[PATCH] Implement throw in conditional operator

[Marius Wachtler]

Thanks for your expertise.
Attached you can find an updated patch where I removed the const and made
the FileCheck a little bit more specific.



On Wed, Jun 11, 2014 at 10:55 PM, Richard Smith <richard at metafoo.co.uk>
wrote:

> On Wed, Jun 11, 2014 at 1:25 PM, Marius Wachtler <undingen at gmail.com>
> wrote:
>
>> 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.
>>
>
> GCC doesn't implement one of the relevant DRs; you don't need to make the
> testcase work with it.
>
> 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.
>>
>
> See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1550;
> Clang's behavior is right, this is just a bug with IR generation.
>
> 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
>>>>>>
>>>>>> _______________________________________________
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>>>>>> cfe-commits at cs.uiuc.edu
>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>>>>>>
>>>>>>
>>>>>
>>>>
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>>>>
>>>
>>
>
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