[vm/compiler] Fix unwinding issues

We used to emit an unwinding rule for caller's SP, however
this does not really work as we expected it to work. Instead there is
an unwritten rule that caller's SP is equal to the CFA which overwrites
our unwinding rule (at least in libunwindstack library used by

This leads to unwinding issues on the boundary between C++ and Dart:
when we unwind through the InvokeDartCode stub frame we arrive to
C++ code with SP equal to callee's CFA, which is set to FP by our
unwinding rules. In this case if C++ code does not use FP based frames
we will fail to unwind C++ frame property - because SP will be
incorrect. This manifest in simpleperf profiles as stacks which
abruptly truncate at DartEntry::InvokeCode.

This CL rewrites unwinding rules to fix this: instead of using
FP as CFA we define CFA to be caller's SP (FP+2*kWordSize). This ensures
that unwinding on the boundary between C++ and Dart works correctly:
as CFA will always be consistently set to caller's SP.

Additionally this CL fixes InvokeDartCode stub to remove a value
that this stub pushes above saved return address in PRODUCT mode.

This value acts as a profiler marker used by builtin profiler
which is only included in non-PRODUCT builds - but the presence of this
value interfers with unwinding (caller's SP will be off by 1 due the
presence of this value).

We could produce custom unwinding information for this stub instead, but
just removing this value in PRODUCT builds should solve the problem
as we usually only profile Flutter release builds with simpleperf.

Fixes b/220804295

TEST=manually profiled a Flutter app and confirmed correct flamegraph.

Change-Id: I2094afaab6e54e89625c0b5a89aebc6b9823d67c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/235226
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
7 files changed
tree: 08f85d8b91334c216f8e41ada1a3c941e1b6ae49
  1. .dart_tool/
  2. .github/
  3. benchmarks/
  4. build/
  5. docs/
  6. pkg/
  7. runtime/
  8. samples/
  9. samples-dev/
  10. samples_2/
  11. sdk/
  12. tests/
  13. third_party/
  14. tools/
  15. utils/
  16. .clang-format
  17. .gitattributes
  18. .gitconfig
  19. .gitignore
  20. .gn
  21. .mailmap
  22. .packages
  23. .style.yapf
  24. .vpython
  26. BUILD.gn
  27. CHANGELOG.md
  28. codereview.settings
  29. compile_output.txt
  31. DEPS
  33. OWNERS
  35. PRESUBMIT.py
  36. README.dart-sdk
  37. README.md
  38. sdk_args.gni
  39. SECURITY.md


A client-optimized language for fast apps on any platform

Dart is:

  • Optimized for UI: Develop with a programming language specialized around the needs of user interface creation.

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  • Fast on all platforms: Compile to ARM & x64 machine code for mobile, desktop, and backend. Or compile to JavaScript for the web.

Dart's flexible compiler technology lets you run Dart code in different ways, depending on your target platform and goals:

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