C++20以降のメモリモデル規定で重要な二項関係：

• happens before
  • フルスペック仕様(consume考慮あり)、非推移的
• simply happens before
  • consume操作のない世界において推移的
• strongly happens before
  • 全てがseq_cst操作の世界において推移的

C++17では simply happens before 相当仕様が strongly happens before と呼ばれていた。C++20で名称変更。

Paper

Repairing Sequential Consistency in C/C++11

Common Compiler Optimisations are Invalid in the C11 Memory Model and what we can do about it

WG21

P0668R5: Revising the C++ memory model

Summarizing, we continue to have, as we did before:

(plain) happens-before
　　The fundamental relation used to reason about data races
strong happens before
　　The ordering relation usually promised by libraries to ensure usability and composability in all contexts

The simply-happens-before relation is an atifact of our definition. In the absence of

1. memory_order_consume, and
2. mixed use of both acquire/release and seq_cst operations on the same location

all three definitions coincide.

LWG3941. §[atomics.order] inadvertently prohibits widespread implementation techniques
(https://wg21.cmeerw.net/lwg/issue3941)

The problem here is an error during the attempt to simplify and translate plv.mpi-sws.org/scfix/paper.pdf to standardese. There is no known issue with the underlying paper.

StackOverflow

• What does "strongly happens before" mean?
• What is the significance of 'strongly happens before' compared to '(simply) happens before'?
• What is the difference between "happens before" and "precedes in a single total order" relations for memory_order_seq_cst operations?