From 1da58a6d12719da8cc2035c2f6f9928d2ad61a20 Mon Sep 17 00:00:00 2001

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

Date: Wed, 23 Mar 2022 16:57:16 -0500

Subject: [PATCH] x86: Code cleanup in strchr-avx2 and comment justifying

 branch

Small code cleanup for size: -53 bytes.

Add comment justifying using a branch to do NULL/non-null return.

All string/memory tests pass and no regressions in benchtests.

geometric_mean(N=20) of all benchmarks Original / New: 1.00

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

(cherry picked from commit a6fbf4d51e9ba8063c4f8331564892ead9c67344)

---

 sysdeps/x86_64/multiarch/strchr-avx2.S | 204 +++++++++++++------------

 1 file changed, 107 insertions(+), 97 deletions(-)

diff --git a/sysdeps/x86_64/multiarch/strchr-avx2.S b/sysdeps/x86_64/multiarch/strchr-avx2.S

index 5884726b..89dd2bf7 100644

--- a/sysdeps/x86_64/multiarch/strchr-avx2.S

+++ b/sysdeps/x86_64/multiarch/strchr-avx2.S

@@ -48,13 +48,13 @@

 # define PAGE_SIZE 4096

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

-ENTRY (STRCHR)

+ENTRY_P2ALIGN (STRCHR, 5)

 	/* Broadcast CHAR to YMM0.	*/

 	vmovd	%esi, %xmm0

 	movl	%edi, %eax

 	andl	$(PAGE_SIZE - 1), %eax

 	VPBROADCAST	%xmm0, %ymm0

-	vpxor	%xmm9, %xmm9, %xmm9

+	vpxor	%xmm1, %xmm1, %xmm1

 	/* Check if we cross page boundary with one vector load.  */

 	cmpl	$(PAGE_SIZE - VEC_SIZE), %eax

@@ -62,37 +62,29 @@ ENTRY (STRCHR)

 	/* Check the first VEC_SIZE bytes.	Search for both CHAR and the

 	   null byte.  */

-	vmovdqu	(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqu	(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jz	L(aligned_more)

 	tzcntl	%eax, %eax

 # ifndef USE_AS_STRCHRNUL

-	/* Found CHAR or the null byte.	 */

-	cmp	(%rdi, %rax), %CHAR_REG

-	jne	L(zero)

-# endif

-	addq	%rdi, %rax

-	VZEROUPPER_RETURN

-

-	/* .p2align 5 helps keep performance more consistent if ENTRY()

-	   alignment % 32 was either 16 or 0. As well this makes the

-	   alignment % 32 of the loop_4x_vec fixed which makes tuning it

-	   easier.  */

-	.p2align 5

-L(first_vec_x4):

-	tzcntl	%eax, %eax

-	addq	$(VEC_SIZE * 3 + 1), %rdi

-# ifndef USE_AS_STRCHRNUL

-	/* Found CHAR or the null byte.	 */

+	/* Found CHAR or the null byte.  */

 	cmp	(%rdi, %rax), %CHAR_REG

+	/* NB: Use a branch instead of cmovcc here. The expectation is

+	   that with strchr the user will branch based on input being

+	   null. Since this branch will be 100% predictive of the user

+	   branch a branch miss here should save what otherwise would

+	   be branch miss in the user code. Otherwise using a branch 1)

+	   saves code size and 2) is faster in highly predictable

+	   environments.  */

 	jne	L(zero)

 # endif

 	addq	%rdi, %rax

-	VZEROUPPER_RETURN

+L(return_vzeroupper):

+	ZERO_UPPER_VEC_REGISTERS_RETURN

 # ifndef USE_AS_STRCHRNUL

 L(zero):

@@ -103,7 +95,8 @@ L(zero):

 	.p2align 4

 L(first_vec_x1):

-	tzcntl	%eax, %eax

+	/* Use bsf to save code size.  */

+	bsfl	%eax, %eax

 	incq	%rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

@@ -113,9 +106,10 @@ L(first_vec_x1):

 	addq	%rdi, %rax

 	VZEROUPPER_RETURN

-	.p2align 4

+	.p2align 4,, 10

 L(first_vec_x2):

-	tzcntl	%eax, %eax

+	/* Use bsf to save code size.  */

+	bsfl	%eax, %eax

 	addq	$(VEC_SIZE + 1), %rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

@@ -125,9 +119,10 @@ L(first_vec_x2):

 	addq	%rdi, %rax

 	VZEROUPPER_RETURN

-	.p2align 4

+	.p2align 4,, 8

 L(first_vec_x3):

-	tzcntl	%eax, %eax

+	/* Use bsf to save code size.  */

+	bsfl	%eax, %eax

 	addq	$(VEC_SIZE * 2 + 1), %rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

@@ -137,6 +132,21 @@ L(first_vec_x3):

 	addq	%rdi, %rax

 	VZEROUPPER_RETURN

+	.p2align 4,, 10

+L(first_vec_x4):

+	/* Use bsf to save code size.  */

+	bsfl	%eax, %eax

+	addq	$(VEC_SIZE * 3 + 1), %rdi

+# ifndef USE_AS_STRCHRNUL

+	/* Found CHAR or the null byte.	 */

+	cmp	(%rdi, %rax), %CHAR_REG

+	jne	L(zero)

+# endif

+	addq	%rdi, %rax

+	VZEROUPPER_RETURN

+

+

+

 	.p2align 4

 L(aligned_more):

 	/* Align data to VEC_SIZE - 1. This is the same number of

@@ -146,90 +156,92 @@ L(aligned_more):

 L(cross_page_continue):

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

 	   since data is only aligned to VEC_SIZE.  */

-	vmovdqa	1(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqa	1(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jnz	L(first_vec_x1)

-	vmovdqa	(VEC_SIZE + 1)(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqa	(VEC_SIZE + 1)(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jnz	L(first_vec_x2)

-	vmovdqa	(VEC_SIZE * 2 + 1)(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqa	(VEC_SIZE * 2 + 1)(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jnz	L(first_vec_x3)

-	vmovdqa	(VEC_SIZE * 3 + 1)(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqa	(VEC_SIZE * 3 + 1)(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jnz	L(first_vec_x4)

-	/* Align data to VEC_SIZE * 4 - 1.	*/

-	addq	$(VEC_SIZE * 4 + 1), %rdi

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

+	/* Align data to VEC_SIZE * 4 - 1.  */

+	incq	%rdi

+	orq	$(VEC_SIZE * 4 - 1), %rdi

 	.p2align 4

 L(loop_4x_vec):

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

-	vmovdqa	(%rdi), %ymm5

-	vmovdqa	(VEC_SIZE)(%rdi), %ymm6

-	vmovdqa	(VEC_SIZE * 2)(%rdi), %ymm7

-	vmovdqa	(VEC_SIZE * 3)(%rdi), %ymm8

+	vmovdqa	1(%rdi), %ymm6

+	vmovdqa	(VEC_SIZE + 1)(%rdi), %ymm7

 	/* Leaves only CHARS matching esi as 0.	 */

-	vpxor	%ymm5, %ymm0, %ymm1

 	vpxor	%ymm6, %ymm0, %ymm2

 	vpxor	%ymm7, %ymm0, %ymm3

-	vpxor	%ymm8, %ymm0, %ymm4

-	VPMINU	%ymm1, %ymm5, %ymm1

 	VPMINU	%ymm2, %ymm6, %ymm2

 	VPMINU	%ymm3, %ymm7, %ymm3

-	VPMINU	%ymm4, %ymm8, %ymm4

-	VPMINU	%ymm1, %ymm2, %ymm5

-	VPMINU	%ymm3, %ymm4, %ymm6

+	vmovdqa	(VEC_SIZE * 2 + 1)(%rdi), %ymm6

+	vmovdqa	(VEC_SIZE * 3 + 1)(%rdi), %ymm7

+

+	vpxor	%ymm6, %ymm0, %ymm4

+	vpxor	%ymm7, %ymm0, %ymm5

+

+	VPMINU	%ymm4, %ymm6, %ymm4

+	VPMINU	%ymm5, %ymm7, %ymm5

-	VPMINU	%ymm5, %ymm6, %ymm6

+	VPMINU	%ymm2, %ymm3, %ymm6

+	VPMINU	%ymm4, %ymm5, %ymm7

-	VPCMPEQ	%ymm6, %ymm9, %ymm6

-	vpmovmskb %ymm6, %ecx

+	VPMINU	%ymm6, %ymm7, %ymm7

+

+	VPCMPEQ	%ymm7, %ymm1, %ymm7

+	vpmovmskb %ymm7, %ecx

 	subq	$-(VEC_SIZE * 4), %rdi

 	testl	%ecx, %ecx

 	jz	L(loop_4x_vec)

-

-	VPCMPEQ	%ymm1, %ymm9, %ymm1

-	vpmovmskb %ymm1, %eax

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpmovmskb %ymm2, %eax

 	testl	%eax, %eax

 	jnz	L(last_vec_x0)

-	VPCMPEQ	%ymm5, %ymm9, %ymm2

-	vpmovmskb %ymm2, %eax

+	VPCMPEQ	%ymm3, %ymm1, %ymm3

+	vpmovmskb %ymm3, %eax

 	testl	%eax, %eax

 	jnz	L(last_vec_x1)

-	VPCMPEQ	%ymm3, %ymm9, %ymm3

-	vpmovmskb %ymm3, %eax

+	VPCMPEQ	%ymm4, %ymm1, %ymm4

+	vpmovmskb %ymm4, %eax

 	/* rcx has combined result from all 4 VEC. It will only be used

 	   if the first 3 other VEC all did not contain a match.  */

 	salq	$32, %rcx

 	orq	%rcx, %rax

 	tzcntq	%rax, %rax

-	subq	$(VEC_SIZE * 2), %rdi

+	subq	$(VEC_SIZE * 2 - 1), %rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

 	cmp	(%rdi, %rax), %CHAR_REG

@@ -239,10 +251,11 @@ L(loop_4x_vec):

 	VZEROUPPER_RETURN

-	.p2align 4

+	.p2align 4,, 10

 L(last_vec_x0):

-	tzcntl	%eax, %eax

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

+	/* Use bsf to save code size.  */

+	bsfl	%eax, %eax

+	addq	$-(VEC_SIZE * 4 - 1), %rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

 	cmp	(%rdi, %rax), %CHAR_REG

@@ -251,16 +264,11 @@ L(last_vec_x0):

 	addq	%rdi, %rax

 	VZEROUPPER_RETURN

-# ifndef USE_AS_STRCHRNUL

-L(zero_end):

-	xorl	%eax, %eax

-	VZEROUPPER_RETURN

-# endif

-	.p2align 4

+	.p2align 4,, 10

 L(last_vec_x1):

 	tzcntl	%eax, %eax

-	subq	$(VEC_SIZE * 3), %rdi

+	subq	$(VEC_SIZE * 3 - 1), %rdi

 # ifndef USE_AS_STRCHRNUL

 	/* Found CHAR or the null byte.	 */

 	cmp	(%rdi, %rax), %CHAR_REG

@@ -269,18 +277,23 @@ L(last_vec_x1):

 	addq	%rdi, %rax

 	VZEROUPPER_RETURN

+# ifndef USE_AS_STRCHRNUL

+L(zero_end):

+	xorl	%eax, %eax

+	VZEROUPPER_RETURN

+# endif

 	/* Cold case for crossing page with first load.	 */

-	.p2align 4

+	.p2align 4,, 8

 L(cross_page_boundary):

 	movq	%rdi, %rdx

 	/* Align rdi to VEC_SIZE - 1.  */

 	orq	$(VEC_SIZE - 1), %rdi

-	vmovdqa	-(VEC_SIZE - 1)(%rdi), %ymm8

-	VPCMPEQ	%ymm8, %ymm0, %ymm1

-	VPCMPEQ	%ymm8, %ymm9, %ymm2

-	vpor	%ymm1, %ymm2, %ymm1

-	vpmovmskb %ymm1, %eax

+	vmovdqa	-(VEC_SIZE - 1)(%rdi), %ymm2

+	VPCMPEQ	%ymm2, %ymm0, %ymm3

+	VPCMPEQ	%ymm2, %ymm1, %ymm2

+	vpor	%ymm3, %ymm2, %ymm3

+	vpmovmskb %ymm3, %eax

 	/* Remove the leading bytes. sarxl only uses bits [5:0] of COUNT

 	   so no need to manually mod edx.  */

 	sarxl	%edx, %eax, %eax

@@ -291,13 +304,10 @@ L(cross_page_boundary):

 	xorl	%ecx, %ecx

 	/* Found CHAR or the null byte.	 */

 	cmp	(%rdx, %rax), %CHAR_REG

-	leaq	(%rdx, %rax), %rax

-	cmovne	%rcx, %rax

-# else

-	addq	%rdx, %rax

+	jne	L(zero_end)

 # endif

-L(return_vzeroupper):

-	ZERO_UPPER_VEC_REGISTERS_RETURN

+	addq	%rdx, %rax

+	VZEROUPPER_RETURN

 END (STRCHR)

-# endif

+#endif

-- 

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