ARM: Add optimized ARMv7 strcmp implementation

Add an optimized implementation of strcmp for ARMv7-A cores. This
implementation is significantly faster than the current generic C
implementation, particularly for strings of 16 bytes and longer.

Tested with the glibc string tests for arm-linux-gnueabihf and
armeb-linux-gnueabihf.

The code was written by ARM, who have agreed to assign the copyright
to the FSF for integration into glibc.

ChangeLog:

2014-05-09  Will Newton  <will.newton@linaro.org>

	* sysdeps/arm/armv7/strcmp.S: New file.
	* NEWS: Mention addition of ARMv7 optimized strcmp.

1 files changed, 492 insertions, 0 deletions

diff --git a/sysdeps/arm/armv7/strcmp.S b/sysdeps/arm/armv7/strcmp.S
new file mode 100644
index 0000000..6c75c11
--- /dev/null
+++ b/sysdeps/arm/armv7/strcmp.S
@@ -0,0 +1,492 @@
+/* strcmp implementation for ARMv7-A, optimized for Cortex-A15.
+   Copyright (C) 2012-2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <arm-features.h>
+#include <sysdep.h>
+
+/* Implementation of strcmp for ARMv7 when DSP instructions are
+   available.  Use ldrd to support wider loads, provided the data
+   is sufficiently aligned.  Use saturating arithmetic to optimize
+   the compares.  */
+
+/* Build Options:
+   STRCMP_PRECHECK: Run a quick pre-check of the first byte in the
+   string.  If comparing completely random strings the pre-check will
+   save time, since there is a very high probability of a mismatch in
+   the first character: we save significant overhead if this is the
+   common case.  However, if strings are likely to be identical (e.g.
+   because we're verifying a hit in a hash table), then this check
+   is largely redundant.  */
+
+#define STRCMP_PRECHECK	1
+
+	/* This version uses Thumb-2 code.  */
+	.thumb
+	.syntax unified
+
+#ifdef __ARM_BIG_ENDIAN
+# define S2LO lsl
+# define S2LOEQ lsleq
+# define S2HI lsr
+# define MSB 0x000000ff
+# define LSB 0xff000000
+# define BYTE0_OFFSET 24
+# define BYTE1_OFFSET 16
+# define BYTE2_OFFSET 8
+# define BYTE3_OFFSET 0
+#else /* not  __ARM_BIG_ENDIAN */
+# define S2LO lsr
+# define S2LOEQ lsreq
+# define S2HI lsl
+# define BYTE0_OFFSET 0
+# define BYTE1_OFFSET 8
+# define BYTE2_OFFSET 16
+# define BYTE3_OFFSET 24
+# define MSB 0xff000000
+# define LSB 0x000000ff
+#endif /* not  __ARM_BIG_ENDIAN */
+
+/* Parameters and result.  */
+#define src1		r0
+#define src2		r1
+#define result		r0	/* Overlaps src1.  */
+
+/* Internal variables.  */
+#define tmp1		r4
+#define tmp2		r5
+#define const_m1	r12
+
+/* Additional internal variables for 64-bit aligned data.  */
+#define data1a		r2
+#define data1b		r3
+#define data2a		r6
+#define data2b		r7
+#define syndrome_a	tmp1
+#define syndrome_b	tmp2
+
+/* Additional internal variables for 32-bit aligned data.  */
+#define data1		r2
+#define data2		r3
+#define syndrome	tmp2
+
+
+	/* Macro to compute and return the result value for word-aligned
+	   cases.  */
+	.macro strcmp_epilogue_aligned synd d1 d2 restore_r6
+#ifdef __ARM_BIG_ENDIAN
+	/* If data1 contains a zero byte, then syndrome will contain a 1 in
+	   bit 7 of that byte.  Otherwise, the highest set bit in the
+	   syndrome will highlight the first different bit.  It is therefore
+	   sufficient to extract the eight bits starting with the syndrome
+	   bit.  */
+	clz	tmp1, \synd
+	lsl	r1, \d2, tmp1
+	.if \restore_r6
+	ldrd	r6, r7, [sp, #8]
+	.endif
+	lsl	\d1, \d1, tmp1
+	lsr	result, \d1, #24
+	ldrd	r4, r5, [sp], #16
+	cfi_remember_state
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	cfi_restore (r6)
+	cfi_restore (r7)
+	sub	result, result, r1, lsr #24
+	bx	lr
+#else
+	/* To use the big-endian trick we'd have to reverse all three words.
+	   that's slower than this approach.  */
+	rev	\synd, \synd
+	clz	tmp1, \synd
+	bic	tmp1, tmp1, #7
+	lsr	r1, \d2, tmp1
+	.if \restore_r6
+	ldrd	r6, r7, [sp, #8]
+	.endif
+	lsr	\d1, \d1, tmp1
+	and	result, \d1, #255
+	and	r1, r1, #255
+	ldrd	r4, r5, [sp], #16
+	cfi_remember_state
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	cfi_restore (r6)
+	cfi_restore (r7)
+	sub	result, result, r1
+
+	bx	lr
+#endif
+	.endm
+
+	.text
+	.p2align	5
+.Lstrcmp_start_addr:
+#if STRCMP_PRECHECK == 1
+.Lfastpath_exit:
+	sub	r0, r2, r3
+	bx	lr
+	nop
+#endif
+ENTRY (strcmp)
+#if STRCMP_PRECHECK == 1
+	ldrb	r2, [src1]
+	ldrb	r3, [src2]
+	cmp	r2, #1
+	it	cs
+	cmpcs	r2, r3
+	bne	.Lfastpath_exit
+#endif
+	strd	r4, r5, [sp, #-16]!
+	cfi_def_cfa_offset (16)
+	cfi_offset (r4, -16)
+	cfi_offset (r5, -12)
+	orr	tmp1, src1, src2
+	strd	r6, r7, [sp, #8]
+	cfi_offset (r6, -8)
+	cfi_offset (r7, -4)
+	mvn	const_m1, #0
+	lsl	r2, tmp1, #29
+	cbz	r2, .Lloop_aligned8
+
+.Lnot_aligned:
+	eor	tmp1, src1, src2
+	tst	tmp1, #7
+	bne	.Lmisaligned8
+
+	/* Deal with mutual misalignment by aligning downwards and then
+	   masking off the unwanted loaded data to prevent a difference.  */
+	and	tmp1, src1, #7
+	bic	src1, src1, #7
+	and	tmp2, tmp1, #3
+	bic	src2, src2, #7
+	lsl	tmp2, tmp2, #3	/* Bytes -> bits.  */
+	ldrd	data1a, data1b, [src1], #16
+	tst	tmp1, #4
+	ldrd	data2a, data2b, [src2], #16
+	/* In thumb code we can't use MVN with a register shift, but
+	   we do have ORN.  */
+	S2HI	tmp1, const_m1, tmp2
+	orn	data1a, data1a, tmp1
+	orn	data2a, data2a, tmp1
+	beq	.Lstart_realigned8
+	orn	data1b, data1b, tmp1
+	mov	data1a, const_m1
+	orn	data2b, data2b, tmp1
+	mov	data2a, const_m1
+	b	.Lstart_realigned8
+
+	/* Unwind the inner loop by a factor of 2, giving 16 bytes per
+	   pass.  */
+	.p2align 5,,12  /* Don't start in the tail bytes of a cache line.  */
+	.p2align 2	/* Always word aligned.  */
+.Lloop_aligned8:
+	ldrd	data1a, data1b, [src1], #16
+	ldrd	data2a, data2b, [src2], #16
+.Lstart_realigned8:
+	uadd8	syndrome_b, data1a, const_m1	/* Only want GE bits,  */
+	eor	syndrome_a, data1a, data2a
+	sel	syndrome_a, syndrome_a, const_m1
+	cbnz	syndrome_a, .Ldiff_in_a
+	uadd8	syndrome_b, data1b, const_m1	/* Only want GE bits.  */
+	eor	syndrome_b, data1b, data2b
+	sel	syndrome_b, syndrome_b, const_m1
+	cbnz	syndrome_b, .Ldiff_in_b
+
+	ldrd	data1a, data1b, [src1, #-8]
+	ldrd	data2a, data2b, [src2, #-8]
+	uadd8	syndrome_b, data1a, const_m1	/* Only want GE bits,  */
+	eor	syndrome_a, data1a, data2a
+	sel	syndrome_a, syndrome_a, const_m1
+	uadd8	syndrome_b, data1b, const_m1	/* Only want GE bits.  */
+	eor	syndrome_b, data1b, data2b
+	sel	syndrome_b, syndrome_b, const_m1
+	/* Can't use CBZ for backwards branch.  */
+	orrs	syndrome_b, syndrome_b, syndrome_a /* Only need if s_a == 0 */
+	beq	.Lloop_aligned8
+
+.Ldiff_found:
+	cbnz	syndrome_a, .Ldiff_in_a
+
+.Ldiff_in_b:
+	strcmp_epilogue_aligned syndrome_b, data1b, data2b 1
+
+.Ldiff_in_a:
+	cfi_restore_state
+	strcmp_epilogue_aligned syndrome_a, data1a, data2a 1
+
+	cfi_restore_state
+.Lmisaligned8:
+	tst	tmp1, #3
+	bne	.Lmisaligned4
+	ands	tmp1, src1, #3
+	bne	.Lmutual_align4
+
+	/* Unrolled by a factor of 2, to reduce the number of post-increment
+	   operations.  */
+.Lloop_aligned4:
+	ldr	data1, [src1], #8
+	ldr	data2, [src2], #8
+.Lstart_realigned4:
+	uadd8	syndrome, data1, const_m1	/* Only need GE bits.  */
+	eor	syndrome, data1, data2
+	sel	syndrome, syndrome, const_m1
+	cbnz	syndrome, .Laligned4_done
+	ldr	data1, [src1, #-4]
+	ldr	data2, [src2, #-4]
+	uadd8	syndrome, data1, const_m1
+	eor	syndrome, data1, data2
+	sel	syndrome, syndrome, const_m1
+	cmp	syndrome, #0
+	beq	.Lloop_aligned4
+
+.Laligned4_done:
+	strcmp_epilogue_aligned syndrome, data1, data2, 0
+
+.Lmutual_align4:
+	cfi_restore_state
+	/* Deal with mutual misalignment by aligning downwards and then
+	   masking off the unwanted loaded data to prevent a difference.  */
+	lsl	tmp1, tmp1, #3	/* Bytes -> bits.  */
+	bic	src1, src1, #3
+	ldr	data1, [src1], #8
+	bic	src2, src2, #3
+	ldr	data2, [src2], #8
+
+	/* In thumb code we can't use MVN with a register shift, but
+	   we do have ORN.  */
+	S2HI	tmp1, const_m1, tmp1
+	orn	data1, data1, tmp1
+	orn	data2, data2, tmp1
+	b	.Lstart_realigned4
+
+.Lmisaligned4:
+	ands	tmp1, src1, #3
+	beq	.Lsrc1_aligned
+	sub	src2, src2, tmp1
+	bic	src1, src1, #3
+	lsls	tmp1, tmp1, #31
+	ldr	data1, [src1], #4
+	beq	.Laligned_m2
+	bcs	.Laligned_m1
+
+#if STRCMP_PRECHECK == 0
+	ldrb	data2, [src2, #1]
+	uxtb	tmp1, data1, ror #BYTE1_OFFSET
+	subs	tmp1, tmp1, data2
+	bne	.Lmisaligned_exit
+	cbz	data2, .Lmisaligned_exit
+
+.Laligned_m2:
+	ldrb	data2, [src2, #2]
+	uxtb	tmp1, data1, ror #BYTE2_OFFSET
+	subs	tmp1, tmp1, data2
+	bne	.Lmisaligned_exit
+	cbz	data2, .Lmisaligned_exit
+
+.Laligned_m1:
+	ldrb	data2, [src2, #3]
+	uxtb	tmp1, data1, ror #BYTE3_OFFSET
+	subs	tmp1, tmp1, data2
+	bne	.Lmisaligned_exit
+	add	src2, src2, #4
+	cbnz	data2, .Lsrc1_aligned
+#else  /* STRCMP_PRECHECK */
+	/* If we've done the pre-check, then we don't need to check the
+	   first byte again here.  */
+	ldrb	data2, [src2, #2]
+	uxtb	tmp1, data1, ror #BYTE2_OFFSET
+	subs	tmp1, tmp1, data2
+	bne	.Lmisaligned_exit
+	cbz	data2, .Lmisaligned_exit
+
+.Laligned_m2:
+	ldrb	data2, [src2, #3]
+	uxtb	tmp1, data1, ror #BYTE3_OFFSET
+	subs	tmp1, tmp1, data2
+	bne	.Lmisaligned_exit
+	cbnz	data2, .Laligned_m1
+#endif
+
+.Lmisaligned_exit:
+	mov	result, tmp1
+	ldr	r4, [sp], #16
+	cfi_remember_state
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	cfi_restore (r6)
+	cfi_restore (r7)
+	bx	lr
+
+#if STRCMP_PRECHECK == 1
+.Laligned_m1:
+	add	src2, src2, #4
+#endif
+.Lsrc1_aligned:
+	cfi_restore_state
+	/* src1 is word aligned, but src2 has no common alignment
+	   with it.  */
+	ldr	data1, [src1], #4
+	lsls	tmp1, src2, #31		/* C=src2[1], Z=src2[0].  */
+
+	bic	src2, src2, #3
+	ldr	data2, [src2], #4
+	bhi	.Loverlap1		/* C=1, Z=0 => src2[1:0] = 0b11.  */
+	bcs	.Loverlap2		/* C=1, Z=1 => src2[1:0] = 0b10.  */
+
+	/* (overlap3) C=0, Z=0 => src2[1:0] = 0b01.  */
+.Loverlap3:
+	bic	tmp1, data1, #MSB
+	uadd8	syndrome, data1, const_m1
+	eors	syndrome, tmp1, data2, S2LO #8
+	sel	syndrome, syndrome, const_m1
+	bne	4f
+	cbnz	syndrome, 5f
+	ldr	data2, [src2], #4
+	eor	tmp1, tmp1, data1
+	cmp	tmp1, data2, S2HI #24
+	bne	6f
+	ldr	data1, [src1], #4
+	b	.Loverlap3
+4:
+	S2LO	data2, data2, #8
+	b	.Lstrcmp_tail
+
+5:
+	bics	syndrome, syndrome, #MSB
+	bne	.Lstrcmp_done_equal
+
+	/* We can only get here if the MSB of data1 contains 0, so
+	   fast-path the exit.  */
+	ldrb	result, [src2]
+	ldrd	r4, r5, [sp], #16
+	cfi_remember_state
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	/* R6/7 Not used in this sequence.  */
+	cfi_restore (r6)
+	cfi_restore (r7)
+	neg	result, result
+	bx	lr
+
+6:
+	cfi_restore_state
+	S2LO	data1, data1, #24
+	and	data2, data2, #LSB
+	b	.Lstrcmp_tail
+
+	.p2align 5,,12	/* Ensure at least 3 instructions in cache line.  */
+.Loverlap2:
+	and	tmp1, data1, const_m1, S2LO #16
+	uadd8	syndrome, data1, const_m1
+	eors	syndrome, tmp1, data2, S2LO #16
+	sel	syndrome, syndrome, const_m1
+	bne	4f
+	cbnz	syndrome, 5f
+	ldr	data2, [src2], #4
+	eor	tmp1, tmp1, data1
+	cmp	tmp1, data2, S2HI #16
+	bne	6f
+	ldr	data1, [src1], #4
+	b	.Loverlap2
+4:
+	S2LO	data2, data2, #16
+	b	.Lstrcmp_tail
+5:
+	ands	syndrome, syndrome, const_m1, S2LO #16
+	bne	.Lstrcmp_done_equal
+
+	ldrh	data2, [src2]
+	S2LO	data1, data1, #16
+#ifdef __ARM_BIG_ENDIAN
+	lsl	data2, data2, #16
+#endif
+	b	.Lstrcmp_tail
+
+6:
+	S2LO	data1, data1, #16
+	and	data2, data2, const_m1, S2LO #16
+	b	.Lstrcmp_tail
+
+	.p2align 5,,12	/* Ensure at least 3 instructions in cache line.  */
+.Loverlap1:
+	and	tmp1, data1, #LSB
+	uadd8	syndrome, data1, const_m1
+	eors	syndrome, tmp1, data2, S2LO #24
+	sel	syndrome, syndrome, const_m1
+	bne	4f
+	cbnz	syndrome, 5f
+	ldr	data2, [src2], #4
+	eor	tmp1, tmp1, data1
+	cmp	tmp1, data2, S2HI #8
+	bne	6f
+	ldr	data1, [src1], #4
+	b	.Loverlap1
+4:
+	S2LO	data2, data2, #24
+	b	.Lstrcmp_tail
+5:
+	tst	syndrome, #LSB
+	bne	.Lstrcmp_done_equal
+	ldr	data2, [src2]
+6:
+	S2LO	data1, data1, #8
+	bic	data2, data2, #MSB
+	b	.Lstrcmp_tail
+
+.Lstrcmp_done_equal:
+	mov	result, #0
+	ldrd	r4, r5, [sp], #16
+	cfi_remember_state
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	/* R6/7 not used in this sequence.  */
+	cfi_restore (r6)
+	cfi_restore (r7)
+	bx	lr
+
+.Lstrcmp_tail:
+	cfi_restore_state
+#ifndef __ARM_BIG_ENDIAN
+	rev	data1, data1
+	rev	data2, data2
+	/* Now everything looks big-endian...  */
+#endif
+	uadd8	tmp1, data1, const_m1
+	eor	tmp1, data1, data2
+	sel	syndrome, tmp1, const_m1
+	clz	tmp1, syndrome
+	lsl	data1, data1, tmp1
+	lsl	data2, data2, tmp1
+	lsr	result, data1, #24
+	ldrd	r4, r5, [sp], #16
+	cfi_def_cfa_offset (0)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	/* R6/7 not used in this sequence.  */
+	cfi_restore (r6)
+	cfi_restore (r7)
+	sub	result, result, data2, lsr #24
+	bx	lr
+END (strcmp)
+libc_hidden_builtin_def (strcmp)