Comment # 1 on bug 1187864 from Aaron Puchert

(In reply to Fabian Vogt from comment #0)
> Using the "-fno-semantic-interposition" compiler option, interprocedural
> optimization can be enabled for exported symbols [...]
The way I see it, -fno-semantic-interposition is justified by the "one
definition rule" in both C and C++. Having different functions with the same
(qualified) name (and signature for C++) is not allowed in a program. There is
still a non-standard "back door": __attribute__((weak)). So if necessary it's
still possible to define overridable functions, but other libraries can't just
override anything they want. It's also kind of on-by-default in Clang (not
literally, but Clang does IPO on default visibility functions) and on other
platforms, so there is hopefully not a lot of software out there that relies on
semantic interposition.

> [...] and the linker option "-Bsymbolic(-functions)" allows symbol
> references inside the same object to be resolved directly instead of going
> through runtime relocation. [...]
> 
> However, executables have to be built with that in mind (by compiling them
> like PIC) to avoid features which rely on interposition, like copy
> relocations. The "-fno-direct-access-external-data" option achieves that
> FWICT.
In the thread where Fangrui Song aka maskray proposed to enable these flags for
building Clang [1], the time difference between -Bsymbolic and
-Bsymbolic-functions was minor. For Qt it might very well look different
though.

Address uniqueness is probably not a big issue, but it certainly can be an
issue, as the standards guarantee it. So I think we could enable
-Bsymbolic-functions on a per-package basis, but probably not globally.
Avoiding copy relocations for -Bsymbolic might be too high a price to pay, at
least in general. I don't know how common default visibility variables are, and
whether they're typically used more often by the library or the program. With
modern libraries I'd expect there not be a lot of exported variables, so maybe
it just doesn't matter that much.

So I'd be in favor of enabling -fno-semantic-interposition by default, but not
-Bsymbolic[-functions]. We can enable that where it matters and where we think
it's safe.

I played with -Bsymbolic in the LLVM build once, but I'm mostly compiling C++
where the compile jobs take so long that the initial relocation processing
doesn't change much. (Since we're building LLVM with Clang, we already have IPO
for default visibility functions.)

[1] https://lore.kernel.org/lkml/20210501235549.vugtjeb7dmd5xell@google.com/