[C] Silence unreachable -Watomic-access diagnostics (#140064)

Accessing the member of a structure or union which is _Atomic-qualified
is undefined behavior in C. We currently diagnose that with a warning
that defaults to an error. In turn, this means we reject a valid program
if the access it not reachable because of the error. e.g.,

  if (0)
    SomeAtomicStruct.Member = 12; // Was diagnosed

This silences the diagnostic if the member access is not reachable.

clang/docs/ReleaseNotes.rst

@@ -533,6 +533,19 @@ Improvements to Clang's diagnostics
 - A new off-by-default warning ``-Wms-bitfield-padding`` has been added to alert to cases where bit-field
   packing may differ under the MS struct ABI (#GH117428).
 
+- ``-Watomic-access`` no longer fires on unreachable code. e.g.,
+
+  .. code-block:: c
+
+    _Atomic struct S { int a; } s;
+    void func(void) {
+      if (0)
+        s.a = 12; // Previously diagnosed with -Watomic-access, now silenced
+      s.a = 12; // Still diagnosed with -Watomic-access
+      return;
+      s.a = 12; // Previously diagnosed, now silenced
+    }
+
 
 - A new ``-Wcharacter-conversion`` warns where comparing or implicitly converting
   between different Unicode character types (``char8_t``, ``char16_t``, ``char32_t``).

clang/lib/Sema/SemaExprMember.cpp

@@ -1385,7 +1385,7 @@ static ExprResult LookupMemberExpr(Sema &S, LookupResult &R,
   // lvalue. Because this is inherently unsafe as an atomic operation, the
   // warning defaults to an error.
   if (const auto *ATy = BaseType->getAs<AtomicType>()) {
-    S.DiagRuntimeBehavior(OpLoc, nullptr,
+    S.DiagRuntimeBehavior(OpLoc, BaseExpr.get(),
                           S.PDiag(diag::warn_atomic_member_access));
     BaseType = ATy->getValueType().getUnqualifiedType();
     BaseExpr = ImplicitCastExpr::Create(

clang/test/Sema/atomic-expr.c

@@ -114,6 +114,23 @@ void func_16(void) {
   (void)sizeof(xp->val);
   (void)sizeof(y.ival);
   (void)sizeof(yp->ival);
+
+  // Also, do not diagnose in unreachable code paths.
+  {
+    if (0) {
+      x.val = 12;
+      xp->val = 12;
+      (void)y.ival;
+      (void)yp->ival;
+    }
+
+    return;
+
+    x.val = 12;
+    xp->val = 12;
+    (void)y.ival;
+    (void)yp->ival;
+  }
 }
 
 // Ensure that we correctly implement assignment constraints from C2x 6.5.16.1.