On Wed, Jan 6, 2021 at 10:32 PM Fangrui Song <i...@maskray.me> wrote: > > On Sat, Dec 26, 2020 at 7:39 AM H.J. Lu <hjl.to...@gmail.com> wrote: > > > > On Sat, Dec 26, 2020 at 7:32 AM Florian Weimer <f...@deneb.enyo.de> wrote: > > > > > > * Fangrui Song: > > > > > > > Hi, I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 which > > > > proposes -fdirect-access-external-data to address some x86-64 > > > > GCC/binutils pain[1] and also benefit non-x86 architectures (also see > > > > [1] > > > > it can prevent copy relocations). > > > > > > > > [1] Mentioned in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112#c2 > > > > > > > > Since I am going to add this option to Clang and I hope (once GCC > > > > decides to > > > > implement this option the two compilers can use the same option name), > > > > I bring > > > > it to your attention. > > > > > > One worry I have is that people start building shared objects with > > > direct data access, expecting the main program to be built with > > > indirect access. We already see this today with Qt. It's not really > > > supported well by the toolchain and causes frequent issues. > > > > It can be solved by ABI extension implemented in linker, ld.so and > > compiler. > > > > > Depending on the ELF ABI in question, the new pair of -f options might > > > not actually be meaningful. It really depends on whether you have > > > reasonably-sized displacements available. I think there are some ABIs > > > where the optimization is theoretically possible, but impractical > > > because the ilimit it imposes on data segment (think AArch64 without > > > adrp). > > > > > > > > -- > > H.J. > > Please check out new comments on > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 > > -fdirect-access-external-data is still the best name. The option is > useful to avoid copy relocations / "canonical PLT entry" > (st_shndx=0,st_value!=0) in -fno-pic code. > I will proceed with my Clang patch.
If I understand it correctly, you want to treat all accesses to protected definitions as local access and all read/write accesses to undefined symbols should go through GOT. Branches to undefined symbols can use PLT. -fdirect-access-external-data doesn't reflect it. -- H.J.
