Re: Build with LTO / -flto on macOS

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