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

[Florian Weimer <fweimer@redhat.com>]

* 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
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)?

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

> ---
>  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.

Thanks,
Florian