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.
