On 2014-02-24 10:57, David Brown wrote:
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.
I noticed that even if you have a const variable, e.g.
extern const int const_var;
GCC will read the value of const_var again after a compiler memory barrier
(e.g. __asm__ volatile("" ::: "memory")). So in my case it turned out that
there is not much left to optimize. It seems that I have to live with the
superfluous loads.
Also, you don't say anything about optimisation options - "pure" and
"const" attributes only help if optimisation is enabled.
I use optimization level -O2.
--
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
Fax : +49 89 189 47 41-09
E-Mail : sebastian.huber@xxxxxxxxxxxxxxxxxx
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