[PATCH 2/4] Check -z single-global-definition and -fsingle-global-definition

[H.J. Lu]

On Mon, Jun 21, 2021 at 12:43 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu via Libc-alpha:
>
> > 1. Check linker support for -z single-global-definition.  If
> > GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION is set on any input
> > relocatable files:
> >   a. Don't generate copy relocations.
> >   b. Turn off extern_protected_data.
> >   c. Treate reference to protected symbols with single global definition
> >   as local.
> >   d. Set GNU_PROPERTY_1_NEEDED_SINGLE_GLOBAL_DEFINITION on output.
> >   e. Add -z [no]single-global-definition to control single global
> >   definition.
> > 2. Check compiler support for -fsingle-global-definition:
> >   a. Generate a single global definition marker in relocatable objects.
> >     i. Always use GOT to access undefined data and function symbols,
> >     including in PIE and non-PIE.  These will avoid copy relocations in
> >     executables.
> >     ii. This is compatible with existing executables and shared libraries.
> >   b. In executable and shared library, bind symbols with the STV_PROTECTED
> >      visibility locally:
> >     i. The address of data symbol is the address of data body.
> >     ii. For systems without function descriptor, the function pointer is
> >     the address of function body.
> >     iii. The resulting shared libraries may not be incompatible with
> >     executables which have copy relocations on protected symbols.
> >
> > Size comparison of non-PIE builds with GCC 12 -O2:
> >
> > 1. On x86-64:
> >    text          data     bss     dec     hex filename
> >  190218          9672     416  200306   30e72 ld.so (original)
> >  190258          9336     416  200010   30d4a ld.so (-fsingle-global-definition)
> > 1917384         20232   52424 1990040  1e5d98 libc.so (original)
> > 1919946         20240   52432 1992618  1e67aa libc.so (-fsingle-global-definition)
> >  261734         10339     744  272817   429b1 localedef (original)
> >  233084         41734     648  275466   4340a localedef (-fsingle-global-definition)
>
> I must say these numbers do not make sense to me.  Why do libc.so and
> localedef data sizes increase with -fsingle-global-definition?  Fewer

I haven't investigated them in detail yet.  My main purposes were to
check if anything breaks.

> copy relocations should *reduce* data size on both sides.  localedef
> would have to have nearly 4,000 data relocations to account for the size
> increase.  And shouldn't the GOT overlap with BSS anyway (so not even
> show up in the data column)?

I will take a closer look.

> And I don't think we can build glibc with -fsingle-global-definition
> anyway because it would be an ABI break.

Correct.

> > ---
> >  configure    | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  configure.ac | 37 ++++++++++++++++++++++++++
> >  2 files changed, 109 insertions(+), 1 deletion(-)
> >
> > diff --git a/configure b/configure
> > index 9619c10991..5844dad68f 100755
> > --- a/configure
> > +++ b/configure
> > @@ -732,6 +732,7 @@ infodir
> >  docdir
> >  oldincludedir
> >  includedir
> > +runstatedir
> >  localstatedir
> >  sharedstatedir
> >  sysconfdir
>
> Please regenerate with an unpatched autoconf.
>

Will do.

Thanks.

-- 
H.J.