So there's three sites that needed to be updated to get this to work
properly:
* Strong reference on the feedback vector
https://github.com/kwasimensah/node/blob/c5ff019a4e93891106859cb272ded1197a92c7e5/deps/v8/src/objects/feedback-vector.cc#L397
* Strong reference to the code's dependencies
https://github.com/kwasimensah/node/blob/974ab4060fe3eff74dc0a62a5a27d516738f4c55/deps/v8/src/objects/code.cc#L792
* Tell MarkingVistors to treat these references as strong
https://github.com/kwasimensah/node/blob/12478684aab233942e0d5dc24f195930c8a5e59d/deps/v8/src/compiler/pipeline.cc#L1199
* What you really want is for IsWeakObject to return false
https://github.com/kwasimensah/node/blob/7b48713334469818661fe276cf571de9c7899f2d/deps/v8/src/objects/code-inl.h#L612
I'm working on a repro that's independent of my project and node but I'm
currently seeing a 19% speedup but it's still 7% slower than when I'm not
forcing GC between each benchmark run* (I think because the hidden classes
for temporary objects that are used in optimized code are lost to GC). I
also have to see what tests this breaks.
*I'm purposely forcing GCs because I'm writing a possibly long running node
program and want to make sure mistimed GCs don't cause it to waste time on
spurious recompiles.
On Wed, Feb 15, 2023 at 1:09 AM Jakob Gruber <[email protected]> wrote:
>
>
> On Wed, Feb 15, 2023 at 1:44 AM Kwasi Mensah <[email protected]>
> wrote:
>
>> Hi v8-dev,
>>
>> I was debugging something that was causing me to place extra GCs and saw
>> that it was causing a ton of spurious turbofan recompilation of functions
>> and deopts referencing weak objects. After digging a bit it looks like v8
>> only keeps a weak reference to optimized code in
>> https://github.com/v8/v8/blame/main/src/objects/feedback-vector.cc#L368
>>
>
> Are you sure it would help? The 'deopts referencing weak objects' sound
> unrelated since the referenced objects are probably not optimized code. And
> deoptimized TF code cannot be reused, so I don't see how we could avoid the
> recompilation through a strong reference in the FeedbackVector in this
> case. Please try your proposal locally first before considering adding a
> flag.
>
>
>>
>>
>> It looks like it's done this from the beginning per
>> https://github.com/v8/v8/commit/11a211ff1b81e8d83d1aa4898743c2efb89603bd#diff-cbddb8a6da7810e9fc3647c9cf4f511410c43d11da9f62873bc33dece03a31f5R305
>>
>> I'm writing a CLI using node and I'm willing to trade memory for speed.
>> Is this a contribution that makes sense behind an experimental flag?
>>
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