Re: Build with LTO / -flto on macOS

Hi,

On 2024-06-03 17:07:22 +0200, Wolfgang Walther wrote:
> Peter Eisentraut:
> > It's probably worth clarifying that this option is needed on macOS only
> > if LTO is also enabled.  For standard (non-LTO) builds, the
> > export-dynamic behavior is already the default on macOS (otherwise
> > nothing in PostgreSQL would work).
>
> Right, man page say this:
>
> > Preserves all global symbols in main executables during LTO. Without this
> option, Link Time Optimization is allowed to inline and remove global
> functions. This option is used when a main executable may load a plug-in
> which requires certain symbols from the main executable.

Gah. Apples tendency to just break stuff that has worked across *nix-y
platforms for decades is pretty annoying. They could just have made
--export-dynamic an alias for --export_dynamic, but no, everyone needs a
special macos thingy in their build scripts.

> Peter:
> > I don't think we explicitly offer LTO builds as part of the make build
> > system, so anyone trying this would do it sort of self-service, by
> > passing additional options to configure or make.  In which case they
> > might as well pass the -export_dynamic option along in the same way?
>
> The challenge is that it defeats the purpose of LTO to pass this along to
> everything, e.g. via CFLAGS. The Makefiles set this in LDFLAGS_EX_BE only,
> so it only affects the backend binary. This is not at all obvious and took
> me quite a while to figure out why LTO silently didn't strip symbols from
> other binaries. It does work to explicitly set LDFLAGS_EX_BE, though.
>
> Also, passing the LTO flag on Linux "just works" (clang, not GCC
> necessarily).

It should just work on gcc, or at least has in the recent past.

ISTM if we want to test for -export_dynamic like what you proposed, we should
do so only if --export-dynamic wasn't found. No need to incur the overhead on
!macos.

Greetings,

Andres Freund