https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85740
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Thomas Koenig from comment #5)
> (In reply to Andrew Pinski from comment #1)
> > These functions are not functional equivalent.
> >
> > In the b.c, it records the max location but it is the last element which
> > contains that value. While in c.c, the first element which contains the
> > value is recorded.
>
> I do not understand.
>
> The only difference between the files is
>
> $ diff -u -b b.c c.c
> --- b.c 2018-05-10 22:22:06.276904322 +0200
> +++ c.c 2018-05-10 22:21:50.028801008 +0200
> @@ -14,7 +14,7 @@
> nm = -1;
> for (i=0; i<n; i++)
> {
> - if (__builtin_expect (a[i] > m, 0))
> + if (a[i] > m)
> {
> m = a[i];
> nm = i;
>
> Does this change the semantics of the comparison? If it does, it
> would be a bug in __builtin_expect.
The documentation is hard to understand but you are correct that it just says
we are expecting the value of the comparison to be false.
Anyways the problem here is a bit more complex than just marking the edge as
expect.
For aarch64 we produce csel (which is basically cmov) for both loops:
.L11:
ldr w4, [x5, x2, lsl 2]
cmp w3, w4
csel w0, w0, w2, ge
add x2, x2, 1
csel w3, w3, w4, ge
cmp w1, w2
bgt .L11
