commit 83a986e9fbc301e6056dbc9d9ec6888621b60f67

Author: Noah Goldstein <goldstein.w.n@gmail.com>

Date:   Mon Jun 6 21:11:31 2022 -0700

    x86: Optimize memrchr-evex.S

    The new code:

        1. prioritizes smaller user-arg lengths more.

        2. optimizes target placement more carefully

        3. reuses logic more

        4. fixes up various inefficiencies in the logic. The biggest

           case here is the `lzcnt` logic for checking returns which

           saves either a branch or multiple instructions.

    The total code size saving is: 263 bytes

    Geometric Mean of all benchmarks New / Old: 0.755

    Regressions:

    There are some regressions. Particularly where the length (user arg

    length) is large but the position of the match char is near the

    beginning of the string (in first VEC). This case has roughly a

    20% regression.

    This is because the new logic gives the hot path for immediate matches

    to shorter lengths (the more common input). This case has roughly

    a 35% speedup.

    Full xcheck passes on x86_64.

    Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

    (cherry picked from commit b4209615a06b01c974f47b4998b00e4c7b1aa5d9)

diff --git a/sysdeps/x86_64/multiarch/memrchr-evex.S b/sysdeps/x86_64/multiarch/memrchr-evex.S

index 16bf8e02b1e80c84..bddc89c3754894ed 100644

--- a/sysdeps/x86_64/multiarch/memrchr-evex.S

+++ b/sysdeps/x86_64/multiarch/memrchr-evex.S

@@ -19,319 +19,316 @@

 #if IS_IN (libc)

 # include <sysdep.h>

+# include "evex256-vecs.h"

+# if VEC_SIZE != 32

+#  error "VEC_SIZE != 32 unimplemented"

+# endif

+

+# ifndef MEMRCHR

+#  define MEMRCHR				__memrchr_evex

+# endif

+

+# define PAGE_SIZE			4096

+# define VECMATCH			VEC(0)

+

+	.section SECTION(.text), "ax", @progbits

+ENTRY_P2ALIGN(MEMRCHR, 6)

+# ifdef __ILP32__

+	/* Clear upper bits.  */

+	and	%RDX_LP, %RDX_LP

+# else

+	test	%RDX_LP, %RDX_LP

+# 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.  */

+	leaq	-1(%rdi, %rdx), %rax

+	vpbroadcastb %esi, %VECMATCH

+

+	/* 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), %VECMATCH, %k0

+	kmovd	%k0, %ecx

+

+	/* 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 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

+	ret

-# define VMOVA		vmovdqa64

-

-# define YMMMATCH	ymm16

-

-# define VEC_SIZE 32

-

-	.section .text.evex,"ax",@progbits

-ENTRY (__memrchr_evex)

-	/* Broadcast CHAR to YMMMATCH.  */

-	vpbroadcastb %esi, %YMMMATCH

-

-	sub	$VEC_SIZE, %RDX_LP

-	jbe	L(last_vec_or_less)

-

-	add	%RDX_LP, %RDI_LP

-

-	/* Check the last VEC_SIZE bytes.  */

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x0)

-

-	subq	$(VEC_SIZE * 4), %rdi

-	movl	%edi, %ecx

-	andl	$(VEC_SIZE - 1), %ecx

-	jz	L(aligned_more)

-

-	/* Align data for aligned loads in the loop.  */

-	addq	$VEC_SIZE, %rdi

-	addq	$VEC_SIZE, %rdx

-	andq	$-VEC_SIZE, %rdi

-	subq	%rcx, %rdx

-

-	.p2align 4

-L(aligned_more):

-	subq	$(VEC_SIZE * 4), %rdx

-	jbe	L(last_4x_vec_or_less)

-

-	/* Check the last 4 * VEC_SIZE.  Only one VEC_SIZE at a time

-	   since data is only aligned to VEC_SIZE.  */

-	vpcmpb	$0, (VEC_SIZE * 3)(%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x3)

-

-	vpcmpb	$0, (VEC_SIZE * 2)(%rdi), %YMMMATCH, %k2

-	kmovd	%k2, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x2)

-

-	vpcmpb	$0, VEC_SIZE(%rdi), %YMMMATCH, %k3

-	kmovd	%k3, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x1)

-

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k4

-	kmovd	%k4, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x0)

-

-	/* Align data to 4 * VEC_SIZE for loop with fewer branches.

-	   There are some overlaps with above if data isn't aligned

-	   to 4 * VEC_SIZE.  */

-	movl	%edi, %ecx

-	andl	$(VEC_SIZE * 4 - 1), %ecx

-	jz	L(loop_4x_vec)

-

-	addq	$(VEC_SIZE * 4), %rdi

-	addq	$(VEC_SIZE * 4), %rdx

-	andq	$-(VEC_SIZE * 4), %rdi

-	subq	%rcx, %rdx

+	/* Fits in aligning bytes of first cache line.  */

+L(zero_0):

+	xorl	%eax, %eax

+	ret

-	.p2align 4

-L(loop_4x_vec):

-	/* Compare 4 * VEC at a time forward.  */

-	subq	$(VEC_SIZE * 4), %rdi

-	subq	$(VEC_SIZE * 4), %rdx

-	jbe	L(last_4x_vec_or_less)

-

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k1

-	vpcmpb	$0, VEC_SIZE(%rdi), %YMMMATCH, %k2

-	kord	%k1, %k2, %k5

-	vpcmpb	$0, (VEC_SIZE * 2)(%rdi), %YMMMATCH, %k3

-	vpcmpb	$0, (VEC_SIZE * 3)(%rdi), %YMMMATCH, %k4

-

-	kord	%k3, %k4, %k6

-	kortestd %k5, %k6

-	jz	L(loop_4x_vec)

-

-	/* There is a match.  */

-	kmovd	%k4, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x3)

-

-	kmovd	%k3, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x2)

-

-	kmovd	%k2, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x1)

-

-	kmovd	%k1, %eax

-	bsrl	%eax, %eax

-	addq	%rdi, %rax

+	.p2align 4,, 9

+L(ret_vec_x0_dec):

+	decq	%rax

+L(ret_vec_x0):

+	lzcntl	%ecx, %ecx

+	subq	%rcx, %rax

 	ret

-	.p2align 4

-L(last_4x_vec_or_less):

-	addl	$(VEC_SIZE * 4), %edx

-	cmpl	$(VEC_SIZE * 2), %edx

-	jbe	L(last_2x_vec)

+	.p2align 4,, 10

+L(more_1x_vec):

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x0)

-	vpcmpb	$0, (VEC_SIZE * 3)(%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x3)

+	/* Align rax (pointer to string).  */

+	andq	$-VEC_SIZE, %rax

-	vpcmpb	$0, (VEC_SIZE * 2)(%rdi), %YMMMATCH, %k2

-	kmovd	%k2, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x2)

+	/* Recompute length after aligning.  */

+	movq	%rax, %rdx

-	vpcmpb	$0, VEC_SIZE(%rdi), %YMMMATCH, %k3

-	kmovd	%k3, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x1_check)

-	cmpl	$(VEC_SIZE * 3), %edx

-	jbe	L(zero)

+	/* Need no matter what.  */

+	vpcmpb	$0, -(VEC_SIZE)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k4

-	kmovd	%k4, %eax

-	testl	%eax, %eax

-	jz	L(zero)

-	bsrl	%eax, %eax

-	subq	$(VEC_SIZE * 4), %rdx

-	addq	%rax, %rdx

-	jl	L(zero)

-	addq	%rdi, %rax

-	ret

+	subq	%rdi, %rdx

-	.p2align 4

+	cmpq	$(VEC_SIZE * 2), %rdx

+	ja	L(more_2x_vec)

 L(last_2x_vec):

-	vpcmpb	$0, (VEC_SIZE * 3)(%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x3_check)

+

+	/* Must dec rax because L(ret_vec_x0_test) expects it.  */

+	decq	%rax

 	cmpl	$VEC_SIZE, %edx

-	jbe	L(zero)

-

-	vpcmpb	$0, (VEC_SIZE * 2)(%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

-	testl	%eax, %eax

-	jz	L(zero)

-	bsrl	%eax, %eax

-	subq	$(VEC_SIZE * 2), %rdx

-	addq	%rax, %rdx

-	jl	L(zero)

-	addl	$(VEC_SIZE * 2), %eax

-	addq	%rdi, %rax

+	jbe	L(ret_vec_x0_test)

+

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x0)

+

+	/* Don't use rax for pointer here because EVEX has better encoding with

+	   offset % VEC_SIZE == 0.  */

+	vpcmpb	$0, -(VEC_SIZE * 2)(%rdi, %rdx), %VECMATCH, %k0

+	kmovd	%k0, %ecx

+	/* NB: 64-bit lzcnt. This will naturally add 32 to position.  */

+	lzcntq	%rcx, %rcx

+	cmpl	%ecx, %edx

+	jle	L(zero_0)

+	subq	%rcx, %rax

 	ret

-	.p2align 4

-L(last_vec_x0):

-	bsrl	%eax, %eax

-	addq	%rdi, %rax

+	/* 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), %VECMATCH, %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

-	.p2align 4

-L(last_vec_x1):

-	bsrl	%eax, %eax

-	addl	$VEC_SIZE, %eax

-	addq	%rdi, %rax

+	/* 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):

+	xorl	%eax, %eax

 	ret

-	.p2align 4

-L(last_vec_x2):

-	bsrl	%eax, %eax

-	addl	$(VEC_SIZE * 2), %eax

-	addq	%rdi, %rax

+	.p2align 4,, 8

+L(ret_vec_x1):

+	/* This will naturally add 32 to position.  */

+	bsrl	%ecx, %ecx

+	leaq	-(VEC_SIZE * 2)(%rcx, %rax), %rax

 	ret

-	.p2align 4

-L(last_vec_x3):

-	bsrl	%eax, %eax

-	addl	$(VEC_SIZE * 3), %eax

-	addq	%rdi, %rax

-	ret

+	.p2align 4,, 8

+L(more_2x_vec):

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x0_dec)

-	.p2align 4

-L(last_vec_x1_check):

-	bsrl	%eax, %eax

-	subq	$(VEC_SIZE * 3), %rdx

-	addq	%rax, %rdx

-	jl	L(zero)

-	addl	$VEC_SIZE, %eax

-	addq	%rdi, %rax

-	ret

+	vpcmpb	$0, -(VEC_SIZE * 2)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x1)

-	.p2align 4

-L(last_vec_x3_check):

-	bsrl	%eax, %eax

-	subq	$VEC_SIZE, %rdx

-	addq	%rax, %rdx

-	jl	L(zero)

-	addl	$(VEC_SIZE * 3), %eax

-	addq	%rdi, %rax

-	ret

+	/* Need no matter what.  */

+	vpcmpb	$0, -(VEC_SIZE * 3)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	.p2align 4

-L(zero):

-	xorl	%eax, %eax

+	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), %VECMATCH, %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

-L(last_vec_or_less_aligned):

-	movl	%edx, %ecx

-

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k1

-

-	movl	$1, %edx

-	/* Support rdx << 32.  */

-	salq	%cl, %rdx

-	subq	$1, %rdx

-

-	kmovd	%k1, %eax

-

-	/* Remove the trailing bytes.  */

-	andl	%edx, %eax

-	testl	%eax, %eax

-	jz	L(zero)

-

-	bsrl	%eax, %eax

-	addq	%rdi, %rax

+	.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

-L(last_vec_or_less):

-	addl	$VEC_SIZE, %edx

-

-	/* Check for zero length.  */

-	testl	%edx, %edx

-	jz	L(zero)

-

-	movl	%edi, %ecx

-	andl	$(VEC_SIZE - 1), %ecx

-	jz	L(last_vec_or_less_aligned)

-

-	movl	%ecx, %esi

-	movl	%ecx, %r8d

-	addl	%edx, %esi

-	andq	$-VEC_SIZE, %rdi

+	.p2align 4,, 8

+L(ret_vec_x2):

+	bsrl	%ecx, %ecx

+	leaq	-(VEC_SIZE * 3)(%rcx, %rax), %rax

+	ret

-	subl	$VEC_SIZE, %esi

-	ja	L(last_vec_2x_aligned)

+	.p2align 4,, 8

+L(ret_vec_x3):

+	bsrl	%ecx, %ecx

+	leaq	-(VEC_SIZE * 4)(%rcx, %rax), %rax

+	ret

-	/* Check the last VEC.  */

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k1

-	kmovd	%k1, %eax

+	.p2align 4,, 8

+L(more_4x_vec):

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x2)

-	/* Remove the leading and trailing bytes.  */

-	sarl	%cl, %eax

-	movl	%edx, %ecx

+	vpcmpb	$0, -(VEC_SIZE * 4)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	movl	$1, %edx

-	sall	%cl, %edx

-	subl	$1, %edx

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x3)

-	andl	%edx, %eax

-	testl	%eax, %eax

-	jz	L(zero)

+	/* 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)

-	bsrl	%eax, %eax

-	addq	%rdi, %rax

-	addq	%r8, %rax

-	ret

+	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(last_vec_2x_aligned):

-	movl	%esi, %ecx

-

-	/* Check the last VEC.  */

-	vpcmpb	$0, VEC_SIZE(%rdi), %YMMMATCH, %k1

+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), %VECMATCH, %k1

+

+	/* VEC(2/3) will have zero-byte where we found a CHAR.  */

+	vpxorq	(VEC_SIZE * 2)(%rax), %VECMATCH, %VEC(2)

+	vpxorq	(VEC_SIZE * 1)(%rax), %VECMATCH, %VEC(3)

+	vpcmpb	$0, (VEC_SIZE * 0)(%rax), %VECMATCH, %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	%VEC(2), %VEC(3), %VEC(3){%k1}{z}

+	vptestnmb %VEC(3), %VEC(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), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	movl	$1, %edx

-	sall	%cl, %edx

-	subl	$1, %edx

+	cmpl	$(VEC_SIZE * 2), %edx

+	jbe	L(last_2x_vec)

-	kmovd	%k1, %eax

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x0_dec)

-	/* Remove the trailing bytes.  */

-	andl	%edx, %eax

-	testl	%eax, %eax

-	jnz	L(last_vec_x1)

+	vpcmpb	$0, (VEC_SIZE * -2)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	/* Check the second last VEC.  */

-	vpcmpb	$0, (%rdi), %YMMMATCH, %k1

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x1)

-	movl	%r8d, %ecx

+	/* Used no matter what.  */

+	vpcmpb	$0, (VEC_SIZE * -3)(%rax), %VECMATCH, %k0

+	kmovd	%k0, %ecx

-	kmovd	%k1, %eax

+	cmpl	$(VEC_SIZE * 3), %edx

+	ja	L(last_vec)

-	/* Remove the leading bytes.  Must use unsigned right shift for

-	   bsrl below.  */

-	shrl	%cl, %eax

-	testl	%eax, %eax

-	jz	L(zero)

+	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):

+	ret

-	bsrl	%eax, %eax

-	addq	%rdi, %rax

-	addq	%r8, %rax

+	.p2align 4,, 6

+L(loop_end):

+	kmovd	%k1, %ecx

+	notl	%ecx

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x0_end)

+

+	vptestnmb %VEC(2), %VEC(2), %k0

+	kmovd	%k0, %ecx

+	testl	%ecx, %ecx

+	jnz	L(ret_vec_x1_end)

+

+	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.  */

+	salq	$32, %rcx

+	orq	%rsi, %rcx

+	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

 	ret

-END (__memrchr_evex)

+

+END(MEMRCHR)

 #endif