Submitter: Batty
Submission Date: 2011-10-14
Source: WG 14
Reference Document: N1584
Version: 1.5
Date: April 2014
Subject: Visible sequences of side effects are
redundant
Summary
It has been mathematically proved that a simplification can be made to the memory model as it is specified in the final draft of the C++11 standard. Essentially, the restriction defining visible sequence of side effects (vsse) is redundant and can be removed with no ill effects. The main motivation for doing this is that the current restriction is misleading. 5.1.2.4p22 defines vsse's:The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every subsequent side effect, it is not the case that B happens before it. 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.The wording of this paragraph makes it seem as if the vsse identifies the writes that an atomic read is allowed to read from, but this is not the case. There can be writes in the vsse that cannot be read due to the coherence requirements (to be included in C, 1.10p15 through 1.10p18 in C++ N3291). Consequently this is even more confusing than it at first appears.
Also propose changing 5.1.2.4p22 to the following:
The value of an atomic object M, as determined by evaluation B, shall be the value stored by some side effect A that modifies M, where B does not happen before A.With a note to remind the reader of the coherence requirements:
NOTE: The set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements belowIf the committee is concerned about allowing a differing text from C++11, then a note could be added to assure the reader:
NOTE: Although the rules for multi-threaded executions differ here from those of C++11, the executions they allow are precisely the same. Visible sequences of side effects are a redundant restriction.
Suggested Technical Corrigendum
See above.Committee Discussion
Feb 2012 meeting
- The changes seem reasonable, there is a concern about having C and C++ differ.
- Should be contingent on Defect 402.
Committee Discussion
Oct 2012 meeting
- There was no work done on this defect between meetings.
- Seems more complicated than thought at the last meeting.
- It is not clear if WG 21 has considered these changes.
- Clark Nelson took an action item to liaise with WG 21.
Committee Discussion
This item has become WG21 Core issue 1466Apr 2013 meeting
Committee Discussion
There has been no discussion or action from WG21.Oct 2013 meeting
Committee Discussion
These changes have been proposed for the C++ working draft:
Change 1.10 paragraph 14 as follows:
The visible sequence of side effects on an atomic object M, with respect to a value computation B of M, is a maximal contiguous sub-sequence of side effects in the modification order of M, where the first side effect is visible with respect to B, and for every side effect, it is not the case that B happens before it.The value of an atomic object M, as determined by evaluation B, shall be the value stored by someoperation in the visible sequence of M with respect to Bside effect A that modifies M, where B does not happen before A. [Note:It can be shown that the visible sequence of side effects of a value computation is unique givenThe set of side effects that a given evaluation might take its value from is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. —end note]
1.10p20 should be changed as follows:
[ Note: Thevisible sequence of side effectsvalue observed by a load of an atomic depends on the "happens before" relation, which depends on the values observed by loads of atomics, which we are restricting here. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the "happens before" relation derived as described above, satisfy the resulting constraints as imposed here. —end note ]
I think 1.10p22 should be changed as follows:
[ Note: Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the"visible sequence"coherence rules. —end note ]
I believe the 29.3p3 wording should change as follows:
- the result of the last modification A of M that precedes B in S, if it exists, or
- if A exists, the result of some modification of M
in the visible sequence of side effects with respect to Bthat is notmemory_order_seq_cstand that does not happen before A, or- if A does not exist, the result of some modification of M
in the visible sequence of side effects with respect to Bthat is notmemory_order_seq_cst.
Apr 2014 meeting
Committee Discussion
WG21 liaison has been asked to ascertain status of this w.r.t. C++14 and to provide a suggested TC.