C++ - [Concurrency 3] Memory Model - Rico贾若童的博客

Introduction

The C++ memory model was formally introduced in C++11 mainly for multithreading. Before C++11:

Threading was platform-dependent.
Behavior of shared memory and data races was undefined and left up to the OS/hardware/compiler.
- 🫠️ “Pthreads lib assume no data race.” Pthreads are a thin wrapper over OS-level primitives and don’t prevent data races. It’s the programmer’s job to synchronize correctly.

C++11 added:

A standard memory model.
- Compared to Pthreads, the C++11 memory model enforces stricter semantics, which helps correctness but might introduce overhead compared to bare-metal threading APIs like Pthreads.
- ❗️ “Memory ordering: Thou shalt not modify the behavior of a single-threaded program”. The compiler is free to reorder instructions for optimization as long as the observable behavior of a single-threaded program doesn’t change — this is called the as-if rule. But in multithreaded programs, these reorderings can cause data races unless explicitly synchronized, which is why C++11 introduced a well-defined memory model.
std::thread, std::mutex, std::atomic types

This brought C++ more in line with Java/C# in terms of native threading support.

Example: Memory Reordering by the Compiler

C++ compilers can reorder instructions as part of optimization — as long as the observable behavior in a single-threaded program is preserved (per the as-if rule). This can lead to subtle issues in multi-threaded programs.

source

cpp code

  int A, B;
  void foo()
  {
      A = B + 1;
      B = 0;
  }

without optimization:

  $ gcc -S -masm=intel foo.c
  $ cat foo.s
  ...
  mov     eax, DWORD PTR _B  (redo this at home...)
  add     eax, 1
  mov     DWORD PTR _A, eax
  mov     DWORD PTR _B, 0
  ...

With Optimization:

  $ gcc -O2 -S -masm=intel foo.c
  $ cat foo.s
  ...
  mov     eax, DWORD PTR B
  mov     DWORD PTR B, 0
  add     eax, 1
  mov     DWORD PTR A, eax
  ...

Why this matters: One can see that it’s possible that B (which may be atomic) can be ready before A (which may/may not be atomic). In a multithreaded context, this reordering can cause issues. For example, another thread might observe B == 0 while A has not yet been updated. This can violate intended synchronization logic.

How to prevent: Use std::atomic with proper memory ordering (e.g., memory_order_seq_cst) to prevent unwanted reordering.

Atomics act as compiler and CPU fences, ensuring ordering constraints are respected where required.

C++ - [Concurrency 3] Memory Model

Introduction

Example: Memory Reordering by the Compiler

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