[PATCH 4/5] arm: Implement armv6 optimized strrchr

[Richard Henderson <rth at twiddle dot net>]

---
 ports/sysdeps/arm/armv6/strrchr.S | 129 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 129 insertions(+)
 create mode 100644 ports/sysdeps/arm/armv6/strrchr.S

diff --git a/ports/sysdeps/arm/armv6/strrchr.S b/ports/sysdeps/arm/armv6/strrchr.S
new file mode 100644
index 0000000..ddd4f7f
--- /dev/null
+++ b/ports/sysdeps/arm/armv6/strrchr.S
@@ -0,0 +1,129 @@
+/* strrchr -- find the last occurence of C in a nul-terminated string
+   Copyright (C) 2013 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 <sysdep.h>
+
+	.syntax unified
+	.text
+
+ENTRY (strrchr)
+	@ r0 = start of string
+	@ r1 = character to match
+	@ returns NULL for no match, or a pointer to the match
+
+	mov	r3, r0
+	mov	r0, #0
+	uxtb	r1, r1
+
+	@ Loop a few times until we're aligned.
+	tst	r3, #7
+	beq	2f
+1:	ldrb	r2, [r3], #1
+	cmp	r2, r1			@ Find the character
+	it	eq
+	subeq	r0, r3, #1
+	cmp	r2, #0			@ Find EOS
+	it	eq
+	bxeq	lr
+	tst	r3, #7			@ Find the aligment point
+	bne	1b
+
+	@ So now we're aligned.  Now we actually need a stack frame.
+2:	push	{ r4, r5, r6, r7 }
+	cfi_adjust_cfa_offset (16)
+	cfi_rel_offset (r4, 0)
+	cfi_rel_offset (r5, 4)
+	cfi_rel_offset (r6, 8)
+	cfi_rel_offset (r7, 12)
+
+	orr	r1, r1, r1, lsl #8	@ Replicate C to all bytes
+#ifdef ARCH_HAS_T2
+	movw	ip, #0x0101
+	movt	ip, #0x0101
+#else
+	ldr	ip, =0x01010101
+#endif
+	orr	r1, r1, r1, lsl #16
+	mov	r2, #0			@ No found bits yet
+
+	@ Loop searching for EOS and C, 8 bytes at a time.
+	@ Any time we find a match in a word, we copy the address of
+	@ the word to r0, and the found bits to r2.
+3:	ldrd	r4, r5, [r3], #8
+	@ Subtracting (unsigned saturating) from 1 means result of 1 for
+	@ any byte that was originally zero and 0 otherwise.  Therefore
+	@ we consider the lsb of each byte the "found" bit.
+	uqsub8	r6, ip, r4		@ Find EOS
+	uqsub8	r7, ip, r5
+	eor	r4, r4, r1		@ Convert C bytes to 0
+	eor	r5, r5, r1
+	uqsub8	r4, ip, r4		@ Find C
+	uqsub8	r5, ip, r5
+	cmp	r6, #0			@ Found EOS, first word
+	bne	4f
+	cmp	r4, #0			@ Handle C, first word
+	itt	ne
+	subne	r0, r3, #8
+	movne	r2, r4
+	cmp	r7, #0			@ Found EOS, second word
+	bne	5f
+	cmp	r5, #0			@ Handle C, second word
+	itt	ne
+	subne	r0, r3, #4
+	movne	r2, r5
+	b	3b
+
+	@ Found EOS in second word; fold to first word.
+5:	add	r3, r3, #4		@ Dec pointer to 2nd word, with below
+	mov	r4, r5			@ Overwrite first word C found
+	mov	r6, r7			@ Overwrite first word EOS found
+
+	@ Found EOS.  Zap found C after EOS.
+4:	sub	r3, r3, #8		@ Decrement pointer to first word
+#ifdef __ARMEB__
+	@ Byte swap to be congruent with LE, which is easier from here on.
+	rev	r6, r6			@ Byte swap found EOS,
+	rev	r4, r4			@ ... this found C
+	rev	r2, r2			@ ... prev found C
+#endif
+	sub	r7, r6, #1		@ Toggle EOS lsb and below
+	eor	r6, r6, r7		@ All bits below and including lsb
+	ands	r4, r4, r6		@ Zap C above EOS
+	itt	ne
+	movne	r2, r4			@ Copy to result, if still non-zero
+	movne	r0, r3
+
+	pop	{ r4, r5, r6, r7 }
+	cfi_adjust_cfa_offset (-16)
+	cfi_restore (r4)
+	cfi_restore (r5)
+	cfi_restore (r6)
+	cfi_restore (r7)
+
+	@ Adjust the result pointer if we found a word containing C.
+	cmp	r2, #0
+	clz	r2, r2			@ Find the bit offset of the last C
+	itt	ne
+	rsbne	r2, r2, #32		@ Convert to a count from the right
+	addne	r0, r0, r2, lsr #3	@ Convert to byte offset and add.
+	bx	lr
+
+END (strrchr)
+
+weak_alias (strrchr, rindex)
+libc_hidden_builtin_def (strrchr)
-- 
1.8.1.2