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x86: Shrink code size of memchr-avx2.S

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This is not meant as a performance optimization. The previous code was
far to liberal in aligning targets and wasted code size unnecissarily.

The total code size saving is: 59 bytes

There are no major changes in the benchmarks.
Geometric Mean of all benchmarks New / Old: 0.967

Full xcheck passes on x86_64.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit 6dcbb7d95d)

x86: Fix page cross case in rawmemchr-avx2 [BZ #29234]

commit 6dcbb7d95d
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Mon Jun 6 21:11:33 2022 -0700

    x86: Shrink code size of memchr-avx2.S

Changed how the page cross case aligned string (rdi) in
rawmemchr. This was incompatible with how
`L(cross_page_continue)` expected the pointer to be aligned and
would cause rawmemchr to read data start started before the
beginning of the string. What it would read was in valid memory
but could count CHAR matches resulting in an incorrect return
value.

This commit fixes that issue by essentially reverting the changes to
the L(page_cross) case as they didn't really matter.

Test cases added and all pass with the new code (and where confirmed
to fail with the old code).
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit 2c9af8421d)
This commit is contained in:
Noah Goldstein 2022-06-06 21:11:33 -07:00 committed by Sunil K Pandey
parent 58ca9728a8
commit 992086b659
3 changed files with 117 additions and 52 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

57
string/test-rawmemchr.c
View File

@ -18,6 +18,7 @@
<https://www.gnu.org/licenses/>. */
#include <assert.h>
#include <support/xunistd.h>
#define TEST_MAIN
#define TEST_NAME "rawmemchr"
@ -51,13 +52,45 @@ do_one_test (impl_t *impl, const char *s, int c, char *exp_res)
}
}
static void
do_test_bz29234 (void)
{
size_t i, j;
char *ptr_start;
char *buf = xmmap (0, 8192, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1);
memset (buf, -1, 8192);
ptr_start = buf + 4096 - 8;
/* Out of range matches before the start of a page. */
memset (ptr_start - 8, 0x1, 8);
for (j = 0; j < 8; ++j)
{
for (i = 0; i < 128; ++i)
{
ptr_start[i + j] = 0x1;
FOR_EACH_IMPL (impl, 0)
do_one_test (impl, (char *) (ptr_start + j), 0x1,
ptr_start + i + j);
ptr_start[i + j] = 0xff;
}
}
xmunmap (buf, 8192);
}
static void
do_test (size_t align, size_t pos, size_t len, int seek_char)
{
size_t i;
char *result;
align &= 7;
align &= getpagesize () - 1;
if (align + len >= page_size)
return;
@ -115,6 +148,13 @@ do_random_tests (void)
}
}
if (align)
{
p[align - 1] = seek_char;
if (align > 4)
p[align - 4] = seek_char;
}
assert (pos < len);
size_t r = random ();
if ((r & 31) == 0)
@ -130,6 +170,13 @@ do_random_tests (void)
result, p);
ret = 1;
}
if (align)
{
p[align - 1] = seek_char;
if (align > 4)
p[align - 4] = seek_char;
}
}
}
@ -151,14 +198,22 @@ test_main (void)
do_test (i, 64, 256, 23);
do_test (0, 16 << i, 2048, 0);
do_test (i, 64, 256, 0);
do_test (getpagesize () - i, 64, 256, 23);
do_test (getpagesize () - i, 64, 256, 0);
}
for (i = 1; i < 32; ++i)
{
do_test (0, i, i + 1, 23);
do_test (0, i, i + 1, 0);
do_test (getpagesize () - 7, i, i + 1, 23);
do_test (getpagesize () - i / 2, i, i + 1, 23);
do_test (getpagesize () - i, i, i + 1, 23);
}
do_random_tests ();
do_test_bz29234 ();
return ret;
}

1
sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
View File

@ -2,6 +2,7 @@
# define MEMCHR __memchr_avx2_rtm
#endif
#define COND_VZEROUPPER COND_VZEROUPPER_XTEST
#define ZERO_UPPER_VEC_REGISTERS_RETURN \
ZERO_UPPER_VEC_REGISTERS_RETURN_XTEST

111
sysdeps/x86_64/multiarch/memchr-avx2.S
View File

@ -57,7 +57,7 @@
# define CHAR_PER_VEC (VEC_SIZE / CHAR_SIZE)
.section SECTION(.text),"ax",@progbits
ENTRY (MEMCHR)
ENTRY_P2ALIGN (MEMCHR, 5)
# ifndef USE_AS_RAWMEMCHR
/* Check for zero length. */
# ifdef __ILP32__
@ -87,12 +87,14 @@ ENTRY (MEMCHR)
# endif
testl %eax, %eax
jz L(aligned_more)
tzcntl %eax, %eax
bsfl %eax, %eax
addq %rdi, %rax
VZEROUPPER_RETURN
L(return_vzeroupper):
ZERO_UPPER_VEC_REGISTERS_RETURN
# ifndef USE_AS_RAWMEMCHR
.p2align 5
.p2align 4
L(first_vec_x0):
/* Check if first match was before length. */
tzcntl %eax, %eax
@ -100,58 +102,31 @@ L(first_vec_x0):
/* NB: Multiply length by 4 to get byte count. */
sall $2, %edx
# endif
xorl %ecx, %ecx
COND_VZEROUPPER
/* Use branch instead of cmovcc so L(first_vec_x0) fits in one fetch
block. branch here as opposed to cmovcc is not that costly. Common
usage of memchr is to check if the return was NULL (if string was
known to contain CHAR user would use rawmemchr). This branch will be
highly correlated with the user branch and can be used by most
modern branch predictors to predict the user branch. */
cmpl %eax, %edx
leaq (%rdi, %rax), %rax
cmovle %rcx, %rax
VZEROUPPER_RETURN
jle L(null)
addq %rdi, %rax
ret
# endif
.p2align 4,, 10
L(first_vec_x1):
bsfl %eax, %eax
incq %rdi
addq %rdi, %rax
VZEROUPPER_RETURN
# ifndef USE_AS_RAWMEMCHR
/* First in aligning bytes here. */
L(null):
xorl %eax, %eax
ret
# endif
.p2align 4
L(cross_page_boundary):
/* Save pointer before aligning as its original value is
necessary for computer return address if byte is found or
adjusting length if it is not and this is memchr. */
movq %rdi, %rcx
/* Align data to VEC_SIZE - 1. ALGN_PTR_REG is rcx for memchr
and rdi for rawmemchr. */
orq $(VEC_SIZE - 1), %ALGN_PTR_REG
VPCMPEQ -(VEC_SIZE - 1)(%ALGN_PTR_REG), %ymm0, %ymm1
vpmovmskb %ymm1, %eax
# ifndef USE_AS_RAWMEMCHR
/* Calculate length until end of page (length checked for a
match). */
leaq 1(%ALGN_PTR_REG), %rsi
subq %RRAW_PTR_REG, %rsi
# ifdef USE_AS_WMEMCHR
/* NB: Divide bytes by 4 to get wchar_t count. */
shrl $2, %esi
# endif
# endif
/* Remove the leading bytes. */
sarxl %ERAW_PTR_REG, %eax, %eax
# ifndef USE_AS_RAWMEMCHR
/* Check the end of data. */
cmpq %rsi, %rdx
jbe L(first_vec_x0)
# endif
testl %eax, %eax
jz L(cross_page_continue)
tzcntl %eax, %eax
addq %RRAW_PTR_REG, %rax
L(return_vzeroupper):
ZERO_UPPER_VEC_REGISTERS_RETURN
.p2align 4
L(first_vec_x1):
tzcntl %eax, %eax
incq %rdi
addq %rdi, %rax
VZEROUPPER_RETURN
.p2align 4
L(first_vec_x2):
tzcntl %eax, %eax
@ -340,7 +315,7 @@ L(first_vec_x1_check):
incq %rdi
addq %rdi, %rax
VZEROUPPER_RETURN
.p2align 4
.p2align 4,, 6
L(set_zero_end):
xorl %eax, %eax
VZEROUPPER_RETURN
@ -428,5 +403,39 @@ L(last_vec_x3):
VZEROUPPER_RETURN
# endif
.p2align 4
L(cross_page_boundary):
/* Save pointer before aligning as its original value is necessary for
computer return address if byte is found or adjusting length if it
is not and this is memchr. */
movq %rdi, %rcx
/* Align data to VEC_SIZE - 1. ALGN_PTR_REG is rcx for memchr
and rdi for rawmemchr. */
orq $(VEC_SIZE - 1), %ALGN_PTR_REG
VPCMPEQ -(VEC_SIZE - 1)(%ALGN_PTR_REG), %ymm0, %ymm1
vpmovmskb %ymm1, %eax
# ifndef USE_AS_RAWMEMCHR
/* Calculate length until end of page (length checked for a match). */
leaq 1(%ALGN_PTR_REG), %rsi
subq %RRAW_PTR_REG, %rsi
# ifdef USE_AS_WMEMCHR
/* NB: Divide bytes by 4 to get wchar_t count. */
shrl $2, %esi
# endif
# endif
/* Remove the leading bytes. */
sarxl %ERAW_PTR_REG, %eax, %eax
# ifndef USE_AS_RAWMEMCHR
/* Check the end of data. */
cmpq %rsi, %rdx
jbe L(first_vec_x0)
# endif
testl %eax, %eax
jz L(cross_page_continue)
bsfl %eax, %eax
addq %RRAW_PTR_REG, %rax
VZEROUPPER_RETURN
END (MEMCHR)
#endif