This CL introduces the following changes to the experimental
implementation of the object start bitmap, that is evaluated as
a mechanism for resolving inner pointers (behind the flag
v8_enable_conservative_stack_scanning):
- Manually iterate through page objects, instead of using the
PagedSpaceObjectIterator, for performance (avoid calling
MakeHeapIterable all the time) and to simplify the handling
of filler objects.
- Clear bits when reusing evacuated pages of the new space.
- Use the cage base to iterate correctly through code objects.
- Introduce a method for verifying the validity of the object
start bitmap.
- Minor fixes, additional checks and cleanup.
Bug: v8:12851
Change-Id: I245937ffe6f4b53c4c2dcf5126e8836aec4dc79e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3675099
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80869}
This CL adds to the existing experimental implementation of the
object start bitmap, that is evaluated as a mechanism for resolving
inner pointers (behind the flag v8_enable_conservative_stack_scanning).
It fixes method ObjectStartBitmap::FindBasePtr to ensure that the
correct base pointer is returned, even if the bitmap is not fully
populated (e.g., with object evacuation or inline object allocation).
This method now recalculates the part of the bitmap that is
required for returning the correct result, by iterating through
objects of the page. A special constructor has been introduced to the
PagedSpaceObjectIterator for this purpose.
It also moves the existing inline methods of ObjectStartBitmap to a
new -inl.h header file, to avoid circular dependencies.
Bug: v8:12851
Change-Id: Iabd0df020bee3bb63ef9d4888591b25d24d79dd9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3641179
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80538}
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: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69615}
