On 22/06/16 10:08, Matthias Pfaller wrote:
On 06/22/2016 09:48 AM, Sebastian Huber wrote:
On 22/06/16 09:42, Mason wrote:
On 22/06/2016 09:00, Sebastian Huber wrote:
is there a way to tell GCC that a global variable is immutable after
initialization? For example
struct {
int (*f)(void);
} s;
int f(void)
{
int a;
int b;
a = (*s.f)();
b = (*s.f)();
return a + b;
}
yields on ARMv8 for example
f:
push {r4, r5, r6, lr}
movw r4, #:lower16:s
movt r4, #:upper16:s
ldr r3, [r4]
blx r3
ldr r3, [r4] <- I would like to get rid of this load here
mov r5, r0
blx r3
add r0, r5, r0
pop {r4, r5, r6, pc}
The
a = (*s.f)();
is a call to a global function, so GCC must assume that s might have
changed afterwards. I would like to get rid of the second load of s.f.
Is there a special attribute to tell GCC that s is essentially
immutable? I cannot use the const qualifier, since the structure is
initialized during system start.
A simple solution would be storing the function pointer in a local
variable.
Thus, the compiler "knows" you intended to call the same function twice.
typedef int func(void);
struct { func *f; } s;
int f(void)
{
func *g = s.f;
int a = g();
int b = g();
return a + b;
}
push {r3, r4, r5, lr}
movw r3, #:lower16:s
movt r3, #:upper16:s
ldr r4, [r3]
blx r4
mov r5, r0
blx r4
add r0, r0, r5
pop {r3, r4, r5, pc}
Regards.
My problem is that the actual (*s.f)() call is an implementation detail.
I have a function which returns a timestamp via an architecture specific
way, e.g. on PowerPC this is something like this:
static inline int timestamp(void)
{
int t;
__asm__(... t ...).
return t;
}
On SPARC this is something like this:
static inline int timestamp(void)
{
return (*s.f)();
}
So, I cannot use a local variable for this particular use case.
So you are saying that your code fragment with the two calls to s.f() is
actually the result of gcc inlining the call to timestamp?
Yes.
--
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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