AntonBikineev added a comment. In D134456#3809581 <https://reviews.llvm.org/D134456#3809581>, @aaron.ballman wrote:
> I'm on the fence about this... the attributes were added as optimization > hints, but for somewhat interesting use cases. The obvious one of "I think > this path is likely/unlikely and so I'll help the compiler figure it out" was > one such use case, but honestly that's akin to `inline` and `register` in > terms of how likely it is the user will make a better decision than the > optimizer. But the more interesting use case was "The optimizer is going to > consider this to be the unlikely path, but I need it to be optimized because > I need my failure code to fail as fast as possible". Basically, some folks > wanted to use this to override the otherwise-reasonable decisions from the > optimizer. Based on that, I'm not certain we should let PGO win -- the user > is telling you "optimize based on something I know more about than you", and > having a profile that covers the code path doesn't necessarily change that. > WDYT? I don't think that the second case is how things works today. PGO is still preferred over the attributes. However, the current behavior is somewhat strange: - if the probability is infinitesimal (e.g. 0.00001%), the compiler will ignore the attribute, - if the probability is zero (the branch never hit in the profile), the compiler will consider the attribute. ================ Comment at: clang/lib/CodeGen/CGStmt.cpp:815 // Prefer the PGO based weights over the likelihood attribute. // When the build isn't optimized the metadata isn't used, so don't generate ---------------- rnk wrote: > I lean towards implementing the intended behavior reflected in this comment > here, which is that PGO data overrides programmer annotations in case of > conflict. We should also check the PGO docs to see if this is documented. If > we've already made promises about how this is supposed to work, I'd prefer to > trust our past decisions rather than revisiting them. As Hans pointed out in a Chrome bug: the behaviour is documented here: https://clang.llvm.org/docs/AttributeReference.html#likely-and-unlikely: "These attributes have no effect on the generated code when using PGO (Profile-Guided Optimization) or at optimization level 0." Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D134456/new/ https://reviews.llvm.org/D134456 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
