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Re: PATCH: PR target/53416: Wrong code when optimising loop involving _rdrand32_step
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 <jakub@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?
Jakub