libc: scalar strlen() in RISC-V assembly

Includes a scalar implementation of strlen() for the RISC-V
architecture and changes to the corresponding manpage.

Performance was benchamarked using before and after:
https://github.com/clausecker/strperf

os: FreeBSD
arch: riscv
        │ strlen_baseline │             strlen_scalar              │
        │     sec/op      │   sec/op     vs base                   │
Short        541.2µ ± 17%   401.6µ ± 0%  -25.78% (p=0.000 n=21+20)
Mid          249.6µ ±  3%   191.9µ ± 0%  -23.13% (p=0.000 n=21+20)
Long         124.6µ ±  0%   110.7µ ± 0%  -11.13% (p=0.000 n=21+20)
geomean      256.3µ         204.3µ       -20.26%

        │ strlen_baseline │              strlen_scalar               │
        │       B/s       │      B/s       vs base                   │
Short       220.3Mi ± 14%    296.8Mi ± 0%  +34.74% (p=0.000 n=21+20)
Mid         477.6Mi ±  3%    621.3Mi ± 0%  +30.09% (p=0.000 n=21+20)
Long        956.9Mi ±  0%   1076.7Mi ± 0%  +12.52% (p=0.000 n=21+20)
geomean     465.2Mi          583.4Mi       +25.40%

MFC after:	1 month
MFC to:		stable/15
Approved by:	mhorne, markj (mentor)
Reviewed by:	fuz
Sponsored by:	Google LLC (GSoC 2024)
Differential Revision:	https://reviews.freebsd.org/D45693

lib/libc/riscv/string/Makefile.inc

@@ -1,4 +1,5 @@
 MDSRCS+= \
 	memchr.S \
 	memset.S \
+	strlen.S \
 	strrchr.S

lib/libc/riscv/string/strlen.S

@@ -0,0 +1,77 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2024 Strahinja Stanisic <strajabot@FreeBSD.org>
+ */
+
+#include <machine/asm.h>
+
+/*
+ * https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord
+ * uses haszero(v) (((v) - 0x01010101UL) & ~(v) & 0x80808080UL)
+ * which evalutates > 0 when there is zero in v
+ *
+ * register a0 - char *s
+ */
+ENTRY(strlen)
+	/*
+	 * register a0 - char *str_start
+	 * register a1 - char *str_ptr
+	 * register a2 - char[8] iter
+	 */
+
+	/* load constants for haszero */
+	li t0, 0x0101010101010101
+	slli t1, t0, 7				# 0x8080808080808080, avoid li
+
+	/* check alignment of str_start */
+	andi a1, a0, ~0b111
+	ld a2, (a1)
+	beq a1, a0, .Lhas_zero
+
+	/* fill bytes before str_start with non-zero */
+	slli t2, a0, 3
+	addi t3, t2, -64
+	neg t3, t3
+	srl t3, t0, t3
+	or a2, a2, t3
+
+	/* unrolled iteration of haszero */
+	not t2, a2
+	sub a2, a2, t0
+	and a2, a2, t2
+	and a2, a2, t1
+
+	bnez a2, .Lfind_zero
+
+.Lloop_has_zero:
+	ld a2, 8(a1)
+	addi a1, a1, 8	# move ptr to next 8byte
+.Lhas_zero:
+	not t2, a2
+	sub a2, a2, t0
+	and a2, a2, t2
+	and a2, a2, t1
+
+	beqz a2, .Lloop_has_zero
+
+.Lfind_zero:
+	/* use (iter & -iter) to isolate lowest set bit */
+	sub a3, zero, a2	#a3 = -iter
+	and t1, a2, a3		#t1 = (iter & -iter)
+
+	li t0, 0x0001020304050607
+	srli t1, t1, 7
+	/*
+	 * lowest set bit is 2^(8*k)
+	 * multiplying by it shifts the idx array in t0 by k bytes to the left
+	 */
+	mul	t1, t1, t0
+	/* highest byte contains idx of first zero */
+	srli t1, t1, 56
+
+	add a1, a1, t1
+	sub a0, a1, a0
+	ret
+END(strlen)
+