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Document number: N3993

Date: 2014-05-23

On Parallel Invocations of Functions in Parallelism TS

by Artur Laksberg

Excerpt:

Abstract

This document proposes a change to N3989 (Technical Specification for C++ Extensions for Parallelism).

Introduction

At the Issaquah meeting in February 2014, Hans Boehm raised the following issue regarding N3850 (Working Draft, Technical Specification for C++ Extensions for Parallelism):

It seems to me that the execution policies need to be a bit more precise about which calls to parameter functions they can make. Something needs to specify that sort may make parallel invocations to swap, but only if the arguments for concurrent calls don’t overlap. I didn’t quickly find such text. In general, “non-racing” calls can be made concurrently, but “racing” ones cannot.

Upon further discussion, it was decided that the resolution of this issue warrants a separate paper that can be presented for discussion in SG1 during the Rapperswil meeting.

Proposed Resolution

First, we introduce changes to 17.6.5.9 [res.on.data.races]/8 that state that an implementation cannot introduce a data race on objects accessed during the execution of the algorithms.

Additionally, instead of talking about “applying user-defined function objects”, we define an umbrella term “element access functions”, which includes the functions required by the specification, as well as the user-provided function objects. We then define the behavior in terms of “invoking the element access functions”.