Re: [PATCH] PR c/86407 - Add option to ignore fndecl attributes on function pointers

[Alex Henrie <alexhenrie24@gmail.com>]

On Fri, May 31, 2019 at 1:38 AM Richard Biener <rguenther@suse.de> wrote:
>
> On Thu, 30 May 2019, Alex Henrie wrote:
>
> > In Wine we need a way to (without warnings) put ms_hook_prologue into
> > a macro that is applied to functions, function pointers, and function
> > pointer typedefs. It sounds like you're saying that you will not
> > accept a patch that silences or splits off warnings about using
> > ms_hook_prologue with function pointers and function pointer typedefs.
> > So how do you think Wine's problem should be solved?
>
> I think ms_hook_prologue should be allowed to apply to function types
> and function decls.  If you say it should apply to function pointers
> then I suppose you want to have it apply to the pointed to function
> of the function pointer - but that's not possible without an indirection
> via a function pointer typedef IIRC.

No, if ms_hook_prologue is applied to a function pointer, it shouldn't
do anything except maybe trigger some optimization of the code around
the indirect function call.

> I also have the following which _may_ motivate that attributes
> currently not applying to function types (because they only
> affect function definitions) should also apply there:
>
> typedef int  (myfun)  (int *) __attribute__((nonnull(1)));
> myfun x;
> int x(int *p) { return p != (int*)0; }
>
> this applies nonnull to the function definition of 'x' but
> I put the attribute on the typedef.  I didn't manage to
> do without the myfun x; declaration.

That is a great example and another compelling reason to allow
"fndecl" attributes in more places.

> > It seems to me that any information about the target of a function
> > pointer, even the flatten attribute or the ms_hook_prologue attribute,
> > provides information that could be useful for optimizing the code
> > around the indirect function call. That sounds like a compelling
> > argument for allowing these attributes in more places without
> > warnings.
>
> Sure.  Can you write down the three cases after macro expansion
> here to clarify what you need?  Esp. say what the attribute should
> apply to.  Just silencing the warning without actually achieving
> what you want would be bad I think ;)

Essentially, the following needs to compile without warnings:

#define WINAPI __attribute__((__stdcall__)) \
               __attribute__((__ms_hook_prologue__))

typedef unsigned int (WINAPI *APPLICATION_RECOVERY_CALLBACK)(void*);

void WINAPI foo()
{
    APPLICATION_RECOVERY_CALLBACK bar;
    unsigned int (WINAPI *baz)(void*);
}

-Alex