based on your clarification ~ 1dc108e
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To give some more background, a deopt happens when we're running optimized machine code generated by the optimizing compiler (TurboFan in V8's case), and one of the assumptions in the optimized code turns out to be invalid. In such cases we have to deopt and go back to running bytecode in the interpreter (Ignition in V8's case). (Our JSConf talk includes some more detail and visualizations that make this more clear, in case you're interested.)
This V8 bug however occurs before we even get to optimized code.
You can verify this yourself by running our reduced test case with --no-opt: it still reproduces.
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Resolves #14365