[PATCH] D51084: Implement -Watomic-implicit-seq-cst

[JF Bastien via Phabricator via cfe-commits]

jfb added a comment.

In https://reviews.llvm.org/D51084#1216913, @rjmccall wrote:

> It says the type of the assignment expression, not the type of the LHS.
>
> C11 [6.5.16]p2: "The type of an assignment expression is the type the left operand would have after lvalue conversion."
>
> C11 [6.3.2.1]p2: "...this is called lvalue conversion. If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic version of the type of the lvalue; otherwise, the value has the type of the lvalue."
>
> The RHS is not converted to have atomic type.


Right that would be nonsensical because you'd need to load it atomically. The C `_Atomic` semantics match what C++ `std::atomic` does:

  _Atomic(int) atom;
  void ass(int i) {
      atom = i;
  }

is the same as:

  #include <atomic>
  std::atomic<int> atom;
  void ass(int i) {
      atom = i;
  }

They're meant to be interchangeable.

This warning is meant to warn on C's implicit `seq_cst`. When I get to writing the C++ version of the warning I'll warn about C++'s implicit `seq_cst`.


Repository:
  rC Clang

https://reviews.llvm.org/D51084