[Bug dynamic-link/19367] Improve branch prediction on Silvermont

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

https://sourceware.org/bugzilla/show_bug.cgi?id=19367

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=b9eb92ab05204df772eb4929eccd018637c9f3e9

commit b9eb92ab05204df772eb4929eccd018637c9f3e9
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Oct 21 14:44:23 2015 -0700

    Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT

    According to Silvermont software optimization guide, for 64-bit
    applications, branch prediction performance can be negatively impacted
    when the target of a branch is more than 4GB away from the branch.  Add
    the Prefer_MAP_32BIT_EXEC bit so that mmap will try to map executable
    pages with MAP_32BIT first.  NB: MAP_32BIT will map to lower 2GB, not
    lower 4GB, address.  Prefer_MAP_32BIT_EXEC reduces bits available for
    address space layout randomization (ASLR), which is always disabled for
    SUID programs and can only be enabled by setting environment variable,
    LD_PREFER_MAP_32BIT_EXEC.

    On Fedora 23, this patch speeds up GCC 5 testsuite by 3% on Silvermont.

        [BZ #19367]
        * sysdeps/unix/sysv/linux/wordsize-64/mmap.c: New file.
        * sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h: Likewise.
        * sysdeps/unix/sysv/linux/x86_64/64/mmap.c: Likewise.
        * sysdeps/x86/cpu-features.h (bit_Prefer_MAP_32BIT_EXEC): New.
        (index_Prefer_MAP_32BIT_EXEC): Likewise.

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

Summary of changes:
 sysdeps/unix/sysv/linux/wordsize-64/mmap.c      |   40 ++++++++++++++++++++
 sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h |   44 +++++++++++++++++++++++
 sysdeps/unix/sysv/linux/x86_64/64/mmap.c        |   37 +++++++++++++++++++
 sysdeps/x86/cpu-features.h                      |    3 ++
 4 files changed, 124 insertions(+), 0 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/wordsize-64/mmap.c
 create mode 100644 sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
 create mode 100644 sysdeps/unix/sysv/linux/x86_64/64/mmap.c

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