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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.
38 lines
1.4 KiB
C
38 lines
1.4 KiB
C
/* Linux mmap system call. x86-64 version.
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Copyright (C) 2015 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <ldsodefs.h>
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/* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable pages
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with MAP_32BIT first. */
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#define MMAP_PREPARE(addr, len, prot, flags, fd, offset) \
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if ((addr) == NULL \
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&& ((prot) & PROT_EXEC) != 0 \
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&& HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC)) \
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{ \
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__ptr_t ret = (__ptr_t) INLINE_SYSCALL (mmap, 6, (addr), (len), \
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(prot), \
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(flags) | MAP_32BIT, \
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(fd), (offset)); \
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if (ret != MAP_FAILED) \
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return ret; \
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}
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#include <sysdeps/unix/sysv/linux/wordsize-64/mmap.c>
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