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CppCon 2015 Racing The File System--Niall Douglas

By Adrien Hamelin | Aug 22, 2016 01:44 PM | Tags: boost advanced

Have you registered for CppCon 2016 in September? Don’t delay – Registration is open now.

While we wait for this year’s event, we’re featuring videos of some of the 100+ talks from CppCon 2015 for you to enjoy. Here is today’s feature:

Racing The File System

by Niall Douglas

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

Almost every programmer knows about and fears race conditions on memory where one strand of execution may concurrently update data in use by another strand of execution, leading to an inconsistent and usually dangerous inconsistent read of program state. Almost every programmer therefore is aware of mutexes, memory ordering, semaphores and the other techniques used to serialise access to memory.

Interestingly, most programmers are but vaguely aware of potential race conditions on the filing system, and as a result write code which assumes that the filing system does not suddenly change out from underneath you when you are working on it. This assumption of a static filing system introduces many potential security bugs never mind ways of crashing your program, and of course creating data loss and corruption.

This workshop will cover some of the ways in which filing system races can confound, and what portable idioms and patterns you should employ to prevent misoperation, even across networked Samba shares. Finally, an introduction of the proposed Boost library AFIO will be made which can help application developers writing filing system race free code portably.

Quick Q: Efficiency of postincrement v.s. preincrement in C++

By Adrien Hamelin | Aug 15, 2016 12:55 PM | Tags: performance advanced

Quick A: The difference is only marginal with optimizations enabled.

Recently on SO:

Efficiency of postincrement v.s. preincrement in C++

It is true - although perhaps overly strict. Pre increment doesn't necessarily introduce a data dependency - but it can.

A trivial example for exposition:

a = b++ * 2;

Here, the increment can be executed in parallel with the multiplication. The operands of both the increment and the multiplication are immediately available and do not depend on the result of either operation.

Another example:

a = ++b * 2;

Here, the multiplication must be executed after the increment, because one of the operands of the multiplication depends on the result of the increment.

Of course, these statements do slightly different things, so the compiler might not always be able to transform the program from one form to the other while keeping the semantics the same - which is why using the post increment might make a slight difference in performance.

A practical example, using a loop:

for(int i= 0; arr[i++];)
    count++;

for(int i=-1; arr[++i];)
    count++;

One might think that the latter is necessarily faster if they reason that "post-increment makes a copy" - which would have been very true in the case of non-fundamental types. However, due to the data dependency (and because int is a fundamental type with no overload function for increment operators), the former can theoretically be more efficient. Whether it is depends on the cpu architecture, and the ability of the optimizer.

For what it's worth - in a trivial program, on x86 arch, using g++ compiler with optimization enabled, the above loops had identical assembly output, so they are perfectly equivalent in that case.

 

Rules of thumb:

If the counter is a fundamental type and the result of increment is not used, then it makes no difference whether you use post/pre increment.

If the counter is not a fundamental type and the result of the increment is not used and optimizations are disabled, then pre increment may be more efficient. With optimizations enabled, there is no difference.

If the counter is a fundamental type and the result of increment is used, then post increment can theoretically be marginally more efficient - in some cpu architecture - in some context - using some compiler.

If the counter is a complex type and the result of the increment is used, then pre increment is typically faster than post increment. Also see R Sahu's answer regarding this case.

Memory consistency made simple(ish)--Glennan Carnie

By Adrien Hamelin | Jul 29, 2016 01:21 PM | Tags: advanced

How to synchronize between threqds or

Memory consistency made simple(ish)

by Glennan Carnie

From the article:

The C++11 memory consistency model is probably one of the most significant aspects of Modern C++; and yet probably one of the least well-understood.  I think the reason is simple:  it’s really difficult to understand what the problem actually is.

The memory consistency problem is a concurrency problem.  That is, it’s a problem that occurs when we start writing multi-threaded code.  More specifically, it’s a parallelism problem – the real subtleties occur when you have two or more processors executing code...

 

Zeroing Memory is Hard (VC++ 2015 arrays)--Bruce Dawson

By Adrien Hamelin | Jul 20, 2016 02:08 PM | Tags: performance advanced

Here is a curious behaviour:

Zeroing Memory is Hard (VC++ 2015 arrays)

by Bruce Dawson

From the article:

Quick, what’s the difference between these two C/C++ definitions of initialized local variables?

char buffer[32] = { 0 };
char buffer[32] = {};

One difference is that the first is legal in C and C++, whereas the second is only legal in C++.

Okay, so let’s focus our attention on C++. What do these two definitions mean?

CppCon 2015 Integrating generators EDSL's for Spirit X3 (WIP)--Feliple Magno de Almeida

By Adrien Hamelin | Jul 20, 2016 02:05 PM | Tags: boost advanced

Have you registered for CppCon 2016 in September? Don’t delay – Registration is open now.

While we wait for this year’s event, we’re featuring videos of some of the 100+ talks from CppCon 2015 for you to enjoy. Here is today’s feature:

Integrating generators EDSL's for Spirit X3 (WIP)

by Feliple Magno de Almeida

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

Based on the presentation I made on C++Now 2015 for Developing EDSL's for Boost.Spirit V2, present the development of generators for Boost.Spirit X3 (next version of boost spirit) and how that can be used for higher abstraction EDSL's while, through template metaprogramming, create parsers and generators automatically from the same grammar, using CORBA format as an example, while dealing with endianness, alignment and asymmetric grammars. This work is based on the library mORBid (https://github.com/expertisesolutions...) and (https://github.com/expertisesolutions...).

Meeting C++ 2016: closing keynote & full schedule

By Meeting C++ | Jul 20, 2016 02:01 AM | Tags: intermediate experimental c++17 c++14 boost advanced

With the announcement of the closing keynote the full schedule for Meeting C++ 2016 stands!

Closing keynote & full schedule of Meeting C++ 2016

by Jens Weller

From the article:

Since mid of June the program of the 5th Meeting C++ conference was taking shape. With the selection of the talks it was also clear in which tracks they go, so that the schedule it self was almost ready, except a last detail: the closing keynote.

The closing keynote will be held by Louis Dionne on "C++ metaprogramming: evolution and future directions".

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Library--JWAKELY

By Adrien Hamelin | Jul 19, 2016 02:02 PM | Tags: experimental advanced

C++ continues to evolve:

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Library

by JWAKELY

From the article:

The recent WG21 meeting in Oulu, Finland, was an especially busy one for the Library Working Group. Every day was spent working through the list of proposals intended for inclusion in C++17, and we also had three “evening” sessions that ran well past the evening (until nearly midnight, although the sun was still up to trick us into working late)...

 

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Core Language--Jason Merrill

By Adrien Hamelin | Jul 19, 2016 02:00 PM | Tags: experimental advanced

C++ continues to evolve:

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Core Language

by Jason Merrill

From the article:

It was quite a trek to get to Oulu, Finland for the June 2016 C++ Standards Committee meeting, but we were warmly received and the meeting went well once we arrived. We had very pleasant weather most of the week, and it was fun to experience the midnight sun, even though it played havoc with my sleep schedule.

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Parallelism and Concurrency--Torvald Rie

By Adrien Hamelin | Jul 19, 2016 01:55 PM | Tags: experimental advanced

C++ continues to evolve:

Red Hat at the ISO C++ Standards Meeting (June 2016, Oulu): Parallelism and Concurrency

by Torvald Riegel

From the article:

Several Red Hat engineers recently attended the JTC1/SC22/WG21 C++ Standards Committee meetings in Oulu, Finland.  This post focuses on the sessions of SG1 (the standards committee sub-group 1 – for concurrency and parallelism) as well as on coroutines-related sessions. Jason already gave an overview of the meeting in his post.