Re: C Metaprogramming

[Daniel Santos <danielfsantos at att dot net>]

Thanks for your response!

On 06/19/2012 08:40 AM, Richard Guenther wrote:
> On Tue, Jun 19, 2012 at 4:41 AM, Daniel Santos <danielfsantos@att.net> wrote:
>> So before filing any feature request bugs, I figured I should bring my
>> discussion here first, as I believe some enhancements to gcc can better
>> enable this type of programming.
> The question this boils down to is - what is a constant?  The current
> implementation is pretty clear (see builtins.c:fold_builtin_constant_p)
> and excludes any addresses of entities (being functions or variables).
> Which probably asks for a new builtin to verify this, not over-loading
> __builtin_constant_p.
To be honest, I haven't peeked at the gcc sources yet for fear I would
want to get too involved! :)  I'll look at this.  And yes, it would
certainly make sense to have a separate builtin for this.
>
>> First off, we need a mechanism to verify constness of a function pointer
>> at build time and generate an error when the value is non-const, similar
>> to the construct:
>>
>> #define BUILD_BUG_ON_NON_CONST(arg)                   \
>> do {                                                  \
>>    extern void __value_non_constant(void)            \
>>        __attribute__((error("value not constant"))); \
>>    if (!__builtin_constant_p(arg))                   \
>>        __not_constant_error();                       \
>> } while (0)
>>
>> Second, it will be helpful to have a mechanism to generate a -Winline
>> warning when a function call by pointer cannot be inlined.  Obviously,
>> this is impossible if you can't first determine that it's a const
>> function pointer and resolve that to the actual function.  I'm sorry to
>> say that I don't have a suggestion for doing this other than some
>> __builtin_isinlineable() type of function (that accepts a function
>> pointer).  Perhaps solving the above problem and assuring that, after
>> this test for function pointer constness succeeds and the const function
>> pointer can be resolved to an inline function, the compiler can just
>> emit a warning as it would when a normal function call to an inline
>> cannot be inlined.  Here's an example:
>>
>> #include <stdio.h>
>> static inline int myfunc(int a) {
>>    return a + 42;
>> }
>>
>> static inline void jack(int (*const fnptr)(int)) {
>>    /* macro from above, except working for function pointers */
>>    BUILD_BUG_ON_NON_CONST(fnptr);
>>
>>    /* if the above test passes, any inability to inline myfunc should
>>     * produce a -Winline warning
>>     */
> I think that would be essentially the same as declaring myfunc
> with attribute always-inline.  You'd get an error if the function pointer
> turns into a direct call and is not inlined.
I haven't tried this yet, but I will.  However, I thought that
always_inline just caused the function to be inlined at all optimization
levels (including none).  If not, then it's incorrectly documented.

Thanks,
Daniel