r225116 - Volatile reads are side-effecting operations, but in the general case of access through a volatile-qualified type, we're not certain of the underlying object's side-effects on access.

Aaron Ballman aaron at aaronballman.com
Sat Jan 3 09:00:13 PST 2015


Author: aaronballman
Date: Sat Jan  3 11:00:12 2015
New Revision: 225116

URL: http://llvm.org/viewvc/llvm-project?rev=225116&view=rev
Log:
Volatile reads are side-effecting operations, but in the general case of access through a volatile-qualified type, we're not certain of the underlying object's side-effects on access.

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);

Modified:
    cfe/trunk/lib/AST/Expr.cpp
    cfe/trunk/test/SemaCXX/warn-unused-value.cpp

Modified: cfe/trunk/lib/AST/Expr.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/Expr.cpp?rev=225116&r1=225115&r2=225116&view=diff
==============================================================================
--- cfe/trunk/lib/AST/Expr.cpp (original)
+++ cfe/trunk/lib/AST/Expr.cpp Sat Jan  3 11:00:12 2015
@@ -3021,6 +3021,13 @@ bool Expr::HasSideEffects(const ASTConte
   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())

Modified: cfe/trunk/test/SemaCXX/warn-unused-value.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/warn-unused-value.cpp?rev=225116&r1=225115&r2=225116&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/warn-unused-value.cpp (original)
+++ cfe/trunk/test/SemaCXX/warn-unused-value.cpp Sat Jan  3 11:00:12 2015
@@ -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
 }
 }





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