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.
 
Index: glibc-2.17-c758a686/sysdeps/unix/sysv/linux/wordsize-64/mmap.c
===================================================================
--- /dev/null
+++ glibc-2.17-c758a686/sysdeps/unix/sysv/linux/wordsize-64/mmap.c
@@ -0,0 +1,40 @@
+/* Linux mmap system call.  64-bit version.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <errno.h>
+#include <sysdep.h>
+
+/* An architecture may override this.  */
+#ifndef MMAP_PREPARE
+# define MMAP_PREPARE(addr, len, prot, flags, fd, offset)
+#endif
+
+__ptr_t
+__mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
+{
+  MMAP_PREPARE (addr, len, prot, flags, fd, offset);
+  return (__ptr_t) INLINE_SYSCALL (mmap, 6, addr, len, prot, flags,
+				   fd, offset);
+}
+
+weak_alias (__mmap, mmap)
+weak_alias (__mmap, mmap64)
+weak_alias (__mmap, __mmap64)
Index: glibc-2.17-c758a686/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
===================================================================
--- /dev/null
+++ glibc-2.17-c758a686/sysdeps/unix/sysv/linux/x86_64/64/dl-librecon.h
@@ -0,0 +1,44 @@
+/* Optional code to distinguish library flavours.  x86-64 version.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_LIBRECON_H
+
+#include <sysdeps/unix/sysv/linux/dl-librecon.h>
+
+/* Recognizing extra environment variables.  For 64-bit applications,
+   branch prediction performance may 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).  Prefer_MAP_32BIT_EXEC
+   is always disabled for SUID programs and can be enabled by setting
+   environment variable, LD_PREFER_MAP_32BIT_EXEC.  */
+#define EXTRA_LD_ENVVARS \
+  case 21:							      \
+    if (memcmp (envline, "PREFER_MAP_32BIT_EXEC", 21) == 0)	      \
+      GLRO(dl_x86_cpu_features).feature[index_Prefer_MAP_32BIT_EXEC]  \
+	= bit_Prefer_MAP_32BIT_EXEC;				      \
+    break;
+
+/* Extra unsecure variables.  The names are all stuffed in a single
+   string which means they have to be terminated with a '\0' explicitly.  */
+#define EXTRA_UNSECURE_ENVVARS \
+  "LD_PREFER_MAP_32BIT_EXEC\0"
+
+#endif /* dl-librecon.h */
Index: glibc-2.17-c758a686/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
===================================================================
--- /dev/null
+++ glibc-2.17-c758a686/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
@@ -0,0 +1,37 @@
+/* Linux mmap system call.  x86-64 version.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+
+/* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable pages
+   with MAP_32BIT first.  */
+#define MMAP_PREPARE(addr, len, prot, flags, fd, offset)		\
+  if ((addr) == NULL							\
+      && ((prot) & PROT_EXEC) != 0					\
+      && HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC))			\
+    {									\
+      __ptr_t ret = (__ptr_t) INLINE_SYSCALL (mmap, 6, (addr), (len),	\
+					      (prot),			\
+					      (flags) | MAP_32BIT,	\
+					      (fd), (offset));		\
+      if (ret != MAP_FAILED)						\
+	return ret;							\
+    }
+
+#include <sysdeps/unix/sysv/linux/wordsize-64/mmap.c>
Index: glibc-2.17-c758a686/sysdeps/x86/cpu-features.h
===================================================================
--- glibc-2.17-c758a686.orig/sysdeps/x86/cpu-features.h
+++ glibc-2.17-c758a686/sysdeps/x86/cpu-features.h
@@ -32,6 +32,7 @@
 #define bit_AVX_Fast_Unaligned_Load	(1 << 11)
 #define bit_AVX512F_Usable		(1 << 12)
 #define bit_AVX512DQ_Usable		(1 << 13)
+#define bit_Prefer_MAP_32BIT_EXEC	(1 << 16)
 
 /* CPUID Feature flags.  */
 
@@ -91,6 +92,7 @@
 # define index_AVX_Fast_Unaligned_Load	FEATURE_INDEX_1*FEATURE_SIZE
 # define index_AVX512F_Usable		FEATURE_INDEX_1*FEATURE_SIZE
 # define index_AVX512DQ_Usable		FEATURE_INDEX_1*FEATURE_SIZE
+# define index_Prefer_MAP_32BIT_EXEC	FEATURE_INDEX_1*FEATURE_SIZE
 
 # if defined (_LIBC) && !IS_IN (nonlib)
 #  ifdef __x86_64__
@@ -267,6 +269,7 @@ extern const struct cpu_features *__get_
 # define index_AVX_Fast_Unaligned_Load	FEATURE_INDEX_1
 # define index_AVX512F_Usable		FEATURE_INDEX_1
 # define index_AVX512DQ_Usable		FEATURE_INDEX_1
+# define index_Prefer_MAP_32BIT_EXEC	FEATURE_INDEX_1
 
 #endif	/* !__ASSEMBLER__ */