On Tue, Apr 7, 2020 at 3:54 PM David Howells <dhowells@xxxxxxxxxx> wrote: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94527 > > With regard to this, I've got back "not sure what Linus was talking about WRT > DSE, if he's got examples he could pass along, they'd be appreciated" I'll do that. We have real examples in the kernel, although they probably aren't all that _important_. I don't see that comment on the bugzilla, but I'll put the stupid example in there too. One such example would be kernel/async.c: async_run_entry_fn(), where we have /* 2) remove self from the pending queues */ spin_lock_irqsave(&async_lock, flags); list_del_init(&entry->domain_list); list_del_init(&entry->global_list); /* 3) free the entry */ kfree(entry); atomic_dec(&entry_count); and while it's good form to do "list_del_init()" on those fields in entry, the fact that we then do "kfree(entry)" right afterwards means that the stores that re-initialize the entry list are dead. So _if_ we had some way to tell the compiler that "hey, kfree(ptr) kills the lifetime of that object", the compiler could eliminate the dead stores. I think that dead store elimination is perhaps less important than if the compiler could warn about us stupidly using the dead storage afterwards, but I mention it as a "it can actually matter for code generation" example too. Now, the above is a particularly stupid example, because if we cared, we could just turn the "list_del_init()" into a plain "list_del()", and simply not do the unnecessary re-initialization of the list entry after removing it. But I picked a stupid example because it's easy to understand. Less stupidly, we sometimes have "cleanup" functions that get rid of things, and are called before you free the underlying storage. And there, the cleanup function might be used in general, and not only just before freeing. So the re-initialization could make sense in that context, but might again be just dead stores for the actual final freeing case. Is this a big deal? No it's not. But it's not really any different from the dead store elimination that gcc already does for local variables on stack. Linus
