Tess Strickland
d5649fc9f6
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>
native_stack_traces
This package provides libraries and a utility for decoding non-symbolic stack traces generated by an AOT-compiled Dart application.
Converting stack traces
In some modes of AOT compilation, information on mapping execution points to source locations is no longer stored in the Dart image. Instead, this information is translated to separately stored debugging information. This debugging information can then be stripped from the application before shipping.
However, there is a drawback. Stack traces generated by such an application no longer includes file, function, and line number information (i.e., symbolic stack traces). Instead, stack trace frames simply include program counter information. Thus, to find the source information for these frames, we must use the debugging information. This means either keeping the original unstripped application, or saving the debugging information into a separate file.
Given this debugging information, the libraries in this package can turn
non-symbolic stack traces back into symbolic stack traces. In addition, this
package includes a command line tool decode whose output is the same as its
input except that non-symbolic stack traces are translated.
Using decode
Take the following Dart code, which we put in throws.dart. The inlining
pragmas are here just to ensure that bar is inlined into foo and that foo
is not inlined into bar, to illustrate how inlined code is handled in the
translated output.
@pragma('vm:prefer-inline')
bar() => throw null;
@pragma('vm:never-inline')
foo() => bar();
main() => foo();
Now we run the following commands:
# Make sure that we have the native_stack_traces package.
$ pub get native_stack_traces
$ pub global activate native_stack_traces
# We compile the example program, removing the source location information
# from the snapshot and saving the debugging information into throws.debug.
$ dart2native -k aot -S throws.debug -o throws.aotsnapshot throws.dart
# Run the program, saving the error output to throws.err.
$ dartaotruntime throws.aotsnapshot 2>throws.err
# Using the saved debugging information, we can translate the stack trace
# contained in throws.err to its symbolic form.
$ pub global run native_stack_traces:decode translate -d throws.debug -i throws.err
# We can also just pipe the output of running the program directly into
# the utility.
$ dartaotruntime throws.aotsnapshot |& \
pub global run native_stack_traces:decode translate -d throws.debug
Features and bugs
Please file feature requests and bugs at the issue tracker.