[Bug c/110007] Implement support for Clang’s __builtin_unpredictable()

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110007

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Yao from comment #3)
> (In reply to Andrew Pinski from comment #2)
> > (In reply to Richard Yao from comment #0)
> > > Having the ability to specify __builtin_unpredictable() as a hint to
> > > encourage the compiler to use cmov would be useful for implementing
> > > algorithms like binary search that have unpredictable branches.
> > > __builtin_expect_with_probability() looks like a possible substitute, but
> > > Clang does not support it and it does not always work as described in
> > > #110001.
> > 
> > PR 110001 has nothing to do with __builtin_expect_with_probability and
> 
> I mentioned it briefly in PR 110001, but I guess I should make it explicit.
> See line 58 here:
> 
> https://gcc.godbolt.org/z/ef3Yfchzv
> 
> That is made into a jump, when all that is necessary is cmov, which is what
> Clang generates. In the example I had posted in PR 110001, I had not used
> __builtin_expect_with_probability() because it made no difference, but here
> I am using it to show that cmov is not used.



If we look at the -fdump-tree-optimized-lineno dump we see why it was not
turned into a cmov:
  [/app/example.c:58:12 discrim 2] if (a_111 <= key_128(D))
    goto <bb 28>; [50.00%]
  else
    goto <bb 23>; [50.00%]

  <bb 23> [local count: 447392427]:
  [/app/example.c:75:40] _268 = (long unsigned int) i_82;
  [/app/example.c:75:40] _271 = _268 * 4;
  [/app/example.c:75:34] _274 = array_89(D) + _271;
  [/app/example.c:5:6] pretmp_277 = *_274;
  goto <bb 28>; [100.00%]


There is a load moved inside the conditional.