`$derived` value re-computed each time it is read even if no dependencies have changed

[hpeebles]

Describe the bug

This is more of an annoyance than a bug since the correct value ends up being returned, but unnecessary computation takes place (which in my use case is causing significant performance degradations).

The issue occurs when a unowned derived value depends on some state, then that state is updated, but the derived value evaluates to the same thing based on the new state. From this point on the derived value is always flagged as MAYBE_DIRTY and so is re-computed each time it is read.

Reproduction

https://svelte.dev/playground/hello-world?version=5.30.1#H4sIAAAAAAAAE7VSTU_jMBD9K6MRUlMpBMrRpJWQQMteirQLJ8zBSSbFwh1H9rgsqvLfV1EoX9sVB8T1ffn52VtksyZUeE7BbqiBjXGJINBh7dddEmowx9Y6iqhutyhP3SAeAMx31rOuK-KGnAxYZSLtw2vPQiwRFZaxDraThWYtbeJarGcIibMpbAdMiyMBA3M4iGKEstn09BWvBrwZ2xbVU5ZNYb7YGbXUnqN3VDi_yjSOl7C8KopC4y5GSyBJgcFACbPjZ7R_oflj0OS3UAfHClobohyKXQ8TmQZ8C83b4SbTPeZ3CjXJodqn2ln_f_5MQeoaIwTBexnjcoj3PrnmzYvtbzTMeXL6je1O1DjJuzwwK2P5pSR7-bToV8r9S51fLS9GutdcHr3-PC6rJOIZPNfO1g_zbUjcL37dLOH68gKuL38uf5RHo2ahGXMU-iOoJCTq73IUY92j5QZVa1yk_i82oS0vSQMAAA==

Logs

Step 0: first-time read of derived value
computing...
derived value: true

Step 1: update root value, should re-compute derived value
computing...
derived value: true

Step 2: read derived value again, should not re-compute derived value
computing...
derived value: true

System Info

System:
    OS: macOS 15.4.1
    CPU: (14) arm64 Apple M3 Max
    Memory: 1.02 GB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.17.0 - ~/.nvm/versions/node/v20.17.0/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 10.8.2 - ~/.nvm/versions/node/v20.17.0/bin/npm
  Browsers:
    Chrome: 136.0.7103.114
    Safari: 18.4

Severity

annoyance

[paoloricciuti]

Duplicato of #15414 not much we can do unfortunately but as you said it's just a minor inconvenience

#15414 (comment)

[hpeebles]

We're having to revert back to using stores because of the performance hit caused by this.
We have derived values that are updated very infrequently but are read a high volume of times and they are being recomputed each time.
Essentially we need a way of having eagerly evaluated derived values.
We could switch the values to $state and then use $effects to update them, but the docs recommend against that.
Any ideas for best way to solve this problem?