On Thu, Nov 17, 2022 at 1:21 PM maskray--- via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > > +.. 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 ...
The x86_64 discussion about this is here https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 . I think clang changing the ABI is just broken and should think twice before we do it for GCC. And there is a lot of visibility protected issues filed in GCC bug databases specifically about copy relocs too. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56527 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37611 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520 https://sourceware.org/bugzilla/show_bug.cgi?id=28875 https://sourceware.org/bugzilla/show_bug.cgi?id=28877 I also suspect clang's behavior is still broken too. Thanks, Andrew > > 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`.
