Re: [PATCH] AArch64: Add support for -mdirect-extern-access

["maskray@google.com" <maskray@google.com>]

> +.. option:: -mdirect-extern-access, -mno-direct-extern-access
> +
> +  Use direct accesses for external data symbols.  It avoids a GOT indirection
> +  on all external data symbols with :option:`-fpie` or :option:`-fPIE`.  This is
> +  useful for executables linked with :option:`-static` or :option:`-static-pie`.
> +  With :option:`-fpic` or :option:`-fPIC`, it only affects accesses to protected
> +  data symbols.  It has no effect on non-position independent code.  The default
> +  is :option:`-mno-direct-extern-access`.
> +
> +  .. warning::
> +
> +    Use :option:`-mdirect-extern-access` either in shared libraries or in
> +    executables, but not in both.  Protected symbols used both in a shared
> +    library and executable may cause linker errors or fail to work correctly.

I think current GCC and Clang's behavior is:

* -mdirect-extern-access is the default for -fno-pic. This is to enable optimizations for -static programs but may introduce copy relocations.
* -mno-direct-extern-access is the default for -fpie and -fpic. This uses some GOT-generating relocations which can be optimized out (lld, see https://maskray.me/blog/2021-08-29-all-about-global-offset-table) but the instruction is nevertheless slightly longer.

(-mdirect-extern-access for -fpic probably doesn't make sense.)

The option I introduced to Clang is -fdirect-access-external-data
(see https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected).
If -mdirect-extern-access gets more popular, I can add a Clang alias.
But I am opposed to forcing a GNU property for -mdirect-extern-access/-mno-direct-extern-access.

FWIW I used https://gist.github.com/MaskRay/c03a90922003df666551589f1629df22 to test my Clang changes related to -fno-semantic-interposition
on various visibility attributes x non-weak/weak x nopic/pie/pic x dllimport/not x ...

On 2022-11-17, Ramana Radhakrishnan wrote:
>On Thu, Nov 17, 2022 at 5:30 PM Richard Sandiford via Gcc-patches
><gcc-patches@gcc.gnu.org> wrote:
>>
>> Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
>> > Hi Richard,
>> >
>> >> Can you go into more detail about:
>> >>
>> >>    Use :option:`-mdirect-extern-access` either in shared libraries or in
>> >>    executables, but not in both.  Protected symbols used both in a shared
>> >>    library and executable may cause linker errors or fail to work correctly
>> >>
>> >> If this is LLVM's default for PIC (and by assumption shared libraries),
>> >> is it then invalid to use -mdirect-extern-access for any PIEs that
>> >> are linked against those shared libraries and use protected symbols
>> >> from those libraries?  How would a user know that one of the shared
>> >> libraries they're linking against was built in this way?
>> >
>> > Yes, the usage model is that you'd either use it for static PIE or only on
>> > data that is not shared. If you get it wrong them you'll get the copy
>> > relocation error.
>>
>> Thanks.  I think I'm still missing something though.  If, for the
>> non-executable case, people should only use the feature on data that
>> is not shared, why do we need to relax the binds-local condition for
>> protected symbols on -fPIC?  Oughtn't the symbol to be hidden rather
>> than protected if the data isn't shared?
>>
>> I can understand the reasoning for the PIE changes but I'm still
>> struggling with the PIC-but-not-PIE bits.
>
>I think I'm with Richard S on hidden vs protected on first reading. I
>can see why this works out of the box and can even be default for
>static-pie.
>
>Any reason why this is not on by default - it's early enough in the
>stage3 cycle and we can always flip the defaults if there are more
>problems found.
>
>You probably need a rebase for the documentation bits,.
>
>regards
>Ramana
>
>
>Ramana


+  is :option:`-mno-direct-extern-access`.