https://bugs.llvm.org/show_bug.cgi?id=34842
Bug ID: 34842
Summary: void cast: expression not evaluated for its side
effects
Product: clang
Version: unspecified
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: f.holle...@gmx.net
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
I usually use a (void) cast to express that a expression should be evaluated
for its side effect, but that the result is discarded.
Example:
extern volatile uint32_t* uart_rx_reg;
void foo()
{
// do something here
:
// uart_rx_reg is accessed for its side effect to empty the receive
register
(void) *uart_rx_reg; // read and discard received character
}
This complies to MISRA C 2012 Rule 2.2, where you can find a similar example.
> A cast to void is assumed to indicate a value that is intentionally not
> being used. The cast is therefore not dead code itself. It is treated
> as using its operand which is therefore also not dead code.
This also complies to ISO/IEC 9899:1999 C standard:
> 6.3.2.2 void
> The (nonexistent) value of a void expression (an expression that has
> type void) shall not be used in any way, and implicit or explicit
> conversions (except to void) shall not be applied to such an expression.
> If an expression of any other type is evaluated as a void expression,
> its value or designator is discarded. (A void expression is evaluated for
> its side effects.)
Unfortunately, clang does not behave this way. It issues a warning and
optimizes away the expression.
https://godbolt.org/g/Ts3L3P
After some discussion on
https://github.com/isocpp/CppCoreGuidelines/issues/1048 I think this is an
abnormality of clang which should be fixed.
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