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Subject: Re: Oct 20 minutes
From: Michael Spear (mfs409_at_[hidden])
Date: 2020-10-21 15:50:55


Thanks Jens. I think writing another paper would subsume the preceding
draft, which focused on allocation. One concern I have is that I don't
really know the right style/tone/level of detail for such a paper for this
audience. Do you know of any examples that I might be able to reference?

- Mike

On Wed, Oct 21, 2020 at 4:48 PM Jens Maurer <Jens.Maurer_at_[hidden]> wrote:

> On 21/10/2020 15.54, Michael Spear via SG5 wrote:
> > If an implementation protects all transactions with a single, global,
> reentrant lock, then there are three pieces to the implementation.
> > - The first is modifying the parser to support the new syntax. This is
> common to any implementation, so whatever effort it entails is really a
> fixed cost for the TS.
> > - The second is some kind of run-time library that maps the beginning
> and end of a lexically scoped transaction to operations for lock acquire
> and lock release. This is quite simple.
>
> Agreed so far.
>
> > - The third is whatever static analysis is needed to warn programmers
> when their atomic blocks do something forbidden. This may be difficult,
> depending on how we specify things. For example, if the compiler *must*
> warn when an exception escapes a transaction, then if the compiler does not
> have access to definition of a function that might throw, how could it give
> such a warning?
> >
> > My guess is that the third piece is going to be "best effort" by the
> compiler writer. Surely some things should produce warnings, but with an
> understanding that there might be false negatives (e.g., we miss a warning
> for a transaction that has undefined behavior, because the transaction
> spans TUs and in some other TU, it uses std::cout or a std::atomic
> variable, or opens a socket, or does something else that can legally
> constitute inter-thread communication).
>
> My guess is there will be close to zero warnings.
> As you correctly observe, there are implementability concerns
> for required warnings on e.g. exceptions escaping an atomic
> block. As a consequence, no warnings will be required.
> (And if we could require warnings, we'd make them errors,
> because if we can say for sure that bad code is executed,
> we'd better refuse to compile.)
>
> > In the case of throw, the expression of exceptions in the intermediate
> representation is via function calls or intrinsics (cxa_throw, cxa_catch,
> LLVM landing pads, etc). The implementation of these is dependent on the
> OS and architecture. For example, IIRC on Linux targets they traverse
> DWARF tables within the binary itself. So, again, making assumptions about
> these is not portable, and correct programs cannot assume anything more
> than the high-level specified behavior. That behavior is little more than
> "nonlocal jump, with destruction of everything on the popped part of the
> run-time stack". Note: if I am wrong, and the behavior of stack unwinding
> is more carefully defined, and that definition includes the possibility of
> stack unwinding being a legal form of inter-thread communication, then my
> whole argument is invalid. Hopefully someone more knowledgeable about
> exceptions in C++ can confirm or refute my claim.
>
> Stack unwinding is properly defined, but it does not have inter-thread
> impact per se. Something like std::lock_guard's destructor unlocks a
> mutex, of course, but these inter-thread communication mechanisms should
> not be allowed in an atomic block regardless.
>
> > In the original TMTS, exceptions are tricky because there need to be
> writes to the exception object, and these writes need to be instrumented in
> a special way, because these writes can escape the transaction even if the
> transaction gets undone. In the proposed TMTS, exceptions cannot escape a
> transaction (and the transaction doesn't ever have to get undone), which
> means that STM can fall back to a single lock whenever it encounters a
> throw. This is easy to do, because the STM instrumentation pass simply
> says "here's a function for which I don't have a definition... I guess I'll
> just serialize the transaction here". cxa_throw is no different than a
> multi-TU transaction.
>
> Agreed.
>
> > If we *wanted* to specify commit-on-exception-escape, it would be
> trivial to support in a reasonable TM implementation. I don't think we
> want to, and I'm in favor of forbidding exceptions from escaping
> transactions. However, the use of exceptions within a transaction does not
> appear to pose an implementation challenge, nor does it appear to pose a
> correctness challenge.
> >
> > So, in summary, I do not see implementation issues with supporting
> throw+catch inside a transaction, or allocation inside a transaction. As
> the minutes state, this will open up a huge portion of the standard library
> to transactions (perhaps only threads, coroutines, mutexes, atomics, and
> iostreams would still be forbidden?).
>
> Sounds about right.
>
> As I said in the call, that's a rather sharp departure from the previous
> approach "limited standard library functions are allowed", so needs an
> accompanying rationale paper, stating approximately what's in your
> e-mail, above.
>
> Jens
>



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