csky: Add C based string functions

Try to access RAM with the largest bit width possible, but without
doing unaligned accesses.

A further improvement could be to use multiple read and writes as the
assembly version was trying to do.

Tested on a BeagleV Starlight with a SiFive U74 core, where the
improvement is noticeable.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Co-developed-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

2 files changed, 137 insertions, 0 deletions

diff --git a/arch/csky/lib/Makefile b/arch/csky/lib/Makefile
index 7fbdbb2c4d12..d0ce6e2d7ab2 100644
--- a/arch/csky/lib/Makefile
+++ b/arch/csky/lib/Makefile
@@ -1,3 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 lib-y  := usercopy.o delay.o
 obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
+ifneq ($(CONFIG_HAVE_EFFICIENT_UNALIGNED_STRING_OPS), y)
+lib-y  += string.o
+endif
diff --git a/arch/csky/lib/string.c b/arch/csky/lib/string.c
new file mode 100644
index 000000000000..d65626fcaeac
--- /dev/null
+++ b/arch/csky/lib/string.c
@@ -0,0 +1,134 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * String functions optimized for hardware which doesn't
+ * handle unaligned memory accesses efficiently.
+ *
+ * Copyright (C) 2021 Matteo Croce
+ */
+
+#include <linux/types.h>
+#include <linux/module.h>
+
+/* Minimum size for a word copy to be convenient */
+#define BYTES_LONG	sizeof(long)
+#define WORD_MASK	(BYTES_LONG - 1)
+#define MIN_THRESHOLD	(BYTES_LONG * 2)
+
+/* convenience union to avoid cast between different pointer types */
+union types {
+	u8 *as_u8;
+	unsigned long *as_ulong;
+	uintptr_t as_uptr;
+};
+
+union const_types {
+	const u8 *as_u8;
+	unsigned long *as_ulong;
+	uintptr_t as_uptr;
+};
+
+void *memcpy(void *dest, const void *src, size_t count)
+{
+	union const_types s = { .as_u8 = src };
+	union types d = { .as_u8 = dest };
+	int distance = 0;
+
+	if (count < MIN_THRESHOLD)
+		goto copy_remainder;
+
+	/* Copy a byte at time until destination is aligned. */
+	for (; d.as_uptr & WORD_MASK; count--)
+		*d.as_u8++ = *s.as_u8++;
+
+	distance = s.as_uptr & WORD_MASK;
+
+	if (distance) {
+		unsigned long last, next;
+
+		/*
+		 * s is distance bytes ahead of d, and d just reached
+		 * the alignment boundary. Move s backward to word align it
+		 * and shift data to compensate for distance, in order to do
+		 * word-by-word copy.
+		 */
+		s.as_u8 -= distance;
+
+		next = s.as_ulong[0];
+		for (; count >= BYTES_LONG; count -= BYTES_LONG) {
+			last = next;
+			next = s.as_ulong[1];
+
+			d.as_ulong[0] = last >> (distance * 8) |
+				next << ((BYTES_LONG - distance) * 8);
+
+			d.as_ulong++;
+			s.as_ulong++;
+		}
+
+		/* Restore s with the original offset. */
+		s.as_u8 += distance;
+	} else {
+		/*
+		 * If the source and dest lower bits are the same, do a simple
+		 * 32/64 bit wide copy.
+		 */
+		for (; count >= BYTES_LONG; count -= BYTES_LONG)
+			*d.as_ulong++ = *s.as_ulong++;
+	}
+
+copy_remainder:
+	while (count--)
+		*d.as_u8++ = *s.as_u8++;
+
+	return dest;
+}
+EXPORT_SYMBOL(memcpy);
+
+/*
+ * Simply check if the buffer overlaps an call memcpy() in case,
+ * otherwise do a simple one byte at time backward copy.
+ */
+void *memmove(void *dest, const void *src, size_t count)
+{
+	if (dest < src || src + count <= dest)
+		return memcpy(dest, src, count);
+
+	if (dest > src) {
+		const char *s = src + count;
+		char *tmp = dest + count;
+
+		while (count--)
+			*--tmp = *--s;
+	}
+	return dest;
+}
+EXPORT_SYMBOL(memmove);
+
+void *memset(void *s, int c, size_t count)
+{
+	union types dest = { .as_u8 = s };
+
+	if (count >= MIN_THRESHOLD) {
+		unsigned long cu = (unsigned long)c;
+
+		/* Compose an ulong with 'c' repeated 4/8 times */
+		cu |= cu << 8;
+		cu |= cu << 16;
+		/* Suppress warning on 32 bit machines */
+		cu |= (cu << 16) << 16;
+
+		for (; count && dest.as_uptr & WORD_MASK; count--)
+			*dest.as_u8++ = c;
+
+		/* Copy using the largest size allowed */
+		for (; count >= BYTES_LONG; count -= BYTES_LONG)
+			*dest.as_ulong++ = cu;
+	}
+
+	/* copy the remainder */
+	while (count--)
+		*dest.as_u8++ = c;
+
+	return s;
+}
+EXPORT_SYMBOL(memset);