On 22/02/14 19:28, Sebastian Huber wrote: > Hello, > > first some background information. I would like to read from a memory > mapped register (some sort of hardware counter). The problem is that > the address of this register is unknown at compile and link time. So > some low-level initialization code determines the register address and > later its value never changes. > > I hoped that the const function attribute is of some help here: > > http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes > > > Here it explicitly says "function is not allowed to read global > memory". Unfortunately the compiler takes this serious. > > I have the following test code: > > extern volatile const int *ip; > > __attribute__((const)) volatile int *get_ip(void); > > /* This function reads from global memory and is declared as const. GCC > ignores the const attribute in this case. */ > __attribute__((const)) static inline volatile int *get_ip_inline(void) > { > return ip; > } Try the "pure" attribute rather than the "const" one. "pure" functions are allowed to read global memory, but not write it (or cause other side-effects), and the compiler can safely call them fewer times than the program says. Also, you don't say anything about optimisation options - "pure" and "const" attributes only help if optimisation is enabled.
