Volatile reads are side-effecting operations, but in the general case of access throu...

Treat volatile accesses as "maybe" instead of "definite" side effects for the purposes of warning on evaluations in an unevaluated context. No longer diagnose on idiomatic code like:

int * volatile v;
(void)sizeof(*v);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225116 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/AST/Expr.cpp b/lib/AST/Expr.cpp
index 818b60ca50bde9137393e6c0628391e22b6ed35e..21e668f68ffd6c080f797104e972c8c1d3c21b21 100644 (file)
--- a/lib/AST/Expr.cpp
+++ b/lib/AST/Expr.cpp
@@ -3021,6 +3021,13 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case CXXReinterpretCastExprClass:
   case CXXConstCastExprClass:
   case CXXFunctionalCastExprClass: {
+    // While volatile reads are side-effecting in both C and C++, we treat them
+    // as having possible (not definite) side-effects. This allows idiomatic
+    // code to behave without warning, such as sizeof(*v) for a volatile-
+    // qualified pointer.
+    if (!IncludePossibleEffects)
+      break;
+
     const CastExpr *CE = cast<CastExpr>(this);
     if (CE->getCastKind() == CK_LValueToRValue &&
         CE->getSubExpr()->getType().isVolatileQualified())

diff --git a/test/SemaCXX/warn-unused-value.cpp b/test/SemaCXX/warn-unused-value.cpp
index 3be2844aba7724c9d63e0e2b0685128328047437..efabd506306814358e1ad79b9e6ee274397a5294 100644 (file)
--- a/test/SemaCXX/warn-unused-value.cpp
+++ b/test/SemaCXX/warn-unused-value.cpp
@@ -90,9 +90,10 @@ void f() {
   Bad b;
   (void)typeid(b.f()); // expected-warning {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
 
-  // A dereference of a volatile pointer is a side effecting operation, despite
-  // it being a reasonable operation.
+  // A dereference of a volatile pointer is a side effecting operation, however
+  // since it is idiomatic code, and the alternatives induce higher maintenance
+  // costs, it is allowed.
   int * volatile x;
-  (void)sizeof(*x); // expected-warning {{expression with side effects has no effect in an unevaluated context}}
+  (void)sizeof(*x); // Ok
 }
 }