Question about FunctionDecl::isVariadic()

[Aaron Ballman via cfe-commits]

On Fri, Oct 2, 2015 at 4:01 PM, mats petersson <mats at planetcatfish.com> wrote:
> Since "varargs" often involve some kind of special passing mechanisms [I've
> seen implementations that build data block and pass a pointer to that,
> rather than passing on the stack, for example], or additional code in the
> recipient function, I would say that `f2()` does not mean `f2(...)`.

I kind of wondered if that was the case. If that's reality, perhaps we
may want to consider naming it FunctionDecl::hasVarArgs() or something
that specifies the difference is in whether we need to care about
packaging up the argument list so it can be used with va_start() and
friends?

~Aaron

>
> --
> Mats
>
> On 2 October 2015 at 20:16, Aaron Ballman via cfe-commits
> <cfe-commits at lists.llvm.org> wrote:
>>
>> Given the following two function declarations:
>>
>> void f1(...);
>> void f2();
>>
>> It makes sense to me that isVariadic() returns true for f1 in both C
>> and C++. It makes sense to me that isVariadic() returns false for f2
>> in C++. I am confused as to why it returns false instead of true for
>> C, however.
>>
>> In C11, 6.7.6.3p9 states: If the list terminates with an ellipsis (,
>> ...), no information about the number or types of the parameters after
>> the comma is supplied.
>>
>> p14 states, in part: "The empty list in a function declarator that is
>> not part of a definition of that function specifies that no
>> information about the number or types of the parameters is supplied."
>>
>> It seems to me that for function *declarations* (not definitions),
>> isVariadic() should return true for f2 in C. Is there a reason it
>> doesn't currently behave that way, or is this a bug?
>>
>> I ask because I was writing an AST matcher for isVariadic() for an
>> in-progress checker, but the checker was failing to catch that f2 was
>> a variadic function. I am not certain whether
>> FunctionDecl::isVariadic() should be changed, whether the AST matcher
>> isVariadic() should be smarter about C code, or whether the checker
>> itself needs to be smarter about this particular behavior in C code.
>> My gut reaction is that FunctionDecl::isVariadic() has a bug, but from
>> looking at code elsewhere, everything seems to assume isVariadic()
>> implies the ellipsis, which makes me think I just need to make my
>> checker smarter.
>>
>> Thanks!
>>
>> ~Aaron
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>