Jeremy Hall schrieb:
Hi,
Looking at the assembler for this speculative code, I saw that without
asm volatile, the second call to rdrand was removed and the value
"low" used twice.
gcc 4.6.1 with any optimization turned on. It looks to me like this
optimization will stop the code working.
My question is: is this my ignorance of inline assembler (most
likely) or is it a bug in gcc which I should report?
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.
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 );
}
