https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110007
--- Comment #3 from Richard Yao <richard.yao at alumni dot stonybrook.edu> --- (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. Earlier in the output, GCC 12.3 uses cmov for every 3rd instruction, so I suspect that this is not a case of two cmovs being too close to each other. Should I file another issue explicitly for that so this could be left for the request that we have __builtin_unpredictable()? > __builtin_expect_with_probability will do exactly the same as > __builtin_unpredictable will do really. __builtin_unpredictable() is easier to read than __builtin_expect_with_probability() and it would be nice for both Clang and GCC to support the same builtins.
