Inline asm - unexpected optimization

[Jeremy Hall]

On 24 August 2011 11:11, Georg-Johann Lay <avr@gjlay.de> wrote:
> It's not a bug in gcc.
>
> The inlne asm has side effects you didn't report to gcc, so you need the
> volatile here.
OK thanks.  I guess there is no way to report this kind of side
effect?  so you have to use volatile or call rdrand twice in the same
asm().
The only spanner in the works is that this problem does NOT occur with
the similar rdtsc instruction.  I wondered if rdtsc was flagged within
gcc in some way to say its output was volatile, and that perhaps this
had yet to be done with rdrand (as its a new instruction not available
for a few months until Ivy Bridge comes out).

>
>> I know its a new instruction.  The retry loop etc is removed to
>> simplify the test case.
>>
>> #include <stdio.h>
>> typedef unsigned long long qword;
>> // #define asm __asm__ __volatile__
>>
>> int
>> main( int argc, char *argv[] )
>> {
>>  qword result;
>>
>>    /* for 32 bit mode call rdrand twice */
>>  unsigned int low, high;
>>  asm( "rdrand %0" : "=r" (low) );
>>  asm( "rdrand %0" : "=r" (high) );
>>  result = ((qword)high << 32U) | low;
>>
>>  printf("64 bit random number: %llu\n", result );
>> }
>