tree 08f85d8b91334c216f8e41ada1a3c941e1b6ae49
parent 1cdfaa85878a42444e026b823fdb356c6fd35612
author Vyacheslav Egorov <vegorov@google.com> 1646580562 +0000
committer Commit Bot <commit-bot@chromium.org> 1646580562 +0000

[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
Android).

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>