Why Does `std::atomic` Use `XCHG` for Sequentially Consistent Stores?
Nov 24, 2024 am 01:37 AMWhy std::atomic Uses XCHG for Sequential Consistency Stores
Background
In the realm of multithreading, the std::atomic class provides a means for concurrent access to shared data across threads while ensuring data integrity. Its store member function allows for writing values to an atomic variable with specified memory ordering semantics.
In the case of sequential consistency (std::memory_order_seq_cst), x86 architectures employ an xchg instruction to implement atomic stores. This instruction performs a simultaneous exchange of values rather than a simple store operation.
Motivation for XCHG
While it might appear that a straightforward store instruction coupled with a memory barrier (e.g., _ReadWriteBarrier() or asm volatile("" ::: "memory");) would suffice for sequential consistency, using xchg has several advantages:
1. Full Memory Barrier: xchg acts as a complete memory fence on x86 due to its implicit lock prefix. This ensures that all memory operations before and after xchg are ordered, effectively preventing memory reordering.
2. Release Semantics are Insufficient: A normal store operation on x86 exhibits release semantics, which allows reordering with subsequent operations, including acquire loads. Sequential consistency, on the other hand, demands that such reordering is prohibited.
Performance Considerations
The choice between xchg and mov mfence for atomic stores has performance implications:
- Skylake: mfence stalls out-of-order execution of ALU instructions, while xchg does not. However, xchg carries a false dependency on the previously loaded value.
- AMD: The hardware optimization manual recommends using xchg for sequential consistency stores.
- GCC/Clang Optimization: Modern compilers typically prefer xchg over mov mfence.
Alternative for Thread Fences
Apart from using xchg for atomic stores, other options for implementing atomic thread fences (also with the seq_cst memory ordering) include:
- lock add to the stack
- lock or dword [rsp], 0
Distinguishing Release and Acquire
It is important to note that:
- A store with sequential consistency does not imply acquire semantics.
- asm volatile("" ::: "memory"); is a compiler barrier only and does not enforce sequential consistency.
- Emulating sequential consistency with weaker order operations and fences may not align entirely with the C abstract machine model.
Conclusion
In summary, std::atomic stores with sequential consistency on x86 architectures utilize xchg due to its full memory barrier effect and compatibility with the requirements of sequential consistency. While alternative implementations exist, xchg remains a widely adopted solution for its efficiency and compliance with industry recommendations.
The above is the detailed content of Why Does `std::atomic` Use `XCHG` for Sequentially Consistent Stores?. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

Yes, function overloading is a polymorphic form in C, specifically compile-time polymorphism. 1. Function overload allows multiple functions with the same name but different parameter lists. 2. The compiler decides which function to call at compile time based on the provided parameters. 3. Unlike runtime polymorphism, function overloading has no extra overhead at runtime, and is simple to implement but less flexible.

C has two main polymorphic types: compile-time polymorphism and run-time polymorphism. 1. Compilation-time polymorphism is implemented through function overloading and templates, providing high efficiency but may lead to code bloating. 2. Runtime polymorphism is implemented through virtual functions and inheritance, providing flexibility but performance overhead.

Yes, polymorphisms in C are very useful. 1) It provides flexibility to allow easy addition of new types; 2) promotes code reuse and reduces duplication; 3) simplifies maintenance, making the code easier to expand and adapt to changes. Despite performance and memory management challenges, its advantages are particularly significant in complex systems.

C destructorscanleadtoseveralcommonerrors.Toavoidthem:1)Preventdoubledeletionbysettingpointerstonullptrorusingsmartpointers.2)Handleexceptionsindestructorsbycatchingandloggingthem.3)Usevirtualdestructorsinbaseclassesforproperpolymorphicdestruction.4

People who study Python transfer to C The most direct confusion is: Why can't you write like Python? Because C, although the syntax is more complex, provides underlying control capabilities and performance advantages. 1. In terms of syntax structure, C uses curly braces {} instead of indentation to organize code blocks, and variable types must be explicitly declared; 2. In terms of type system and memory management, C does not have an automatic garbage collection mechanism, and needs to manually manage memory and pay attention to releasing resources. RAII technology can assist resource management; 3. In functions and class definitions, C needs to explicitly access modifiers, constructors and destructors, and supports advanced functions such as operator overloading; 4. In terms of standard libraries, STL provides powerful containers and algorithms, but needs to adapt to generic programming ideas; 5

Polymorphisms in C are divided into runtime polymorphisms and compile-time polymorphisms. 1. Runtime polymorphism is implemented through virtual functions, allowing the correct method to be called dynamically at runtime. 2. Compilation-time polymorphism is implemented through function overloading and templates, providing higher performance and flexibility.

C polymorphismincludescompile-time,runtime,andtemplatepolymorphism.1)Compile-timepolymorphismusesfunctionandoperatoroverloadingforefficiency.2)Runtimepolymorphismemploysvirtualfunctionsforflexibility.3)Templatepolymorphismenablesgenericprogrammingfo

C polymorphismisuniqueduetoitscombinationofcompile-timeandruntimepolymorphism,allowingforbothefficiencyandflexibility.Toharnessitspowerstylishly:1)Usesmartpointerslikestd::unique_ptrformemorymanagement,2)Ensurebaseclasseshavevirtualdestructors,3)Emp
