Hi, On 2024-07-19 11:06:47 -0400, Tom Lane wrote: > 2. Do we really want to encourage people to build with -flto? > > I fear that #2 is actually a pretty serious concern. I think there > are a lot of places where we've assumed semi-implicitly that > compilation file boundaries are optimization barriers, particularly > around stuff like LWLocks and semaphores. I don't really want to > spend time chasing obscure, irreproducible bugs that may appear when > that assumption gets broken. I especially don't want to do it just > because some packager has randomly decided to inject random build > switches.
I don't really buy this argument. It'd be one thing if compilation boundaries actually provided hard guarantees - but they don't, the CPU can reorder things as well, not just the compiler. And the CPU doesn't know about compilation units. If anything, compiler reorderings are *less* obscure than CPU reordering, because the latter is heavily dependent on running on large enough machines with specific microarchitectures. The only case I know where we do rely on compilation units providing some level of boundaries is on compilers where we don't know how to emit a compiler barrier. That's probably a fallback we ought to remove one of these days... > In short: if we want to support LTO, let's do it officially and not > by the back door. But I think somebody needs to make the case that > there are compelling benefits that would justify the nontrivial > amount of risk and work that may ensue. My default position here > is "sorry, we don't support that". FWIW, I've seen pretty substantial wins, particularly in more heavyweight queries. Greetings, Andres Freund
