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x86: Optimize memrchr-evex.S

Browse Source 
Optimizations are:
1. Use the fact that lzcnt(0) -> VEC_SIZE for memchr to save a branch
   in short string case.
2. Save several instructions in len = [VEC_SIZE, 4 * VEC_SIZE] case.
3. Use more code-size efficient instructions.
	- tzcnt ...     -> bsf ...
	- vpcmpb $0 ... -> vpcmpeq ...

Code Size Changes:
memrchr-evex.S      :  -29 bytes

Net perf changes:

Reported as geometric mean of all improvements / regressions from N=10
runs of the benchtests. Value as New Time / Old Time so < 1.0 is
improvement and 1.0 is regression.

memrchr-evex.S      : 0.949 (Mostly from improvements in small strings)

Full results attached in email.

Full check passes on x86-64.
This commit is contained in:
Noah Goldstein 2022-10-18 17:44:06 -07:00
parent b79f8ff26a
commit 4af6844aa5
1 changed files with 360 additions and 250 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

610
sysdeps/x86_64/multiarch/memrchr-evex.S
View File

@ -21,17 +21,19 @@
#if ISA_SHOULD_BUILD (4)
# include <sysdep.h>
# include "x86-evex256-vecs.h"
# if VEC_SIZE != 32
# error "VEC_SIZE != 32 unimplemented"
# ifndef VEC_SIZE
# include "x86-evex256-vecs.h"
# endif
# include "reg-macros.h"
# ifndef MEMRCHR
# define MEMRCHR __memrchr_evex
# define MEMRCHR __memrchr_evex
# endif
# define PAGE_SIZE 4096
# define VMMMATCH VMM(0)
# define PAGE_SIZE 4096
# define VMATCH VMM(0)
.section SECTION(.text), "ax", @progbits
ENTRY_P2ALIGN(MEMRCHR, 6)
@ -43,294 +45,402 @@ ENTRY_P2ALIGN(MEMRCHR, 6)
# endif
jz L(zero_0)
/* Get end pointer. Minus one for two reasons. 1) It is necessary for a
correct page cross check and 2) it correctly sets up end ptr to be
subtract by lzcnt aligned. */
/* Get end pointer. Minus one for three reasons. 1) It is
necessary for a correct page cross check and 2) it correctly
sets up end ptr to be subtract by lzcnt aligned. 3) it is a
necessary step in aligning ptr. */
leaq -1(%rdi, %rdx), %rax
vpbroadcastb %esi, %VMMMATCH
vpbroadcastb %esi, %VMATCH
/* Check if we can load 1x VEC without cross a page. */
testl $(PAGE_SIZE - VEC_SIZE), %eax
jz L(page_cross)
/* Don't use rax for pointer here because EVEX has better encoding with
offset % VEC_SIZE == 0. */
vpcmpb $0, -(VEC_SIZE)(%rdi, %rdx), %VMMMATCH, %k0
kmovd %k0, %ecx
/* Don't use rax for pointer here because EVEX has better
encoding with offset % VEC_SIZE == 0. */
vpcmpeqb (VEC_SIZE * -1)(%rdi, %rdx), %VMATCH, %k0
KMOV %k0, %VRCX
/* Fall through for rdx (len) <= VEC_SIZE (expect small sizes). */
cmpq $VEC_SIZE, %rdx
ja L(more_1x_vec)
L(ret_vec_x0_test):
/* If rcx is zero then lzcnt -> VEC_SIZE. NB: there is a
already a dependency between rcx and rsi so no worries about
false-dep here. */
lzcnt %VRCX, %VRSI
/* If rdx <= rsi then either 1) rcx was non-zero (there was a
match) but it was out of bounds or 2) rcx was zero and rdx
was <= VEC_SIZE so we are done scanning. */
cmpq %rsi, %rdx
/* NB: Use branch to return zero/non-zero. Common usage will
branch on result of function (if return is null/non-null).
This branch can be used to predict the ensuing one so there
is no reason to extend the data-dependency with cmovcc. */
jbe L(zero_0)
/* If ecx is zero (no matches) lzcnt will set it 32 (VEC_SIZE) which
will guarantee edx (len) is less than it. */
lzcntl %ecx, %ecx
cmpl %ecx, %edx
jle L(zero_0)
subq %rcx, %rax
/* If rcx is zero then len must be > RDX, otherwise since we
already tested len vs lzcnt(rcx) (in rsi) we are good to
return this match. */
test %VRCX, %VRCX
jz L(more_1x_vec)
subq %rsi, %rax
ret
/* Fits in aligning bytes of first cache line. */
/* Fits in aligning bytes of first cache line for VEC_SIZE ==
32. */
# if VEC_SIZE == 32
.p2align 4,, 2
L(zero_0):
xorl %eax, %eax
ret
.p2align 4,, 9
L(ret_vec_x0_dec):
decq %rax
L(ret_vec_x0):
lzcntl %ecx, %ecx
subq %rcx, %rax
ret
# endif
.p2align 4,, 10
L(more_1x_vec):
testl %ecx, %ecx
jnz L(ret_vec_x0)
/* Align rax (pointer to string). */
andq $-VEC_SIZE, %rax
L(page_cross_continue):
/* Recompute length after aligning. */
movq %rax, %rdx
subq %rdi, %rax
/* Need no matter what. */
vpcmpb $0, -(VEC_SIZE)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
subq %rdi, %rdx
cmpq $(VEC_SIZE * 2), %rdx
cmpq $(VEC_SIZE * 2), %rax
ja L(more_2x_vec)
L(last_2x_vec):
vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
/* Must dec rax because L(ret_vec_x0_test) expects it. */
decq %rax
cmpl $VEC_SIZE, %edx
jbe L(ret_vec_x0_test)
test %VRCX, %VRCX
jnz L(ret_vec_x0_test)
testl %ecx, %ecx
jnz L(ret_vec_x0)
/* If VEC_SIZE == 64 need to subtract because lzcntq won't
implicitly add VEC_SIZE to match position. */
# if VEC_SIZE == 64
subl $VEC_SIZE, %eax
# else
cmpb $VEC_SIZE, %al
# endif
jle L(zero_2)
/* Don't use rax for pointer here because EVEX has better encoding with
offset % VEC_SIZE == 0. */
vpcmpb $0, -(VEC_SIZE * 2)(%rdi, %rdx), %VMMMATCH, %k0
kmovd %k0, %ecx
/* NB: 64-bit lzcnt. This will naturally add 32 to position. */
/* We adjusted rax (length) for VEC_SIZE == 64 so need seperate
offsets. */
# if VEC_SIZE == 64
vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
# else
vpcmpeqb (VEC_SIZE * -2)(%rdi, %rax), %VMATCH, %k0
# endif
KMOV %k0, %VRCX
/* NB: 64-bit lzcnt. This will naturally add 32 to position for
VEC_SIZE == 32. */
lzcntq %rcx, %rcx
cmpl %ecx, %edx
jle L(zero_0)
subq %rcx, %rax
ret
/* Inexpensive place to put this regarding code size / target alignments
/ ICache NLP. Necessary for 2-byte encoding of jump to page cross
case which in turn is necessary for hot path (len <= VEC_SIZE) to fit
in first cache line. */
L(page_cross):
movq %rax, %rsi
andq $-VEC_SIZE, %rsi
vpcmpb $0, (%rsi), %VMMMATCH, %k0
kmovd %k0, %r8d
/* Shift out negative alignment (because we are starting from endptr and
working backwards). */
movl %eax, %ecx
/* notl because eax already has endptr - 1. (-x = ~(x - 1)). */
notl %ecx
shlxl %ecx, %r8d, %ecx
cmpq %rdi, %rsi
ja L(more_1x_vec)
lzcntl %ecx, %ecx
cmpl %ecx, %edx
jle L(zero_1)
subq %rcx, %rax
ret
/* Continue creating zero labels that fit in aligning bytes and get
2-byte encoding / are in the same cache line as condition. */
L(zero_1):
subl %ecx, %eax
ja L(first_vec_x1_ret)
/* If VEC_SIZE == 64 put L(zero_0) here as we can't fit in the
first cache line (this is the second cache line). */
# if VEC_SIZE == 64
L(zero_0):
# endif
L(zero_2):
xorl %eax, %eax
ret
.p2align 4,, 8
L(ret_vec_x1):
/* This will naturally add 32 to position. */
bsrl %ecx, %ecx
leaq -(VEC_SIZE * 2)(%rcx, %rax), %rax
/* NB: Fits in aligning bytes before next cache line for
VEC_SIZE == 32. For VEC_SIZE == 64 this is attached to
L(first_vec_x0_test). */
# if VEC_SIZE == 32
L(first_vec_x1_ret):
leaq -1(%rdi, %rax), %rax
ret
# endif
.p2align 4,, 8
L(more_2x_vec):
testl %ecx, %ecx
jnz L(ret_vec_x0_dec)
vpcmpb $0, -(VEC_SIZE * 2)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
testl %ecx, %ecx
jnz L(ret_vec_x1)
/* Need no matter what. */
vpcmpb $0, -(VEC_SIZE * 3)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
subq $(VEC_SIZE * 4), %rdx
ja L(more_4x_vec)
cmpl $(VEC_SIZE * -1), %edx
jle L(ret_vec_x2_test)
L(last_vec):
testl %ecx, %ecx
jnz L(ret_vec_x2)
/* Need no matter what. */
vpcmpb $0, -(VEC_SIZE * 4)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
lzcntl %ecx, %ecx
subq $(VEC_SIZE * 3 + 1), %rax
subq %rcx, %rax
cmpq %rax, %rdi
ja L(zero_1)
ret
.p2align 4,, 8
L(ret_vec_x2_test):
lzcntl %ecx, %ecx
subq $(VEC_SIZE * 2 + 1), %rax
subq %rcx, %rax
cmpq %rax, %rdi
ja L(zero_1)
ret
.p2align 4,, 8
L(ret_vec_x2):
bsrl %ecx, %ecx
leaq -(VEC_SIZE * 3)(%rcx, %rax), %rax
ret
.p2align 4,, 8
L(ret_vec_x3):
bsrl %ecx, %ecx
leaq -(VEC_SIZE * 4)(%rcx, %rax), %rax
ret
.p2align 4,, 8
L(more_4x_vec):
testl %ecx, %ecx
jnz L(ret_vec_x2)
vpcmpb $0, -(VEC_SIZE * 4)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
testl %ecx, %ecx
jnz L(ret_vec_x3)
/* Check if near end before re-aligning (otherwise might do an
unnecessary loop iteration). */
addq $-(VEC_SIZE * 4), %rax
cmpq $(VEC_SIZE * 4), %rdx
jbe L(last_4x_vec)
decq %rax
andq $-(VEC_SIZE * 4), %rax
movq %rdi, %rdx
/* Get endptr for loop in rdx. NB: Can't just do while rax > rdi because
lengths that overflow can be valid and break the comparison. */
andq $-(VEC_SIZE * 4), %rdx
.p2align 4
L(loop_4x_vec):
/* Store 1 were not-equals and 0 where equals in k1 (used to mask later
on). */
vpcmpb $4, (VEC_SIZE * 3)(%rax), %VMMMATCH, %k1
/* VEC(2/3) will have zero-byte where we found a CHAR. */
vpxorq (VEC_SIZE * 2)(%rax), %VMMMATCH, %VMM(2)
vpxorq (VEC_SIZE * 1)(%rax), %VMMMATCH, %VMM(3)
vpcmpb $0, (VEC_SIZE * 0)(%rax), %VMMMATCH, %k4
/* Combine VEC(2/3) with min and maskz with k1 (k1 has zero bit where
CHAR is found and VEC(2/3) have zero-byte where CHAR is found. */
vpminub %VMM(2), %VMM(3), %VMM(3){%k1}{z}
vptestnmb %VMM(3), %VMM(3), %k2
/* Any 1s and we found CHAR. */
kortestd %k2, %k4
jnz L(loop_end)
addq $-(VEC_SIZE * 4), %rax
cmpq %rdx, %rax
jne L(loop_4x_vec)
/* Need to re-adjust rdx / rax for L(last_4x_vec). */
subq $-(VEC_SIZE * 4), %rdx
movq %rdx, %rax
subl %edi, %edx
L(last_4x_vec):
/* Used no matter what. */
vpcmpb $0, (VEC_SIZE * -1)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
cmpl $(VEC_SIZE * 2), %edx
jbe L(last_2x_vec)
testl %ecx, %ecx
jnz L(ret_vec_x0_dec)
vpcmpb $0, (VEC_SIZE * -2)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
testl %ecx, %ecx
jnz L(ret_vec_x1)
/* Used no matter what. */
vpcmpb $0, (VEC_SIZE * -3)(%rax), %VMMMATCH, %k0
kmovd %k0, %ecx
cmpl $(VEC_SIZE * 3), %edx
ja L(last_vec)
lzcntl %ecx, %ecx
subq $(VEC_SIZE * 2 + 1), %rax
subq %rcx, %rax
cmpq %rax, %rdi
jbe L(ret_1)
xorl %eax, %eax
L(ret_1):
.p2align 4,, 6
L(ret_vec_x0_test):
lzcnt %VRCX, %VRCX
subl %ecx, %eax
jle L(zero_2)
# if VEC_SIZE == 64
/* Reuse code at the end of L(ret_vec_x0_test) as we can't fit
L(first_vec_x1_ret) in the same cache line as its jmp base
so we might as well save code size. */
L(first_vec_x1_ret):
# endif
leaq -1(%rdi, %rax), %rax
ret
.p2align 4,, 6
L(loop_end):
kmovd %k1, %ecx
notl %ecx
testl %ecx, %ecx
jnz L(ret_vec_x0_end)
L(loop_last_4x_vec):
/* Compute remaining length. */
subl %edi, %eax
L(last_4x_vec):
cmpl $(VEC_SIZE * 2), %eax
jle L(last_2x_vec)
# if VEC_SIZE == 32
/* Only align for VEC_SIZE == 32. For VEC_SIZE == 64 we need
the spare bytes to align the loop properly. */
.p2align 4,, 10
# endif
L(more_2x_vec):
/* Length > VEC_SIZE * 2 so check the first 2x VEC for match and
return if either hit. */
vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
test %VRCX, %VRCX
jnz L(first_vec_x0)
vpcmpeqb (VEC_SIZE * -2)(%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
test %VRCX, %VRCX
jnz L(first_vec_x1)
/* Need no matter what. */
vpcmpeqb (VEC_SIZE * -3)(%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
/* Check if we are near the end. */
subq $(VEC_SIZE * 4), %rax
ja L(more_4x_vec)
test %VRCX, %VRCX
jnz L(first_vec_x2_test)
/* Adjust length for final check and check if we are at the end.
*/
addl $(VEC_SIZE * 1), %eax
jle L(zero_1)
vpcmpeqb (VEC_SIZE * -1)(%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
lzcnt %VRCX, %VRCX
subl %ecx, %eax
ja L(first_vec_x3_ret)
L(zero_1):
xorl %eax, %eax
ret
L(first_vec_x3_ret):
leaq -1(%rdi, %rax), %rax
ret
.p2align 4,, 6
L(first_vec_x2_test):
/* Must adjust length before check. */
subl $-(VEC_SIZE * 2 - 1), %eax
lzcnt %VRCX, %VRCX
subl %ecx, %eax
jl L(zero_4)
addq %rdi, %rax
ret
.p2align 4,, 10
L(first_vec_x0):
bsr %VRCX, %VRCX
leaq (VEC_SIZE * -1)(%rdi, %rax), %rax
addq %rcx, %rax
ret
/* Fits unobtrusively here. */
L(zero_4):
xorl %eax, %eax
ret
.p2align 4,, 10
L(first_vec_x1):
bsr %VRCX, %VRCX
leaq (VEC_SIZE * -2)(%rdi, %rax), %rax
addq %rcx, %rax
ret
.p2align 4,, 8
L(first_vec_x3):
bsr %VRCX, %VRCX
addq %rdi, %rax
addq %rcx, %rax
ret
.p2align 4,, 6
L(first_vec_x2):
bsr %VRCX, %VRCX
leaq (VEC_SIZE * 1)(%rdi, %rax), %rax
addq %rcx, %rax
ret
.p2align 4,, 2
L(more_4x_vec):
test %VRCX, %VRCX
jnz L(first_vec_x2)
vpcmpeqb (%rdi, %rax), %VMATCH, %k0
KMOV %k0, %VRCX
test %VRCX, %VRCX
jnz L(first_vec_x3)
/* Check if near end before re-aligning (otherwise might do an
unnecessary loop iteration). */
cmpq $(VEC_SIZE * 4), %rax
jbe L(last_4x_vec)
/* NB: We setup the loop to NOT use index-address-mode for the
buffer. This costs some instructions & code size but avoids
stalls due to unlaminated micro-fused instructions (as used
in the loop) from being forced to issue in the same group
(essentially narrowing the backend width). */
/* Get endptr for loop in rdx. NB: Can't just do while rax > rdi
because lengths that overflow can be valid and break the
comparison. */
# if VEC_SIZE == 64
/* Use rdx as intermediate to compute rax, this gets us imm8
encoding which just allows the L(more_4x_vec) block to fit
in 1 cache-line. */
leaq (VEC_SIZE * 4)(%rdi), %rdx
leaq (VEC_SIZE * -1)(%rdx, %rax), %rax
/* No evex machine has partial register stalls. This can be
replaced with: `andq $(VEC_SIZE * -4), %rax/%rdx` if that
changes. */
xorb %al, %al
xorb %dl, %dl
# else
leaq (VEC_SIZE * 3)(%rdi, %rax), %rax
andq $(VEC_SIZE * -4), %rax
leaq (VEC_SIZE * 4)(%rdi), %rdx
andq $(VEC_SIZE * -4), %rdx
# endif
.p2align 4
L(loop_4x_vec):
/* NB: We could do the same optimization here as we do for
memchr/rawmemchr by using VEX encoding in the loop for access
to VEX vpcmpeqb + vpternlogd. Since memrchr is not as hot as
memchr it may not be worth the extra code size, but if the
need arises it an easy ~15% perf improvement to the loop. */
cmpq %rdx, %rax
je L(loop_last_4x_vec)
/* Store 1 were not-equals and 0 where equals in k1 (used to
mask later on). */
vpcmpb $4, (VEC_SIZE * -1)(%rax), %VMATCH, %k1
/* VEC(2/3) will have zero-byte where we found a CHAR. */
vpxorq (VEC_SIZE * -2)(%rax), %VMATCH, %VMM(2)
vpxorq (VEC_SIZE * -3)(%rax), %VMATCH, %VMM(3)
vpcmpeqb (VEC_SIZE * -4)(%rax), %VMATCH, %k4
/* Combine VEC(2/3) with min and maskz with k1 (k1 has zero bit
where CHAR is found and VEC(2/3) have zero-byte where CHAR
is found. */
vpminub %VMM(2), %VMM(3), %VMM(3){%k1}{z}
vptestnmb %VMM(3), %VMM(3), %k2
addq $-(VEC_SIZE * 4), %rax
/* Any 1s and we found CHAR. */
KORTEST %k2, %k4
jz L(loop_4x_vec)
/* K1 has non-matches for first VEC. inc; jz will overflow rcx
iff all bytes where non-matches. */
KMOV %k1, %VRCX
inc %VRCX
jnz L(first_vec_x0_end)
vptestnmb %VMM(2), %VMM(2), %k0
kmovd %k0, %ecx
testl %ecx, %ecx
jnz L(ret_vec_x1_end)
KMOV %k0, %VRCX
test %VRCX, %VRCX
jnz L(first_vec_x1_end)
KMOV %k2, %VRCX
kmovd %k2, %ecx
kmovd %k4, %esi
/* Combine last 2 VEC matches. If ecx (VEC3) is zero (no CHAR in VEC3)
then it won't affect the result in esi (VEC4). If ecx is non-zero
then CHAR in VEC3 and bsrq will use that position. */
/* Seperate logic for VEC_SIZE == 64 and VEC_SIZE == 32 for
returning last 2x VEC. For VEC_SIZE == 64 we test each VEC
individually, for VEC_SIZE == 32 we combine them in a single
64-bit GPR. */
# if VEC_SIZE == 64
test %VRCX, %VRCX
jnz L(first_vec_x2_end)
KMOV %k4, %VRCX
# else
/* Combine last 2 VEC matches for VEC_SIZE == 32. If rcx (from
VEC(3)) is zero (no CHAR in VEC(3)) then it won't affect the
result in rsi (from VEC(4)). If rcx is non-zero then CHAR in
VEC(3) and bsrq will use that position. */
KMOV %k4, %VRSI
salq $32, %rcx
orq %rsi, %rcx
# endif
bsrq %rcx, %rcx
addq %rcx, %rax
ret
.p2align 4,, 4
L(ret_vec_x0_end):
addq $(VEC_SIZE), %rax
L(ret_vec_x1_end):
bsrl %ecx, %ecx
leaq (VEC_SIZE * 2)(%rax, %rcx), %rax
L(first_vec_x0_end):
/* rcx has 1s at non-matches so we need to `not` it. We used
`inc` to test if zero so use `neg` to complete the `not` so
the last 1 bit represent a match. NB: (-x + 1 == ~x). */
neg %VRCX
bsr %VRCX, %VRCX
leaq (VEC_SIZE * 3)(%rcx, %rax), %rax
ret
.p2align 4,, 10
L(first_vec_x1_end):
bsr %VRCX, %VRCX
leaq (VEC_SIZE * 2)(%rcx, %rax), %rax
ret
# if VEC_SIZE == 64
/* Since we can't combine the last 2x VEC for VEC_SIZE == 64
need return label for it. */
.p2align 4,, 4
L(first_vec_x2_end):
bsr %VRCX, %VRCX
leaq (VEC_SIZE * 1)(%rcx, %rax), %rax
ret
# endif
.p2align 4,, 4
L(page_cross):
/* only lower bits of eax[log2(VEC_SIZE):0] are set so we can
use movzbl to get the amount of bytes we are checking here.
*/
movzbl %al, %ecx
andq $-VEC_SIZE, %rax
vpcmpeqb (%rax), %VMATCH, %k0
KMOV %k0, %VRSI
/* eax was comptued as %rdi + %rdx - 1 so need to add back 1
here. */
leal 1(%rcx), %r8d
/* Invert ecx to get shift count for byte matches out of range.
*/
notl %ecx
shlx %VRCX, %VRSI, %VRSI
/* if r8 < rdx then the entire [buf, buf + len] is handled in
the page cross case. NB: we can't use the trick here we use
in the non page-cross case because we aren't checking full
VEC_SIZE. */
cmpq %r8, %rdx
ja L(page_cross_check)
lzcnt %VRSI, %VRSI
subl %esi, %edx
ja L(page_cross_ret)
xorl %eax, %eax
ret
L(page_cross_check):
test %VRSI, %VRSI
jz L(page_cross_continue)
lzcnt %VRSI, %VRSI
subl %esi, %edx
L(page_cross_ret):
leaq -1(%rdi, %rdx), %rax
ret
END(MEMRCHR)
#endif