• Leszek Swirski's avatar
    Reland^2 "[serializer] Allocate during deserialization" · c4a062a9
    Leszek Swirski authored
    This is a reland of 28a30c57
    which was a reland of 5d7a29c9
    
    The crashes were from calling RegisterDeserializerFinished on a null
    Isolate pointer, for a deserializer that was never initialised
    (specifically, ReadOnlyDeserializer when ROHeap is shared).
    
    Original change's description:
    > Reland "[serializer] Allocate during deserialization"
    >
    > This is a reland of 5d7a29c9
    >
    > This reland shuffles around the order of checks in Heap::AllocateRawWith
    > to not check the new space addresses until it's known that this is a new
    > space allocation. This fixes an UBSan failure during read-only space
    > deserialization, which happens before the new space is initialized.
    >
    > It also fixes some issues discovered by --stress-snapshot, around
    > serializing ThinStrings (which are now elided as part of serialization),
    > handle counts (I bumped the maximum handle count in that check), and
    > clearing map transitions (the map backpointer field needed a Smi
    > uninitialized value check).
    >
    > Original change's description:
    > > [serializer] Allocate during deserialization
    > >
    > > This patch removes the concept of reservations and a specialized
    > > deserializer allocator, and instead makes the deserializer allocate
    > > directly with the Heap's Allocate method.
    > >
    > > The major consequence of this is that the GC can now run during
    > > deserialization, which means that:
    > >
    > >   a) Deserialized objects are visible to the GC, and
    > >   b) Objects that the deserializer/deserialized objects point to can
    > >      move.
    > >
    > > Point a) is mostly not a problem due to previous work in making
    > > deserialized objects "GC valid", i.e. making sure that they have a valid
    > > size before any subsequent allocation/safepoint. We now additionally
    > > have to initialize the allocated space with a valid tagged value -- this
    > > is a magic Smi value to keep "uninitialized" checks simple.
    > >
    > > Point b) is solved by Handlifying the deserializer. This involves
    > > changing any vectors of objects into vectors of Handles, and any object
    > > keyed map into an IdentityMap (we can't use Handles as keys because
    > > the object's address is no longer a stable hash).
    > >
    > > Back-references can no longer be direct chunk offsets, so instead the
    > > deserializer stores a Handle to each deserialized object, and the
    > > backreference is an index into this handle array. This encoding could
    > > be optimized in the future with e.g. a second pass over the serialized
    > > array which emits a different bytecode for objects that are and aren't
    > > back-referenced.
    > >
    > > Additionally, the slot-walk over objects to initialize them can no
    > > longer use absolute slot offsets, as again an object may move and its
    > > slot address would become invalid. Now, slots are walked as relative
    > > offsets to a Handle to the object, or as absolute slots for the case of
    > > root pointers. A concept of "slot accessor" is introduced to share the
    > > code between these two modes, and writing the slot (including write
    > > barriers) is abstracted into this accessor.
    > >
    > > Finally, the Code body walk is modified to deserialize all objects
    > > referred to by RelocInfos before doing the RelocInfo walk itself. This
    > > is because RelocInfoIterator uses raw pointers, so we cannot allocate
    > > during a RelocInfo walk.
    > >
    > > As a drive-by, the VariableRawData bytecode is tweaked to use tagged
    > > size rather than byte size -- the size is expected to be tagged-aligned
    > > anyway, so now we get an extra few bits in the size encoding.
    > >
    > > Bug: chromium:1075999
    > > Change-Id: I672c42f553f2669888cc5e35d692c1b8ece1845e
    > > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404451
    > > Commit-Queue: Leszek Swirski <leszeks@chromium.org>
    > > Reviewed-by: Jakob Gruber <jgruber@chromium.org>
    > > Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
    > > Cr-Commit-Position: refs/heads/master@{#70229}
    >
    > Bug: chromium:1075999
    > Change-Id: Ibc77cc48b3440b4a28b09746cfc47e50c340ce54
    > Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440828
    > Commit-Queue: Leszek Swirski <leszeks@chromium.org>
    > Auto-Submit: Leszek Swirski <leszeks@chromium.org>
    > Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
    > Reviewed-by: Jakob Gruber <jgruber@chromium.org>
    > Cr-Commit-Position: refs/heads/master@{#70267}
    
    Tbr: jgruber@chromium.org,ulan@chromium.org
    Bug: chromium:1075999
    Change-Id: Iaa8dc54895866ada0e34a7c9e8fff9ae1cb13f2d
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2444991Reviewed-by: 's avatarUlan Degenbaev <ulan@chromium.org>
    Commit-Queue: Leszek Swirski <leszeks@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#70279}
    c4a062a9
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