I believe the reordering of the first/last check across the code that
delves into letter frames is an improvement, but a currently undectable
one, since it appears that we don't currently allow ::first-letter
pseudo-elements to break across lines, even in the presence of
multi-character ::first-letters that are broken by
'word-break:break-all'.
The MacOS and Windows profiler cores have a threading structure where one
thread ("sampler thread") collects information from a thread to be profiled
("samplee thread") by suspending it, getting its register state, unwinding its
stack, and then resuming it. This requires kernel-level primitives to perform
the suspend, get-registers and resume steps.
The Linux/Android core is different, because none of those three primitives
exist. Until now, data collection has been done by sending a SIGPROF to the
samplee, and collecting all relevant data within the signal handler. This has
a number of disadvantages:
(1) Current work to rationalise/clean up the threading structure of the
profiler is complicated by the need to reason about/verify two different
schemes.
In particular, the Tick call in the Windows and MacOS implementations will
produce its output on the sampler thread. In the Linux implementation
that is produced on the sampled threads.
(2) Dynamic verification results (primarily, absence of data races and
deadlocks) established for the Linux implementation are less likely to
carry over to the other two implementations, because the threading
structures are different.
(3) It causes a lot of duplicated code in platform-*.cpp. For example
SamplerThread::Run() in the -win32.cpp and -macos.cpp files are very
similar. Ideally all three could be merged into a single file with the
identical logic commoned up.
(4) Running lots of code -- the entire contents of Tick -- in a signal handler
isn't considered good practice. POSIX severely restricts the set of
functions we can safely call from within a signal handler.
This commit changes the Linux implementation by using semaphores to implement
the suspend and resume primitives, and moves the bulk of the data collection
work to the sampler thread. By doing this, it causes the Linux implementation
to have the same threading structure as the other two.
--HG--
extra : rebase_source : 675b6ef76915d164ed263b831dddd6ce0c0e97f3
AbstractThread::GetCurrent() can return null if a thread isn't a
default AbstractThread (MainThread, etc). This doesn't get caught in
MozPromise until we try to check for reliability of dispatching on the
thread. Adding asserts to make things clearer on crashes.
MozReview-Commit-ID: AQJwpdTUiHZ
