On Thu, Nov 17, 2022 at 1:37 PM Andrew Pinski <pins...@gmail.com> wrote: > > 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
Well, I don't think Clang changed ABI regarding -fno-pic/-fpie/-fpic. As I did archaeology on https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected "Reflection on protected data symbols and copy relocations" GCC 5 x86-64 made a change and GCC aarch64 accidentally picked up the change. """ On the GCC side, in -fpic mode, using GOT-generating relocations when accessing a protected variable subverts the point using the protected visibility. The unneeded pessimization is the foremost complaint. The pessimization applies to all ports with #define TARGET_BINDS_LOCAL_P default_binds_local_p_2. aarch64 moved to default_binds_local_p_2 accidentally by https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cbddf64c0243816b45e6680754a251c603245dbc. For GCC<5 (and all versions of Clang), direct accesses to protected variables are produced in -fpic code. Mixing such object files can still silently break copy relocations on protected data symbols. Therefore, GNU ld made the controversial change https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=ca3fe95e469b9daec153caa2c90665f5daaec2b5 to error in -shared mode. """ > > > > 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`. -- 宋方睿
