From: Andrew Pinski <pinskia@gmail.com>
To: "maskray@google.com" <maskray@google.com>
Cc: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
Richard Sandiford <richard.sandiford@arm.com>,
Ramana Radhakrishnan <ramana.gcc@googlemail.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
Subject: Re: [PATCH] AArch64: Add support for -mdirect-extern-access
Date: Thu, 17 Nov 2022 13:37:38 -0800 [thread overview]
Message-ID: <CA+=Sn1moKo4Gy9W9bm2=BOwWZYQ2PmEj_=8OFcq-ukoEyMbyMg@mail.gmail.com> (raw)
In-Reply-To: <20221117212006.dspm45znjyqj6ktf@google.com>
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.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`.
next prev parent reply other threads:[~2022-11-17 21:37 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-11 14:48 Wilco Dijkstra
2022-11-16 10:54 ` Richard Sandiford
2022-11-17 16:42 ` Wilco Dijkstra
2022-11-17 17:30 ` Richard Sandiford
2022-11-17 20:52 ` Ramana Radhakrishnan
2022-11-17 21:20 ` maskray
2022-11-17 21:37 ` Andrew Pinski [this message]
2022-11-17 21:46 ` Fangrui Song
2022-11-17 21:55 ` Andrew Pinski
2022-11-17 22:07 ` Fangrui Song
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