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authorH.J. Lu <hjl.tools@gmail.com>2015-10-21 14:44:23 -0700
committerH.J. Lu <hjl.tools@gmail.com>2015-12-15 13:16:02 -0800
commitb9eb92ab05204df772eb4929eccd018637c9f3e9 (patch)
treef272c9a3e0ef7232d0714cfda7e79b040a757fb9 /sysdeps/x86
parentc9afcaaafaf9126a973883528eed8d0058a264c0 (diff)
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Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT hjl/32bit/master
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.
Diffstat (limited to 'sysdeps/x86')
-rw-r--r--sysdeps/x86/cpu-features.h3
1 files changed, 3 insertions, 0 deletions
diff --git a/sysdeps/x86/cpu-features.h b/sysdeps/x86/cpu-features.h
index 80edbee75c..93bee69eaf 100644
--- a/sysdeps/x86/cpu-features.h
+++ b/sysdeps/x86/cpu-features.h
@@ -33,6 +33,7 @@
 #define bit_AVX512DQ_Usable		(1 << 13)
 #define bit_I586			(1 << 14)
 #define bit_I686			(1 << 15)
+#define bit_Prefer_MAP_32BIT_EXEC	(1 << 16)
 
 /* CPUID Feature flags.  */
 
@@ -97,6 +98,7 @@
 # define index_AVX512DQ_Usable		FEATURE_INDEX_1*FEATURE_SIZE
 # define index_I586			FEATURE_INDEX_1*FEATURE_SIZE
 # define index_I686			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__
@@ -248,6 +250,7 @@ extern const struct cpu_features *__get_cpu_features (void)
 # define index_AVX512DQ_Usable		FEATURE_INDEX_1
 # define index_I586			FEATURE_INDEX_1
 # define index_I686			FEATURE_INDEX_1
+# define index_Prefer_MAP_32BIT_EXEC	FEATURE_INDEX_1
 
 #endif	/* !__ASSEMBLER__ */