strchrnul.S

/*
 * strchrnul - find a character or nul in a string
 *
 * Copyright (c) 2014-2022, Arm Limited.
 * SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception
 */

/* Assumptions:
 *
 * ARMv8-a, AArch64
 * Neon Available.
 */

#include "asmdefs.h"

/* Arguments and results.  */
#define srcin		x0
#define chrin		w1

#define result		x0

#define src		x2
#define	tmp1		x3
#define wtmp2		w4
#define tmp3		x5

#define vrepchr		v0
#define vdata1		v1
#define vdata2		v2
#define vhas_nul1	v3
#define vhas_nul2	v4
#define vhas_chr1	v5
#define vhas_chr2	v6
#define vrepmask	v7
#define vend1		v16

/* Core algorithm.

   For each 32-byte hunk we calculate a 64-bit syndrome value, with
   two bits per byte (LSB is always in bits 0 and 1, for both big
   and little-endian systems).  For each tuple, bit 0 is set iff
   the relevant byte matched the requested character or nul.  Since the
   bits in the syndrome reflect exactly the order in which things occur
   in the original string a count_trailing_zeros() operation will
   identify exactly which byte is causing the termination.  */

/* Locals and temporaries.  */

ENTRY (__strchrnul_aarch64)
	/* Magic constant 0x40100401 to allow us to identify which lane
	   matches the termination condition.  */
	mov	wtmp2, #0x0401
	movk	wtmp2, #0x4010, lsl #16
	dup	vrepchr.16b, chrin
	bic	src, srcin, #31		/* Work with aligned 32-byte hunks.  */
	dup	vrepmask.4s, wtmp2
	ands	tmp1, srcin, #31
	b.eq	L(loop)

	/* Input string is not 32-byte aligned.  Rather than forcing
	   the padding bytes to a safe value, we calculate the syndrome
	   for all the bytes, but then mask off those bits of the
	   syndrome that are related to the padding.  */
	ld1	{vdata1.16b, vdata2.16b}, [src], #32
	neg	tmp1, tmp1
	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
	cmhs	vhas_nul1.16b, vhas_chr1.16b, vdata1.16b
	cmhs	vhas_nul2.16b, vhas_chr2.16b, vdata2.16b
	and	vhas_chr1.16b, vhas_nul1.16b, vrepmask.16b
	and	vhas_chr2.16b, vhas_nul2.16b, vrepmask.16b
	lsl	tmp1, tmp1, #1
	addp	vend1.16b, vhas_chr1.16b, vhas_chr2.16b	// 256->128
	mov	tmp3, #~0
	addp	vend1.16b, vend1.16b, vend1.16b		// 128->64
	lsr	tmp1, tmp3, tmp1

	mov	tmp3, vend1.d[0]
	bic	tmp1, tmp3, tmp1	// Mask padding bits.
	cbnz	tmp1, L(tail)

	.p2align 4
L(loop):
	ld1	{vdata1.16b, vdata2.16b}, [src], #32
	cmeq	vhas_chr1.16b, vdata1.16b, vrepchr.16b
	cmeq	vhas_chr2.16b, vdata2.16b, vrepchr.16b
	cmhs	vhas_nul1.16b, vhas_chr1.16b, vdata1.16b
	cmhs	vhas_nul2.16b, vhas_chr2.16b, vdata2.16b
	orr	vend1.16b, vhas_nul1.16b, vhas_nul2.16b
	umaxp	vend1.16b, vend1.16b, vend1.16b
	mov	tmp1, vend1.d[0]
	cbz	tmp1, L(loop)

	/* Termination condition found.  Now need to establish exactly why
	   we terminated.  */
	and	vhas_chr1.16b, vhas_nul1.16b, vrepmask.16b
	and	vhas_chr2.16b, vhas_nul2.16b, vrepmask.16b
	addp	vend1.16b, vhas_chr1.16b, vhas_chr2.16b		// 256->128
	addp	vend1.16b, vend1.16b, vend1.16b		// 128->64

	mov	tmp1, vend1.d[0]
L(tail):
	/* Count the trailing zeros, by bit reversing...  */
	rbit	tmp1, tmp1
	/* Re-bias source.  */
	sub	src, src, #32
	clz	tmp1, tmp1	/* ... and counting the leading zeros.  */
	/* tmp1 is twice the offset into the fragment.  */
	add	result, src, tmp1, lsr #1
	ret

END (__strchrnul_aarch64)