[PATCH] Implement throw in conditional operator

[Marius Wachtler]

Hi

I added a testcase for the function you mention but I also added const to
the return type because otherwise gcc won't compile it ("error: invalid
initialization of non-const reference of type ‘int&’ from an rvalue of type
‘int’") - clang compiles both.

The generated code from clang for the function looks correct and works as I
would expect it.
Compiling it with g++ 4.8.2 and -O2 will instead produce and object which
will return an address to a temporary in the function and not a reference
to the global val.

I'm not sure if clangs behavior is correct or if it should also only accept
a const reference? If the error is somewhere else I would appreciate some
guidance to where to look around and what the expected behavior is.

The C++11 non final standard I have says (5.16-2):
"If either the second or the third operand has type (possibly cv-qualified)
void, then the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and
function-to-pointer (4.3) standard conversions are performed on the second
and third operands, and one of the following shall hold:
— The second or the third operand (but not both) is a throw-expression
(15.1); the result is of the type
of the other and is a prvalue.
— Both the second and the third operands have type void; the result is of
type void and is a prvalue. [ Note: This includes the case where both
operands are throw-expressions. — end note ]
"
So this means gcc is correct?

Thank you very much
- Marius





On Wed, Jun 11, 2014 at 1:05 AM, Richard Smith <richard at metafoo.co.uk>
wrote:

> Please also include a test that the produced lvalue actually has the right
> value. Something like:
>
> int &test7(bool cond) {
>   return cond ? throw 1 : val;
> }
>
> ... and check that the value returned is @_Z3val
>
> On Tue, Jun 10, 2014 at 3:01 PM, Marius Wachtler <undingen at gmail.com>
> wrote:
>
>> Hi
>> Attached you can find an updates patch.
>>
>>
>>
>>
>> On Tue, Jun 10, 2014 at 11:50 PM, David Majnemer <
>> david.majnemer at gmail.com> wrote:
>>
>>> +
>>>> +  case Expr::CXXThrowExprClass: {
>>>> +    const auto* throwExpr = cast<CXXThrowExpr>(E);
>>>>
>>>
>>> Please stick the star on the RHS. Also, it is customary in LLVM and
>>> clang to spell it like "ThrowExpr"
>>>
>>>
>>>> +    EmitCXXThrowExpr(throwExpr);
>>>> +    const QualType subExprType = throwExpr->getSubExpr()->getType();
>>>>
>>>
>>> Likewise for "subExprType"
>>>
>>> +    llvm::Type *Ty =
>>>> llvm::PointerType::getUnqual(ConvertType(subExprType));
>>>> +    return MakeAddrLValue(llvm::UndefValue::get(Ty), subExprType);
>>>>    }
>>>
>>>
>>>
>>> On Tue, Jun 10, 2014 at 5:19 PM, Marius Wachtler <undingen at gmail.com>
>>> wrote:
>>>
>>>> Hello
>>>>
>>>> Attached you can find my first clang patch which should implement
>>>> "cond ? throw 1 : val".
>>>>
>>>> Without this patch I get: "cannot compile this l-value expression yet"
>>>>
>>>> As this is my first time looking under the hood of clang I'm not sure
>>>> if this is the right approach. I'm Looking forward to your feedback.
>>>>
>>>> - Marius Wachtler
>>>>
>>>> _______________________________________________
>>>> cfe-commits mailing list
>>>> cfe-commits at cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>>>>
>>>>
>>>
>>
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