rjmccall added a comment. In D99791#2672528 <https://reviews.llvm.org/D99791#2672528>, @lebedev.ri wrote:
> @rjmccall thank you for taking a look! > > In D99791#2670333 <https://reviews.llvm.org/D99791#2670333>, @rjmccall wrote: > >> The last major conversation we had about this was this RFC I sent out about >> five years ago: >> >> https://lists.llvm.org/pipermail/llvm-dev/2016-January/094012.html >> >> In that RFC, I specifically argued that we should not do this, and that it >> should only be considered undefined behavior to actually access memory >> through a misaligned pointer (for a particular definition of "access >> memory"). At Apple, we have made that a promise to our internal users, so >> even if we decide to do this, we will need to not do it on Darwin. However, >> as I remember it, the LLVM community did not reach a consensus to adopt my >> recommendation, so in principle we still have the flexibility to start doing >> this. I continue to believe that doing so would be a mistake. At any rate, >> you should start by reading that thread. > > Thank you for the pointer! > I can for sure provide an opt-out, however note that in the end it will cause > performance regressions as compared to the current LLVM optimizations. How so? We do not currently assume that pointer arguments are aligned, or even dereferenceable. I don't mind making stronger assumptions about the C++ `this` argument, but we really should not do this for arbitrary pointer arguments. If you want to put together a UBSan mode that checks this, that's fine, but I do not think there is a path forward for actually optimizing based on this assumption by default. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D99791/new/ https://reviews.llvm.org/D99791 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
