On 06/22/2016 10:21 AM, Sebastian Huber wrote:
>
>
> 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.
In that case your options are very limited :-( Its ugly, but the only
way I can see:
#if defined(PLATFORM_SPARC)
#define timestamp (*s.f)
#endif
now you can use the suggested work around using a local variable:
func *g = timestamp;
Be aware that in the powerpc case gcc will treat the __asm__ as
__attribute__((const)) when it has no inputs. Thus it will emit the code
in __asm__ only once and multiply by two...
--
Matthias Pfaller Software Entwicklung
marco Systemanalyse und Entwicklung GmbH Tel +49 8131 5161 41
Hans-Böckler-Str. 2, D 85221 Dachau Fax +49 8131 5161 66
http://www.marco.de/ Email leo@xxxxxxxx
Geschäftsführer Martin Reuter HRB 171775 Amtsgericht München
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