Re: Specifying different registers in inline asm

Le 2025-11-19 16:19, Richard Earnshaw (foss) a écrit :
[snip]

Thank you all very much, I am slowly (the slowness comes entirely from 
my side) understanding what I should do, both on a conceptual and on a 
practical levels.

I still need a little push.

>> 
>> #define byte_from_user(addr)     ({uint8_t val; __asm__ 
>> __volatile__("LDRBT    %0, [%1]\n" : "=&r"(val) : "r" (addr), 
>> "m"(*addr)); val;})
>> 
>> void dump(const uint8_t * src, size_t count;
>>     for(size_t cntsrc=0;cntsrc<count;cntsrc++){
>>         uint8_t data = byte_from_user(&src[cntsrc]);
>>         printf("%02X ", data);
>>     }
>>     printf("\n");
>> }
>> 
>> So the only way to be theoretically correct is to copy addr to a dummy 
>> variable and say it will come out of the asm block? Like this:
>> 
>> #define byte_from_user(addr)     ({uint8_t val; uintptr_t clob_addr = 
>> addr; __asm__ __volatile__("LDRBT    %0, [%1]\n" : "=&r"(val), 
>> "+r"(clob_addr) : "m"(*clob_addr)); val;})
>> 
> 
> With the '+r' variant you don't need to mention clob_addr twice.  You
> also don't need '&' on the first value now because the compiler will
> never assign two outputs to the same register.
> 
> So I think you're best solution would be:
> 
> 
> static inline uint8_t
> byte_from_user(uint8_t *addr)
> {
>   uint8_t val;
>   void *unused;
>   __asm__ __volatile__ ("LDRBT    %0, [%1]\n"
> 			: "=r"(val), "=r" (unused)
> 			: "1" (addr)
> 			: "memory");
>   return val;
> }
> 
> void dump(const uint8_t * src, size_t count;
>     for(size_t cntsrc=0;cntsrc<count;cntsrc++){
>         uint8_t data = byte_from_user(&src[cntsrc]);
>         printf("%02X ", data);
>     }
>     printf("\n");
> }
> 
> You probably won't need the clobber of 'memory' in addition to the
> 'volatile' qualifier, but you certainly need at least one.
> 
About memory access, I think I made a mistake, *addr is not clobbered, 
but will be read, should that be mentioned in the input list?
In the converse function byte_to_user, *addr would be written, should it 
then be marked as clobber?
Is there a sense in saying that my code only reads/writes *addr and not 
any other memory (i.e. does it make a difference for clobber to write 
"m"(*addr) rather then "memory")?

Cheers,
JH