Optimizing the Unoptimizable: A Journey to Faster C++ Compile Times -- Victor Zverovich

By Blog Staff | Feb 20, 2024 10:49 AM | Tags: None

In this post, Victor talks about bringing compile times of the {fmt} library on par with the C standard I/O library (stdio).

Optimizing the Unoptimizable: A Journey to Faster C++ Compile Times

by Victor Zverovich

From the article:

First some background: {fmt} is a popular open-source formatting library for C++ that provides a better alternative to C++ iostreams and C stdio. It has already surpassed stdio in many areas:

• Type safety with compile-time format string checks available by default since C++20 and as an opt in for C++14/17. Runtime format strings are also safe to use in {fmt} which is impossible to achieve in printf.
• Extensibility: user-defined type can be made formattable and most standard library types such as containers, dates and times are formattable out of the box.
• Performance: {fmt} is significantly faster than common standard library implementations of printf, in some cases by an order of magnitude (e.g. on floating-point formatting).
• Portable Unicode support.

However, one area where stdio remained significantly better was compile times. 

Seastar, ScyllaDB, and C++23

By Jordi Mon Companys | Feb 17, 2024 05:36 AM | Tags: c++20 c++17

Seastar announces that now that C++23 is available, they will support C++23 and C++20 (dropping support for C++17) in accordance with their support policy

Seastar, ScyllaDB, and C++23

By Avi Kivity

From the article:

Seastar is an open-source (Apache 2.0 licensed) C++ framework for I/O intensive asynchronous computing, using the thread-per-core model. Seastar underpins several high- performance distributed systems: ScyllaDB, Redpanda, and Ceph Crimson. Seastar source is available on github. As a C++ framework, Seastar must choose which C++ versions to support. The support policy is last-two-versions. That means that at any given time, the most recently released version as well as the previous one are supported, but earlier versions cannot be expected to work. This policy gives users of the framework three years to upgrade to the next C++ edition while not constraining Seastar to ancient versions of the language.

Now that C++23 has been ratified, Seastar now officially supports C++20 and C++23. The previously supported C++17 is now no longer supported.

The Case of the Vector With an Impossibly Large Size -- Raymond Chen

By Blog Staff | Feb 16, 2024 10:46 AM | Tags: None

In this software troubleshooting case, a customer experienced program crashes, and a detailed analysis of the code revealed several issues. The primary problem stemmed from lazy initialization of a widget list, leading to inconsistent vector states and potential crashes. Additionally, a multithreading issue was identified, highlighting the importance of thread-safety mechanisms in code that can be accessed concurrently.

The Case of the Vector With an Impossibly Large Size

by Raymond Chen

From the article:

A customer had a program that crashed with this stack:

contoso!Widget::GetCost
contoso!StandardWidgets::get_TotalCost+0x12f
rpcrt4!Invoke+0x73
rpcrt4!Ndr64StubWorker+0xb9b
rpcrt4!NdrStubCall3+0xd7
combase!CStdStubBuffer_Invoke+0xdb
combase!ObjectMethodExceptionHandlingAction<<lambda_...> >+0x47
combase!DefaultStubInvoke+0x376
combase!ServerCall::ContextInvoke+0x6f3
combase!ComInvokeWithLockAndIPID+0xacb
combase!ThreadInvoke+0x103
rpcrt4!DispatchToStubInCNoAvrf+0x18
rpcrt4!RPC_INTERFACE::DispatchToStubWorker+0x1a9
rpcrt4!RPC_INTERFACE::DispatchToStubWithObject+0x1a7
rpcrt4!LRPC_SCALL::DispatchRequest+0x308
rpcrt4!LRPC_SCALL::HandleRequest+0xdcb
rpcrt4!LRPC_SASSOCIATION::HandleRequest+0x2c3
rpcrt4!LRPC_ADDRESS::HandleRequest+0x183
rpcrt4!LRPC_ADDRESS::ProcessIO+0x939
rpcrt4!LrpcIoComplete+0xff
ntdll!TppAlpcpExecuteCallback+0x14d
ntdll!TppWorkerThread+0x4b4
kernel32!BaseThreadInitThunk+0x18
ntdll!RtlUserThreadStart+0x21

They wondered if some recent change to Windows was the source of the problem, since it didn’t happen as much in earlier versions of Windows.

 

2024-02 Mailing Available

By Administrator | Feb 15, 2024 11:19 AM | Tags: None

The 2024-02 mailing of new standards papers is now available.

SObjectizer Tales – 19. Thread-safe handlers--Marco Arena

By Marco Arena | Feb 14, 2024 11:31 PM | Tags: sobjectizer intermediate c++20

A new episode of the series about SObjectizer and message passing:

SObjectizer Tales – 19. Thread-safe handlers

by Marco Arena

From the article:

In this episode we explore thread-safe message handlers and revisit the problem of distributing work across multiple threads.

DR20 - The Equality Operator You Are Looking For -- Sandor Dargo

By Blog Staff | Feb 13, 2024 05:34 PM | Tags: None

We delve into how C++20 introduced changes to object comparisons, leading to unexpected behavior and the need for matching operator!= declarations.

DR20 - The Equality Operator You Are Looking For

by Sandor Dargo

From the article:

When I see DR, I immediately think about Disaster Recovery. That’s due to my first corporate job where I worked as a Database Administrator and we had regular exercises to simulate events when datacenters would be unavailable.

When you see DR in the title of a C++ proposal, it’s not about a disaster, it’s more about a bug. DR stands for defect report. But the paper itself is usually not about reporting that there is a problem - that has been already done -, but more about proposing a solution.

What’s more important is that defect reports are not becoming part of the latest standard, they retrospectively change the one that introduced the defect.

A defect of C++20’s equality operator

P2468R2 is addressing a problem that was introduced by C++20. C++20 brought us the spaceship operator (operator<=>), but it further changed the logic of object comparisons. It changed the meaning of == and != and how overload resolution applies to them.

Aggregates: C++17 vs. C++20 -- Andreas Fertig

By Blog Staff | Feb 9, 2024 01:05 PM | Tags: None

Sometimes the small changes between two C++ standards really bite you. Today's post is about when I got bitten by a change to aggregates in C++20.

Aggregates: C++17 vs. C++20

by Andreas Fertig

From the article:

Attendees of my training classes usually assume that I know everything. I can say sorry, but that's not the case. One day in the past, I showed the following example during a class:

struct Point {
  int x;
  int y;
};

Point pt{2, 3};

The class did cover C++17 and C++20. The code of Point is a reduced version for this post. We were talking about C++17's structured bindings. I use Point to show the decomposition using C++ Insights.

I showed the behavior and certain variations while answering questions from the attendees. One question was about move and copy. The question leads me changing the initial code to the following one:

2 3 4 5 6 7 8

struct Point { int x; int y; Point(Point&&) = delete; A }; Point pt{2, 3};

2
3
4
5
6
7
8

struct Point {
  int x;
  int y;

  Point(Point&&) = delete;  A 
};

Point pt{2, 3};

As you can see, in A, I deleted the move constructor. As far as I remember, this topic was more or less the last one for this day. Everything went well and as expected.

The next morning one of the attendees approached me with the question of why the code didn't compile on his machine. My usual answer here is that if C++ were be easy, I wouldn't have the job I have. Somehow this never shuts down such questions. I like curious attendees.

He shared his code via Compiler Explorer. Guess what? His code looked exactly like mine but didn't compile. Here is the error message from the compiler:

Machine Code Explained -- Matt Godbolt

By Blog Staff | Feb 8, 2024 05:34 PM | Tags: None

Explaining machine code from the ground up! 

Machine Code Explained

by Matt Godbolt

From the video:

In this video, Matt Godbolt appears on Computerphile to discuss their fascination with how computers work and their mental model developed in the 1980s, which still helps them understand modern computer systems. They use a simple analogy involving a robot with an abacus and pigeonholes to explain fundamental computer operations and how programs are executed. Matt then demonstrates how machine code, the computer's language, can be represented as a sequence of numbers and stored in memory to instruct the computer in performing tasks, illustrating the basic concept of programming.

SObjectizer Tales – 18. Limiting message chains--Marco Arena

By Marco Arena | Feb 8, 2024 12:36 AM | Tags: sobjectizer intermediate c++20

A new episode of the series about SObjectizer and message passing:

SObjectizer Tales – 18. Limiting message chains

by Marco Arena

From the article:

In this episode we explore limited message chains as an alternative way to regulating message flow.

What are senders good for, anyway? -- Eric Niebler

By Eric Niebler | Feb 6, 2024 12:46 PM | Tags: c++26

In a recent blog post, Eric Niebler gets inside the soon-to-be-standard async programming model and shows why you should care.

What are senders good for, anyway?

From the article:

At the heart of P2300 is a simple, elegant (IMHO) core that brings many benefits, but it’s hard to see that forest for all the trees.

So let’s make this concrete. In this post, I’ll show how to bring a crusty old C-style async API into the world of senders, and why you might want to do that.