On Fri, Feb 09, 2024 at 11:10:10AM -0800, Nick Desaulniers wrote: > I have 100% observed llvm throw out writes to objects declared as > const where folks tried to write via "casting away the const" (since > that's UB) which resulted in boot failures in the Linux kernel > affecting android devices. I can't find the commit in question at the > moment, but seemed to have made some kind of note in it in 2020. > https://android-review.git.corp.google.com/c/platform/prebuilts/clang/host/linux-x86/+/1201901/1/RELEASE_NOTES.md > > That said, I just tried Al's union, and don't observe such optimizations. > https://godbolt.org/z/zrj71E8W5 The really shitty part is not the missing stores; it's reordered loads. If spin_lock(&dentry->d_lock); name = dentry->d_name.name; len = dentry->d_name.len; something(name, len); spin_unlock(&dentry->d_lock); has the compiler go "->d_name is const, so I can bloody well move the load before that spin_lock() call", we really have a problem. Can it reorder the loads of const member wrt e.g. barrier()? If that's the case, this approach is no-go and accessor is the only feasible alternative.
