+.. 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.dijks...@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`.
