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Keeping them provides no measurable benefit and only >>>>> enlarges the generated code. >>>> It's actually worse than this. >>>> >>>> The branch-taken hint has new meaning in Lion Cove cores and later, >>>> along with a warning saying "performance penalty for misuse". >>>> >>>> i.e. "only insert this prefix after profiling". >>> Don't they really have much the same meaning as before? >> Architecturally yes, microarchitecturally very much not. >> >> For a branch known to the predictor, there is no effect.  If a branch >> unknown to the predictor gets decoded, it triggers a frontend flush and >> resteer. >> >> It is only useful for programs large enough to exceed the working set of >> the conditional predictor, and for which certain branches are known to >> be ~always taken. >> >> Putting the prefix on a branch that isn't ~always taken is worse than >> not having the prefix in the first place, hence the warning. > These branches indeed ~always follow the hinted path (not taken in this case). > > So it sounds like this definitely isn't stable@ material, and maybe even begs > the question if dropping the hints is a net positive? The new behaviour only exists for hint-taken. Because it only has any effect for a branch unknown to the predictor, the behaviour without this hint would be as if it were a larger basic block. hint-not-takens have no behaviour since the Pentium 4 that I'm aware of. This change is almost certainly marginal at best.  It's not as if VMREAD/VMWRITE lead to good code gen even at the best of times. ~Andrew