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

John McCall via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Tue Aug 28 17:05:51 PDT 2018

rjmccall added inline comments.

Comment at: lib/Sema/SemaChecking.cpp:10668
+  if (Source->isAtomicType() || Target->isAtomicType())
+    S.Diag(E->getBeginLoc(), diag::warn_atomic_implicit_seq_cst);
jfb wrote:
> rjmccall wrote:
> > Why would the target be an atomic type?  And if it is an atomic type, isn't that an initialization of a temporary?  In what situation does it make sense to order the initialization of a temporary?
> In this case:
> ```
> void bad_assign_1(int i) {
>   atom = i; // expected-warning {{implicit use of sequentially-consistent atomic may incur stronger memory barriers than necessary}}
> }
> ```
> We want to warn on assignment to atomics being implicitly `seq_cst`.
> Though you're right, initialization shouldn't warn because it isn't atomic. The issue is that `ATOMIC_VAR_INIT` is what needs to get used, and that's weird to test. I'll add a test that just assigns (which is what `ATOMIC_VAR_INIT` expands to for clang), and I'll need to update the code to detect that pattern and avoid warning in that case. I guess I have to look at the `Expr` and figure out if the LHS is a `Decl` or something like that.
Do we really implicitly convert the RHS of that assignment to atomic type?  That seems wrong; `_Atomic` is really a type qualifier, and the RHS should not be converted to `_Atomic(T)` any more than it would be converted to `volatile T` for an assignment into a `volatile` l-value.

  rC Clang


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