The AVX2 strcmp implementation uses the 'bzhi' instruction, which
belongs to the BMI2 CPU feature.
NB: It also uses the 'tzcnt' BMI1 instruction, but it is executed as BSF
as BSF if the CPU doesn't support TZCNT, and produces the same result
for non-zero input.
Partially fixes: b77b06e0e2 ("x86: Optimize strcmp-avx2.S")
Partially resolves: BZ #29611
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
1. Add default ISA level selection in non-multiarch/rtld
implementations.
2. Add ISA level build guards to different implementations.
- I.e strcmp-avx2.S which is ISA level 3 will only build if
compiled ISA level <= 3. Otherwise there is no reason to
include it as we will always use one of the ISA level 4
implementations (strcmp-evex.S).
3. Refactor the ifunc selector and ifunc implementation list to use
the ISA level aware wrapper macros that allow functions below the
compiled ISA level (with a guranteed replacement) to be skipped.
Tested with and without multiarch on x86_64 for ISA levels:
{generic, x86-64-v2, x86-64-v3, x86-64-v4}
And m32 with and without multiarch.
This has been missing since the the ifuncs where added.
The performance of SSE4.2 is preferable to to SSE2.
Measured on Tigerlake with N = 20 runs.
Geometric Mean of all benchmarks SSE4.2 / SSE2: 0.906
With SSE2, SSE4.1, AVX2, and EVEX versions very few targets prefer
SSSE3. As a result it is no longer worth it to keep the SSSE3
versions given the code size cost.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
I used these shell commands:
../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")
and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 7061 files FOO.
I then removed trailing white space from math/tgmath.h,
support/tst-support-open-dev-null-range.c, and
sysdeps/x86_64/multiarch/strlen-vec.S, to work around the following
obscure pre-commit check failure diagnostics from Savannah. I don't
know why I run into these diagnostics whereas others evidently do not.
remote: *** 912-#endif
remote: *** 913:
remote: *** 914-
remote: *** error: lines with trailing whitespace found
...
remote: *** error: sysdeps/unix/sysv/linux/statx_cp.c: trailing lines
Remove Prefer_AVX2_STRCMP to enable EVEX strcmp. When comparing 2 32-byte
strings, EVEX strcmp has been improved to require 1 load, 1 VPTESTM, 1
VPCMP, 1 KMOVD and 1 INCL instead of 2 loads, 3 VPCMPs, 2 KORDs, 1 KMOVD
and 1 TESTL while AVX2 strcmp requires 1 load, 2 VPCMPEQs, 1 VPMINU, 1
VPMOVMSKB and 1 TESTL. EVEX strcmp is now faster than AVX2 strcmp by up
to 40% on Tiger Lake and Ice Lake.
Since VZEROUPPER triggers RTM abort while VZEROALL won't, select AVX
optimized string/memory functions with
xtest
jz 1f
vzeroall
ret
1:
vzeroupper
ret
at function exit on processors with usable RTM, but without 256-bit EVEX
instructions to avoid VZEROUPPER inside a transactionally executing RTM
region.
Update ifunc-avx2.h, strchr.c, strcmp.c, strncmp.c and wcsnlen.c to
select the function optimized with 256-bit EVEX instructions using
YMM16-YMM31 registers to avoid RTM abort with usable AVX512VL, AVX512BW
and BMI2 since VZEROUPPER isn't needed at function exit.
For strcmp/strncmp, prefer AVX2 strcmp/strncmp if Prefer_AVX2_STRCMP
is set.
I used these shell commands:
../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")
and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 6694 files FOO.
I then removed trailing white space from benchtests/bench-pthread-locks.c
and iconvdata/tst-iconv-big5-hkscs-to-2ucs4.c, to work around this
diagnostic from Savannah:
remote: *** pre-commit check failed ...
remote: *** error: lines with trailing whitespace found
remote: error: hook declined to update refs/heads/master
Support usable check for all CPU features with the following changes:
1. Change struct cpu_features to
struct cpuid_features
{
struct cpuid_registers cpuid;
struct cpuid_registers usable;
};
struct cpu_features
{
struct cpu_features_basic basic;
struct cpuid_features features[COMMON_CPUID_INDEX_MAX];
unsigned int preferred[PREFERRED_FEATURE_INDEX_MAX];
...
};
so that there is a usable bit for each cpuid bit.
2. After the cpuid bits have been initialized, copy the known bits to the
usable bits. EAX/EBX from INDEX_1 and EAX from INDEX_7 aren't used for
CPU feature detection.
3. Clear the usable bits which require OS support.
4. If the feature is supported by OS, copy its cpuid bit to its usable
bit.
5. Replace HAS_CPU_FEATURE and CPU_FEATURES_CPU_P with CPU_FEATURE_USABLE
and CPU_FEATURE_USABLE_P to check if a feature is usable.
6. Add DEPR_FPU_CS_DS for INDEX_7_EBX_13.
7. Unset MPX feature since it has been deprecated.
The results are
1. If the feature is known and doesn't requre OS support, its usable bit
is copied from the cpuid bit.
2. Otherwise, its usable bit is copied from the cpuid bit only if the
feature is known to supported by OS.
3. CPU_FEATURE_USABLE/CPU_FEATURE_USABLE_P are used to check if the
feature can be used.
4. HAS_CPU_FEATURE/CPU_FEATURE_CPU_P are used to check if CPU supports
the feature.
GCC 9 has gained an enhancement to help detect attribute mismatches
between alias declarations and their targets. It consists of a new
warning, -Wattribute-alias, an enhancement to an existing warning,
-Wmissing-attributes, and a new attribute called copy.
The purpose of the warnings is to help identify either possible bugs
(an alias declared with more restrictive attributes than its target
promises) or optimization or diagnostic opportunities (an alias target
missing some attributes that it could be declared with that might
benefit analysis and code generation). The purpose of the new
attribute is to easily apply (almost) the same set of attributes
to one declaration as those already present on another.
As expected (and intended) the enhancement triggers warnings for
many alias declarations in Glibc code. This change, tested on
x86_64-linux, avoids all instances of the new warnings by making
use of the attribute where appropriate. To fully benefit from
the enhancement Glibc will need to be compiled with
-Wattribute-alias=2 and remaining warnings reviewed and dealt with
(there are a couple of thousand but most should be straightforward
to deal with).
ChangeLog:
* include/libc-symbols.h (__attribute_copy__): Define macro unless
it's already defined.
(_strong_alias): Use __attribute_copy__.
(_weak_alias, __hidden_ver1, __hidden_nolink2): Same.
* misc/sys/cdefs.h (__attribute_copy__): New macro.
* sysdeps/x86_64/multiarch/memchr.c (memchr): Use __attribute_copy__.
* sysdeps/x86_64/multiarch/memcmp.c (memcmp): Same.
* sysdeps/x86_64/multiarch/mempcpy.c (mempcpy): Same.
* sysdeps/x86_64/multiarch/memset.c (memset): Same.
* sysdeps/x86_64/multiarch/stpcpy.c (stpcpy): Same.
* sysdeps/x86_64/multiarch/strcat.c (strcat): Same.
* sysdeps/x86_64/multiarch/strchr.c (strchr): Same.
* sysdeps/x86_64/multiarch/strcmp.c (strcmp): Same.
* sysdeps/x86_64/multiarch/strcpy.c (strcpy): Same.
* sysdeps/x86_64/multiarch/strcspn.c (strcspn): Same.
* sysdeps/x86_64/multiarch/strlen.c (strlen): Same.
* sysdeps/x86_64/multiarch/strncmp.c (strncmp): Same.
* sysdeps/x86_64/multiarch/strncpy.c (strncpy): Same.
* sysdeps/x86_64/multiarch/strnlen.c (strnlen): Same.
* sysdeps/x86_64/multiarch/strpbrk.c (strpbrk): Same.
* sysdeps/x86_64/multiarch/strrchr.c (strrchr): Same.
* sysdeps/x86_64/multiarch/strspn.c (strspn): Same.
Optimize x86-64 strcmp/wcscmp and strncmp/wcsncmp with AVX2. It uses vector
comparison as much as possible. Peak performance observed on a SkyLake
machine: 9x, 3x, 2.5x and 5.5x for strcmp, strncmp, wcscmp and wcsncmp,
respectively. The larger the comparison length, the more benefit using
avx2 functions, except on the strcmp, where peak is observed at length
== 32 bytes. Select AVX2 strcmp/wcscmp on AVX2 machines where vzeroupper
is preferred and AVX unaligned load is fast.
NB: It uses TZCNT instead of BSF since TZCNT produces the same result
as BSF for non-zero input. TZCNT is faster than BSF and is executed
as BSF if machine doesn't support TZCNT.
* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
strcmp-avx2, strncmp-avx2, wcscmp-avx2, wcscmp-sse2, wcsncmp-avx2 and
wcsncmp-sse2.
* sysdeps/x86_64/multiarch/ifunc-impl-list.c
(__libc_ifunc_impl_list): Add tests for __strcmp_avx2,
__strncmp_avx2, __wcscmp_avx2, __wcsncmp_avx2, __wcscmp_sse2
and __wcsncmp_sse2.
* sysdeps/x86_64/multiarch/strcmp.c (OPTIMIZE (avx2)):
(IFUNC_SELECTOR): Return OPTIMIZE (avx2) on AVX 2 machines if
AVX unaligned load is fast and vzeroupper is preferred.
* sysdeps/x86_64/multiarch/strncmp.c: Likewise.
* sysdeps/x86_64/multiarch/strcmp-avx2.S: New file.
* sysdeps/x86_64/multiarch/strncmp-avx2.S: Likewise.
* sysdeps/x86_64/multiarch/wcscmp-avx2.S: Likewise.
* sysdeps/x86_64/multiarch/wcscmp-sse2.S: Likewise.
* sysdeps/x86_64/multiarch/wcscmp.c: Likewise.
* sysdeps/x86_64/multiarch/wcsncmp-avx2.S: Likewise.
* sysdeps/x86_64/multiarch/wcsncmp-sse2.c: Likewise.
* sysdeps/x86_64/multiarch/wcsncmp.c: Likewise.
* sysdeps/x86_64/wcscmp.S (__wcscmp): Add alias only if __wcscmp
is undefined.
Implement strcmp family IFUNC selectors in C.
All internal calls within libc.so can use IFUNC on x86-64 since unlike
x86, x86-64 supports PC-relative addressing to access the GOT entry so
that it can call via PLT without using an extra register. For libc.a,
we can't use IFUNC for functions which are called before IFUNC has been
initialized. Use IFUNC internally reduces the icache footprint since
libc.so and other codes in the process use the same implementations.
This patch uses IFUNC for strcmp family functions within libc.
* sysdeps/x86_64/multiarch/Makefile (sysdep_routines): Add
strcmp-sse2, strcmp-sse4_2, strncmp-sse2, strncmp-sse4_2,
strcasecmp_l-sse2, strcasecmp_l-sse4_2, strcasecmp_l-avx,
strncase_l-sse2, strncase_l-sse4_2 and strncase_l-avx.
* sysdeps/x86_64/multiarch/ifunc-strcasecmp.h: New file.
* sysdeps/x86_64/multiarch/strcasecmp.c: Likewise.
* sysdeps/x86_64/multiarch/strcasecmp_l-avx.S: Likewise.
* sysdeps/x86_64/multiarch/strcasecmp_l-sse2.S: Likewise.
* sysdeps/x86_64/multiarch/strcasecmp_l-sse4_2.S: Likewise.
* sysdeps/x86_64/multiarch/strcasecmp_l.c: Likewise.
* sysdeps/x86_64/multiarch/strcmp-sse2.S: Likewise.
* sysdeps/x86_64/multiarch/strcmp-sse4_2.S: Likewise.
* sysdeps/x86_64/multiarch/strcmp.c: Likewise.
* sysdeps/x86_64/multiarch/strncase.c: Likewise.
* sysdeps/x86_64/multiarch/strncase_l-avx.S : Likewise.
* sysdeps/x86_64/multiarch/strncase_l-sse2.S: Likewise.
* sysdeps/x86_64/multiarch/strncase_l-sse4_2.S: Likewise.
* sysdeps/x86_64/multiarch/strncase_l.c: Likewise.
* sysdeps/x86_64/multiarch/strncmp-sse2.S: Likewise.
* sysdeps/x86_64/multiarch/strncmp-sse4_2.S: Likewise.
* sysdeps/x86_64/multiarch/strncmp.c: Likewise.
* sysdeps/x86_64/multiarch/strcasecmp_l.S: Removed.
* sysdeps/x86_64/multiarch/strcmp.S: Likewise.
* sysdeps/x86_64/multiarch/strncase_l.S: Likewise.
* sysdeps/x86_64/multiarch/strncmp.S: Likewise.
* sysdeps/x86_64/multiarch/strcmp-sse42.S: Include <sysdep.h>.
(STRCMP_SSE42): New. Defined to __strcmp_sse42 if not defined.
[USE_AS_STRCASECMP_L || USE_AS_STRNCASECMP_L]: Include
"locale-defines.h".
(UPDATE_STRNCMP_COUNTER): New.
(SECTION): Likewise.
(GLABEL): Likewise.
(LABEL): Likewise.
* sysdeps/x86_64/multiarch/strncmp-ssse3.S: Rewrite and enable
for libc.a.
