llvm dev - Jun 2021 - [cfe-dev] -fpic ELF default: reclaim some -fno-semantic-interposition optimization opportunities?

On Sat, Jun 05, 2021 at 06:08:57PM -0700, Fāng-ruì Sòng via llvm-dev
wrote:
> On 2021-06-06, Joerg Sonnenberger wrote:
> > On Fri, Jun 04, 2021 at 03:26:53PM -0700, Fāng-ruì Sòng via llvm-dev
wrote:
> > > Fixing the last point is actually easy: let -fno-pic use GOT when
> > > taking the address of an non-definition function.
> > 
> > I'd far prefer to have an attribute to explicitly say that the
address
> > of a given symbol should always be computed indirectly (e.g. via GOT).
> > That gives the explicit control necessary for libraries without
> > penalizing the larger executables like clang.
> > 
> > Joerg
> 
> Taking the address (in code) of a non-definition function is rare,
> rarer after optimization. At least when building clang, I cannot find
> any penalizing.
I was not talking about just functions. I can't even think of a case
where pointer equality for function pointers matters. But the case I
care far more about is being able to avoid copy relocations for global
variables and that's the same problem (loading the address of a symbol).

Joerg

On Sun, Jun 6, 2021 at 7:08 AM Joerg Sonnenberger via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On Sat, Jun 05, 2021 at 06:08:57PM -0700, Fāng-ruì Sòng via llvm-dev wrote:
> > On 2021-06-06, Joerg Sonnenberger wrote:
> > > On Fri, Jun 04, 2021 at 03:26:53PM -0700, Fāng-ruì Sòng via
llvm-dev wrote:
> > > > Fixing the last point is actually easy: let -fno-pic use GOT
when
> > > > taking the address of an non-definition function.
> > >
> > > I'd far prefer to have an attribute to explicitly say that
the address
> > > of a given symbol should always be computed indirectly (e.g. via
GOT).
> > > That gives the explicit control necessary for libraries without
> > > penalizing the larger executables like clang.
> > >
> > > Joerg
> >
> > Taking the address (in code) of a non-definition function is rare,
> > rarer after optimization. At least when building clang, I cannot find
> > any penalizing.
>
> I was not talking about just functions. I can't even think of a case
> where pointer equality for function pointers matters. But the case I
> care far more about is being able to avoid copy relocations for global
> variables and that's the same problem (loading the address of a
symbol).
>
> Joerg
On the Clang side, `-fno-pic -fno-direct-access-external-data` uses
GOT to access a default visibility global variable today.
If all TUs use this option and assembly files do the right thing, copy
relocations can be avoided.

I know some folks prefer eliminating copy relocations for ABI and
security reasons.
I deliberately make the scope narrow to functions because functions
are where we can improve performance.

The x86 ABI maintainer seems to want to do this in a more complex way.
I have commented on
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100593