[cfe-dev] C++ and volatile lvalue-to-rvalue

[Sebastian Redl]

On 13.10.2011 14:24, Sebastian Redl wrote:
> On 13.10.2011 13:43, Abramo Bagnara wrote:
>> I want to put this gcc bug report to clang developer attention.
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364
>>
>> The gcc developers believe that according to the standard
>>
>> volatile int v;
>> x = v = 3;
>>
>> does not imply necessarily a read from v.
>>
>> clang instead add a lvalue-to-rvalue implicit cast outside v = 3
>> assignment (and there is general consensus that lvalue-to-rvalue *is*
>> the marker for memory read access).
>>
> C++11 5.17 [expr.ass]p1 says:
> "The assignment operator (=) and the compound assignment operators all 
> group right-to-left. All require a modifiable lvalue as their left 
> operand and *return an lvalue referring to the left operand*."
> So the result of (v = 3) is an lvalue referring to v, just as the 
> expression (v) alone. Therefore the code is equivalent to the two 
> separate statements:
> v = 3;
> x = v;
>
> Clang is right, GCC is wrong.
To clarify: this is only C++. In C, the situation is different. C99 
6.5.16p3 says:
"An assignment expression has the value of the left operand after the 
assignment, but is not an lvalue."

So in C, GCC is right, and Clang is wrong.

It could be considered a defect of one of the two languages that they 
differ in such a subtle detail.

Sebastian
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