[cpp-threads] cpp-threads Digest, Vol 42, Issue 1

[Mark Batty]

I like this potential simplification a lot and I think it is true. I
am going to attempt to prove it over our mathematical model. It would
be nice if I can, because it simplifies the mathematics of the model
quite a bit.

Mark

On Fri, Dec 3, 2010 at 12:00 PM,  <cpp-threads-request at decadent.org.uk> wrote:
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> Today's Topics:
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>   1. Re: Visible sequence of side effects and coherence
>      requirements (Paul E. McKenney)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 2 Dec 2010 11:27:57 -0800
> From: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Subject: Re: [cpp-threads] Visible sequence of side effects and
>        coherence requirements
> To: C++ threads standardisation <cpp-threads at decadent.org.uk>
> Message-ID: <20101202192757.GM2085 at linux.vnet.ibm.com>
> Content-Type: text/plain; charset=us-ascii
>
> On Tue, Nov 23, 2010 at 11:29:45AM -0500, Alan Stern wrote:
>> Given the coherence requirements expressed in D3196 ("New Paragraphs
>> Following 1.10.p13"), the definition of "visible sequence of side
>> effects" (VSSE) in 1.10.p13 is too broad.  That is, the VSSE on an
>> atomic object M with respect to a value computation B of M may include
>> side effects which could not possibly be the value determined by B.
>>
>> For example, suppose that side effects A1 and A2 both modify M, where
>> A1 occurs before A2 in the modification order of M.  Further suppose
>> that A1 and A2 both happen before B, but neither happens before the
>> other.
>>
>> Then the VSSE on M with respect to B includes both A1 and A2, because
>> A1 is visible with respect to B and B does not happen before either A1
>> or A2.  But the write-read coherence requirement for A2 and B does not
>> allow B to take its value from A1.  Hence A1 shouldn't be in the VSSE,
>> even though the current definition puts it there.
>>
>> In short, although the current definition of VSSE is not incorrect, it
>> could be improved.  For instance, it might be stated as:
>>
>>       The visible sequence of side effects on an atomic object M,
>>       with respect to a value computation B of M, is the contiguous
>>       sub-sequence of side effects in the modification order of M
>>       beginning with the last side effect that happens before B and
>>       continuing up to, but not including, the first side effect
>>       that B happens before, if any.
>>
>> However I'm not going to advocate changing the definition of VSSE.
>> Instead, I suggest removing the whole concept from the formal part of
>> the standard entirely!
>>
>> The VSSE is defined solely for the purpose of this specification:
>>
>>       The value of an atomic object M, as determined by evaluation
>>       B, shall be the value stored by some operation in the visible
>>       sequence of M with respect to B.
>>
>> This does not need to be stated expicitly, because it follows directly
>> from the new coherence requirements.  Here's how:
>>
>> Let X be the last side effect in the modification order of M that
>> happens before B, let Y be the side effect from which B determines the
>> value of M, and let Z the first side effect in the modification order
>> of M that B happens before (if any).
>>
>> The write-read coherence requirement, applied to X and B, states that Y
>> either is X or follows X in the modification order.  The read-write
>> coherence requirement, applied to B and Z, states that Y precedes Z in
>> the modification order.  Together these facts imply that Y belongs to
>> the maximal contiguous sub-sequence defined above (or the sub-sequence
>> defined as in 1.10.p13).
>>
>> (As a special case, if Z doesn't exist then the only requirement is
>> that Y is X or follows X, again agreeing with the definition of the
>> VSSE.  If X doesn't exist then the behavior is undefined or
>> unspecified, because B could read an uninitialized value.)
>>
>> This argument shows that the only reason for introducing the VSSE into
>> the standard has now been superseded.  Therefore I suggest that the
>> existing text of 1.10.p13 be removed completely, leaving only the new
>> coherence requirements.
>
> I of course like the thought of removing unnecessary words from the
> standard.
>
> Mark, Hans, Clark, any thoughts?
>
>                                                        Thanx, Paul
>
>
>
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> End of cpp-threads Digest, Vol 42, Issue 1
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