On Sun, May 20, 2012 at 11:15 AM, Jakub Jelinek <ja...@redhat.com> wrote:
> On Sun, May 20, 2012 at 10:37:13AM -0700, H.J. Lu wrote:
>> On Sun, May 20, 2012 at 10:19 AM, Jakub Jelinek <ja...@redhat.com> wrote:
>> > On Sun, May 20, 2012 at 10:04:26AM -0700, H.J. Lu wrote:
>> >> rdrand<mode>_1 must be marked with unspec_volatile since it returns
>> >> a different value every time. OK for trunk, 4.7 and 4.6?
>> >
>> > A testcase for this would be nice (runtime is not possible, since the
>> > RNG in theory could return the same value twice, but scanning assembly
>> > for a particular number of the rdrand insns would be nice).
>> >
>>
>> For
>>
>> unsigned int number = 0;
>> volatile int result = 0;
>>
>> for (register int i = 0; i < 4; ++i) {
>> result = _rdrand32_step(&number);
>> printf("%d: %d\n", result, number);
>> }
>
> Try it without the loop, unroll it by hand, see if without the patch
> the rdrand insns are still CSEd together?
>
It doesn't:
[hjl@gnu-ivb-1 tmp]$ cat x.c
#include <stdio.h>
int
main(int argc, char **argv)
{
unsigned int number = 0;
volatile int result = 0;
result = __builtin_ia32_rdrand32_step (&number);
printf("%d: %d\n", result, number);
result = __builtin_ia32_rdrand32_step (&number);
printf("%d: %d\n", result, number);
result = __builtin_ia32_rdrand32_step (&number);
printf("%d: %d\n", result, number);
result = __builtin_ia32_rdrand32_step (&number);
printf("%d: %d\n", result, number);
return 0;
}
[hjl@gnu-ivb-1 tmp]$
/export/gnu/import/git/gcc-regression/master/187369/usr/bin/gcc
-mrdrnd -O3 -S x.c
[hjl@gnu-ivb-1 tmp]$ grep rdrand x.s
rdrand %ebx
rdrand %eax
rdrand %eax
rdrand %eax
[hjl@gnu-ivb-1 tmp]$
--
H.J.
