Since we've run out of room for more fields in the Image object header
on 64-bit architectures, the serializer instead creates an ImageHeader
object for precompiled snapshots that is placed at the start of text
segments. The new ImageHeader object contains the following information:
* The offset of the BSS segment from the text segment, previously
stored in the Image object header.
* The relocated address of the text segment in the dynamic shared
object. Due to restrictions when generating assembly snapshots, this
field is only set for ELF snapshots, and so it can also be used to
detect whether a snapshot was compiled to assembly or ELF.
* The offset of the build ID description field from the text segment.
* The length of the build ID description field.
We replace the BSS offset in the Image object header with the offset of
the ImageHeader object within the text segment, so that we can detect
when a given Image has an ImageHeader object available.
There are no methods available on ImageHeader objects, but instead the
Image itself controls access to the information. In particular, the
relocated address method either returns the relocated address
information from the ImageHeader object or from the initialized BSS
depending on the type of snapshot, so the caller need not do this work.
Also, instead of returning the raw offset to the BSS section and having
the caller turn that into an appropriate pointer, the method for
accessing the BSS segment now returns a pointer to the segment.
Bug: https://github.com/dart-lang/sdk/issues/43274
Cq-Include-Trybots: luci.dart.try:vm-precomp-ffi-qemu-linux-release-arm-try,vm-kernel-precomp-android-release-arm64-try,vm-kernel-precomp-android-release-arm_x64-try
Change-Id: I15eae4ad0a088260b127f3d07da79374215b7f56
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/163207
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Tess Strickland
d5649fc9f6