[Bug libc/19745] Avoid extra branch to PLT entry when calling external function

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19745

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- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=16b23e03632623cd81a21fc5a24db546595addac

commit 16b23e03632623cd81a21fc5a24db546595addac
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Mar 4 15:36:18 2016 -0800

    Replace PREINIT_FUNCTION@PLT with *%rax in call

    Since we have loaded address of PREINIT_FUNCTION into %rax, we can
    avoid extra branch to PLT slot.

        [BZ #19745]
        * sysdeps/x86_64/crti.S (_init): Replace PREINIT_FUNCTION@PLT
        with *%rax in call.

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=21683b5a7d9b8460e120e061f66a6ad84fa47e98

commit 21683b5a7d9b8460e120e061f66a6ad84fa47e98
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Mar 4 15:28:59 2016 -0800

    Replace @PLT with @GOTPCREL(%rip) in call

    Since __libc_start_main is called very early, lazy binding isn't relevant
    here.  Use indirect branch via GOT to avoid extra branch to PLT slot.

        [BZ #19745]
        * sysdeps/x86_64/start.S (_start): __libc_start_main@PLT
        with *__libc_start_main@GOTPCREL(%rip) in call.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog              |   12 ++++++++++++
 sysdeps/x86_64/crti.S  |    2 +-
 sysdeps/x86_64/start.S |    6 ++++--
 3 files changed, 17 insertions(+), 3 deletions(-)

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