• Jake Hughes's avatar
    [heap] Add object start bitmap for conservative stack scanning · 5f6aa2e5
    Jake Hughes authored
    With conservative stack scanning enabled, a snapshot of the call stack
    upon entry to GC will be used to determine part of the root-set. When
    the collector walks the stack, it looks at each value and determines
    whether it could be a potential on-heap object pointer. However, unlike
    with Handles, these on-stack pointers aren't guaranteed to point to the
    start of the object: the compiler may decide hide these pointers, and
    create interior pointers in C++ frames which the GC doesn't know about.
    
    The solution to this is to include an object start bitmap in the header
    of each page. Each bit in the bitmap represents a word in the page
    payload which is set when an object is allocated. This means that when
    the collector finds an arbitrary potential pointer into the page, it can
    walk backwards through the bitmap until it finds the relevant object's
    base pointer. To prevent the bitmap becoming stale after compaction, it
    is rebuilt during object sweeping.
    
    This is experimental, and currently only works with inline allocation
    disabled, and single generational collection.
    
    Bug: v8:10614
    Change-Id: I28ebd9562f58f335f8b3c2d1189cdf39feaa1f52
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2375195
    Commit-Queue: Anton Bikineev <bikineev@chromium.org>
    Reviewed-by: 's avatarMichael Achenbach <machenbach@chromium.org>
    Reviewed-by: 's avatarDominik Inführ <dinfuehr@chromium.org>
    Reviewed-by: 's avatarAnton Bikineev <bikineev@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#69615}
    5f6aa2e5
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