1. 12 Apr, 2019 1 commit
    • Benedikt Meurer's avatar
      [map] Support in-place field representation changes. · f11ba854
      Benedikt Meurer authored
      This adds a new flag --modify-field-representation-inplace (enabled by
      default), which lets the runtime perform field representation changes
      for Smi to Tagged or for HeapObject to Tagged in-place instead of
      creating new maps and marking the previous map tree as deprecated.
      
      That means we create (a lot) fewer Maps and DescriptorArrays in the
      beginning and also need to self-heal fewer objects later (migrating
      off the deprecated maps). In TurboFan we just take the "field owner
      dependency" whenever we use the field representation, which is very
      similar to what we already do for the field types. That means if we
      change the representation of a field that we used in optimized code,
      we will simply deoptimize that code and have TurboFan potentially
      later optimize it again with the new field representation.
      
      On the Speedometer2/ElmJS-TodoMVC test, this reduces the total execution
      time from around 415ms to around 352ms, which corresponds to a **15%**
      improvement. The overall Speedometer2 score improves from around 74.1
      to around 78.3 (on local runs with content_shell), corresponding to a
      **5.6%** improvement here. :tada:
      
      On the CNN desktop browsing story, it seems that we reduce map space
      utilization/fragmentation by about 4-5%. But since we allocate a lot
      less (fewer Maps and DescriptorArrays) we also significantly change
      the GC timing, which heavily influences the results here. So take this
      with a grain of salt. :shrug:
      
      Note: For Double fields, this doesn't change anything, meaning they
      still create new maps and deprecate the previous map trees.
      
      Bug: v8:8749, v8:8865, v8:9114
      Change-Id: Ibd70efcb59be982863905663dbfaa89aa5b31e14
      Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
      Doc: http://bit.ly/v8-in-place-field-representation-changes
      Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1565891
      Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
      Commit-Queue: Igor Sheludko <ishell@chromium.org>
      Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
      Reviewed-by: 's avatarIgor Sheludko <ishell@chromium.org>
      Cr-Commit-Position: refs/heads/master@{#60822}
      f11ba854