Thats very interesting, thanks for sharing. mike On Tue, Jun 19, 2012 at 4:41 AM, Daniel Santos <danielfsan...@att.net> wrote: > Yes, my topic sounds crazy huh? But you guys made it possible when you > started optimizing out constant function pointers. (Thank you!!) This > didn't mature to "full power" until 4.6.? (works in 4.6.2, not sure > about earlier 4.6). Now that you can populate a struct with const > pointers to inline functions and pass a pointer to that struct to a > generic inline function, full C type-injection is now possible. (Well, > you probably didn't need to be able to do it via a struct, but it sure > cleans up the interface). > > Example here: http://lwn.net/Articles/500757/ (patches last sent to LKML > on June 7th, don't have them posted elsewhere atm, sorry) > > However, the process of verifying that your calls by > constant-function-pointer (much less, one that is a struct member) is > one of examining the generated code, since __builtin_constant_p will > always return zero when passed a function pointer (or any pointer I've > given it thus far). This simple program demonstrates the problem: > > #include <stdio.h> > static inline int myfunc(int a) { > return a + 42; > } > > static int (*const myptr)(int) = myfunc; > > int main(int argc, char *argv[]) { > printf("is const = %d\n", __builtin_constant_p(myptr)); > /* same result if I dereference it: __builtin_constant_p(*myptr) */ > } > > 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. > > 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 > */ > fnptr(0); > } > > int main(int argc, char *argv[]) { > jack(myfunc); > } > > Thanks > Daniel
-- James Michael DuPont Member of Free Libre Open Source Software Kosova http://flossk.org Contributor FOSM, the CC-BY-SA map of the world http://fosm.org Mozilla Rep https://reps.mozilla.org/u/h4ck3rm1k3
