https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87047
--- Comment #11 from Michael_S <already5chosen at yahoo dot com> --- Sorry for intervening, but IMHO a new __builtin is long overdue. __builtin (In reply to Jakub Jelinek from comment #9) > (In reply to Alexander Monakov from comment #8) > > Well, original_costs is already initialized with COSTS_N_INSNS (2), > > accounting for test and branch that would be removed. So it's not too bad: > > in pr78120.c else_bb is in fact missing. > > I think that is not enough. > > > (on x86 I think we shouldn't encourage if-conversion, by adding data > > dependencies it can lengthen critical paths) > > if-conversion is indeed very problematic on x86-64, but it isn't as easy as > don't try to encourage it, because it slows down many important cases in > that case if jumps are used. We have many PRs where users ask on specific > testcases for more x86 if-conversion, and many PRs where users ask for less > or no if-conversion. if-conversion, except for simplest shortest cases, is problematic on any processor with good branch prediction and not very wide pipeline. Nothing specific to x86-64 here. To something like ARM Cortex-A73 (dual-issue) it applies much stronger than to Skylake or Ryzen. If it was dependent on me then, starting from gcc9, I would apply automatic (i.e. non-PGO) if-conversion only to *really* short sequences. For anything longer, I'd request user hint in form of __builtin_expect_with_probability(, 0.5).
