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Date:   Wed, 18 Nov 2020 23:10:27 +0800
From:   Zhaoxiu Zeng <zengzhaoxiu@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>
Subject: Re: [PATCH 1/3] lib: Introduce copy_from_back()

在 2020/11/17 1:24, Christoph Hellwig 写道:
>> +#define FAST_COPY_SAFEGUARD_SIZE (sizeof(long) * 2 - 1)
>> +
>> +#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>> +
>> +/*
>> + * The caller must ensure that the output buffer has enough space (len + FAST_COPY_SAFEGUARD_SIZE),
>> + * so we can write 2 words per loop without overflowing the output buffer
>> + */
>> +static __always_inline u8 *copy_from_back_fast(u8 *out, size_t dist, size_t len)
> 
> The code is pretty unreadable with these longs lines.  Please stick to
> the normal 80 char limit.
> 

I cleaned up the code.


---
 include/asm-generic/Kbuild           |   1 +
 include/asm-generic/copy_from_back.h | 422 +++++++++++++++++++++++++++
 2 files changed, 423 insertions(+)
 create mode 100644 include/asm-generic/copy_from_back.h

diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
index e78bbb9a07e9..84964a32572c 100644
--- a/include/asm-generic/Kbuild
+++ b/include/asm-generic/Kbuild
@@ -12,6 +12,7 @@ mandatory-y += bugs.h
 mandatory-y += cacheflush.h
 mandatory-y += checksum.h
 mandatory-y += compat.h
+mandatory-y += copy_from_back.h
 mandatory-y += current.h
 mandatory-y += delay.h
 mandatory-y += device.h
diff --git a/include/asm-generic/copy_from_back.h b/include/asm-generic/copy_from_back.h
new file mode 100644
index 000000000000..5628290f0286
--- /dev/null
+++ b/include/asm-generic/copy_from_back.h
@@ -0,0 +1,423 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __COPY_FROM_BACK_H
+#define __COPY_FROM_BACK_H
+
+#include <linux/types.h>
+
+#define FAST_COPY_SAFEGUARD_SIZE (sizeof(long) * 2 - 1)
+
+#define COPY_WORD(ptr) \
+	*(unsigned long *)(ptr) = *(unsigned long *)((ptr) - dist)
+
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+
+/*
+ * The caller must ensure that the output buffer has enough space
+ * (len + FAST_COPY_SAFEGUARD_SIZE), so we can write 2 words per
+ * loop without overflowing the output buffer
+ */
+static __always_inline u8 *copy_from_back_fast(u8 *out, size_t dist, size_t len)
+{
+	u8 *end = out + len;
+
+	/*
+	 * ensure the distance is larger than sizeof(long),
+	 * so we can copy one word per step
+	 */
+	if (unlikely(dist < sizeof(long))) {
+#if BITS_PER_LONG == 32
+		/* extend the distance '1,2,3' to '4,4,6' */
+		out[0] = out[0 - dist];
+		out[1] = out[1 - dist];
+		out[2] = out[2 - dist];
+		out += sizeof(long) - 1;
+		if (out >= end)
+			return end;
+		dist = dist == 3 ? 6 : 4;
+#elif BITS_PER_LONG == 64
+		/* extend the distance '1,2,3,4,5,6,7' to '8,8,9,8,A,C,E' */
+		out[0] = out[0 - dist];
+		out[1] = out[1 - dist];
+		out[2] = out[2 - dist];
+		*(u32 *)(out + 3) = *(u32 *)(out -
+					((0x43213110 >> (dist * 4)) & 0xF));
+		out += sizeof(long) - 1;
+		if (out >= end)
+			return end;
+		dist = (0xECA89880 >> (dist * 4)) & 0xF;
+#else
+#error "Unknown BITS_PER_LONG"
+#endif
+	}
+
+	do {
+		COPY_WORD(out);
+		out += sizeof(long);
+		COPY_WORD(out);
+		out += sizeof(long);
+	} while (out < end);
+
+	return end;
+}
+
+static __always_inline u8 *copy_from_back(u8 *out, size_t dist, size_t len)
+{
+	if (len >= sizeof(long)) {
+		/* extend the distance if nedded */
+		if (unlikely(dist < sizeof(long))) {
+#if BITS_PER_LONG == 32
+			out[0] = out[0 - dist];
+			out[1] = out[1 - dist];
+			out[2] = out[2 - dist];
+			out += sizeof(long) - 1;
+			len -= sizeof(long) - 1;
+			if (len < sizeof(long))
+				goto _last;
+			dist = dist == 3 ? 6 : 4;
+#elif BITS_PER_LONG == 64
+			out[0] = out[0 - dist];
+			out[1] = out[1 - dist];
+			out[2] = out[2 - dist];
+			*(u32 *)(out + 3) = *(u32 *)(out -
+					((0x43213110 >> (dist * 4)) & 0xF));
+			out += sizeof(long) - 1;
+			len -= sizeof(long) - 1;
+			if (len < sizeof(long))
+				goto _last;
+			dist = (0xECA89880 >> (dist * 4)) & 0xF;
+#else
+#error "Unknown BITS_PER_LONG"
+#endif
+		}
+
+		while (len >= sizeof(long) * 2) {
+			len -= sizeof(long) * 2;
+			COPY_WORD(out);
+			out += sizeof(long);
+			COPY_WORD(out);
+			out += sizeof(long);
+		}
+		if (len >= sizeof(long)) {
+			len -= sizeof(long);
+			COPY_WORD(out);
+			out += sizeof(long);
+		}
+		if (len) {
+			out += len;
+			COPY_WORD(out - sizeof(long));
+		}
+	} else {
+		*out = *(out - dist);
+		out++;
+		len--;
+_last:
+		if (len >= 2) {
+			if (dist == 1)
+				dist = 2;
+#if BITS_PER_LONG == 32
+			*(u16 *)out = *(u16 *)(out - dist);
+			out += 2;
+			len -= 2;
+#else
+			do {
+				*(u16 *)out = *(u16 *)(out - dist);
+				out += 2;
+				len -= 2;
+			} while (len >= 2);
+#endif
+		}
+		if (len) {
+			*out = *(out - dist);
+			out++;
+		}
+	}
+
+	return out;
+}
+
+#else /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
+
+#include <asm/unaligned.h>
+
+/*
+ * Endian independent macros for shifting bits
+ */
+#ifdef __BIG_ENDIAN
+# define LSPULL(x, n)	((x) << (n))
+# define LSPUSH(x, n)	((x) >> (n))
+# define LSMASK(n)	(~(~0UL >> (n)))
+# define LSFETCH(x, n)	((x) >> (BITS_PER_LONG - (n)))
+#else
+# define LSPULL(x, n)	((x) >> (n))
+# define LSPUSH(x, n)	((x) << (n))
+# define LSMASK(n)	((1UL << (n)) - 1)
+# define LSFETCH(x, n)	((x) & LSMASK(n))
+#endif
+
+#define COPY_WORD_SHIFT(ptr, l, h) \
+	*(unsigned long *)(ptr) = LSPULL((l), shift_l) | LSPUSH((h), shift_h)
+
+static __always_inline u8 *copy_from_back_fast(u8 *out, size_t dist, size_t len)
+{
+	u8 *end = out + len;
+	unsigned long val;
+	unsigned int n;
+
+	/* extend the distance if nedded */
+	if (unlikely(dist < sizeof(long))) {
+#if BITS_PER_LONG == 32
+		out[0] = out[0 - dist];
+		out[1] = out[1 - dist];
+		out[2] = out[2 - dist];
+		out += sizeof(long) - 1;
+		if (out >= end)
+			return end;
+		dist = dist == 3 ? 6 : 4;
+#elif BITS_PER_LONG == 64
+		out[0] = out[0 - dist];
+		out[1] = out[1 - dist];
+		out[2] = out[2 - dist];
+		out[3] = out[3 - dist];
+		out[4] = out[4 - dist];
+		out[5] = out[5 - dist];
+		out[6] = out[6 - dist];
+		out += sizeof(long) - 1;
+		if (out >= end)
+			return end;
+		dist = (0xECA89880 >> (dist * 4)) & 0xF;
+#else
+#error "Unknown BITS_PER_LONG"
+#endif
+	}
+
+	/* align out addr */
+	n = (uintptr_t)out & (sizeof(long) - 1);
+	if (n) {
+		val = get_unaligned((unsigned long *)(out - dist));
+		put_unaligned(val, (unsigned long *)out);
+		out += sizeof(long);
+		if (out >= end)
+			return end;
+		out -= n;
+	}
+
+	n = dist & (sizeof(long) - 1);
+	if (n) {
+		const unsigned int shift_h = n * 8;
+		const unsigned int shift_l = BITS_PER_LONG - shift_h;
+		unsigned long prev;
+
+		dist -= n;
+		prev = *(unsigned long *)(out - dist - sizeof(long));
+
+		do {
+			val = *(unsigned long *)(out - dist);
+			COPY_WORD_SHIFT(out, prev, val);
+			out += sizeof(long);
+			prev = *(unsigned long *)(out - dist);
+			COPY_WORD_SHIFT(out, val, prev);
+			out += sizeof(long);
+		} while (out < end);
+	} else {
+		do {
+			COPY_WORD(out);
+			out += sizeof(long);
+			COPY_WORD(out);
+			out += sizeof(long);
+		} while (out < end);
+	}
+
+	return end;
+}
+
+static __always_inline u8 *copy_from_back(u8 *out, size_t dist, size_t len)
+{
+	if (len >= sizeof(long)) {
+		unsigned long val;
+		unsigned int n;
+
+		/* extend the distance if nedded */
+		if (unlikely(dist < sizeof(long))) {
+#if BITS_PER_LONG == 32
+			out[0] = out[0 - dist];
+			out[1] = out[1 - dist];
+			out[2] = out[2 - dist];
+			out += sizeof(long) - 1;
+			len -= sizeof(long) - 1;
+			if (len < sizeof(long))
+				goto _last;
+			dist = dist == 3 ? 6 : 4;
+#elif BITS_PER_LONG == 64
+			out[0] = out[0 - dist];
+			out[1] = out[1 - dist];
+			out[2] = out[2 - dist];
+			out[3] = out[3 - dist];
+			out[4] = out[4 - dist];
+			out[5] = out[5 - dist];
+			out[6] = out[6 - dist];
+			out += sizeof(long) - 1;
+			len -= sizeof(long) - 1;
+			if (len < sizeof(long))
+				goto _last;
+			dist = (0xECA89880 >> (dist * 4)) & 0xF;
+#else
+#error "Unknown BITS_PER_LONG"
+#endif
+		}
+
+		/* align out addr */
+		n = (uintptr_t)out & (sizeof(long) - 1);
+		if (n) {
+			val = get_unaligned((unsigned long *)(out - dist));
+			put_unaligned(val, (unsigned long *)out);
+			out += sizeof(long);
+			len -= sizeof(long);
+			if (len < sizeof(long)) {
+				if (len)
+					goto _last;
+				return out;
+			}
+			out -= n;
+			len += n;
+		}
+
+		n = dist & (sizeof(long) - 1);
+		if (n) {
+			const unsigned int shift_h = n * 8;
+			const unsigned int shift_l = BITS_PER_LONG - shift_h;
+			unsigned long prev;
+
+			dist -= n;
+			prev = *(unsigned long *)(out - dist - sizeof(long));
+
+			while (len >= sizeof(long) * 2) {
+				len -= sizeof(long) * 2;
+				val = *(unsigned long *)(out - dist);
+				COPY_WORD_SHIFT(out, prev, val);
+				out += sizeof(long);
+				prev = *(unsigned long *)(out - dist);
+				COPY_WORD_SHIFT(out, val, prev);
+				out += sizeof(long);
+			}
+			if (len >= sizeof(long)) {
+				len -= sizeof(long);
+				val = *(unsigned long *)(out - dist);
+				COPY_WORD_SHIFT(out, prev, val);
+				out += sizeof(long);
+				prev = val;
+			}
+			if (len) {
+				val = LSPULL(prev, shift_l);
+				if (len * 8 > shift_h) {
+					prev = *(unsigned long *)(out - dist);
+					val |= LSPUSH(prev, shift_h);
+				}
+				goto _part;
+			}
+		} else if (dist != sizeof(long)) {
+			while (len >= sizeof(long) * 2) {
+				len -= sizeof(long) * 2;
+				COPY_WORD(out);
+				out += sizeof(long);
+				COPY_WORD(out);
+				out += sizeof(long);
+			}
+			if (len >= sizeof(long)) {
+				len -= sizeof(long);
+				COPY_WORD(out);
+				out += sizeof(long);
+			}
+			if (len) {
+				val = *(unsigned long *)(out - dist);
+				goto _part;
+			}
+		} else {
+			val = *(unsigned long *)(out - sizeof(long));
+			while (len >= sizeof(long) * 2) {
+				len -= sizeof(long) * 2;
+				*(unsigned long *)out = val;
+				out += sizeof(long);
+				*(unsigned long *)out = val;
+				out += sizeof(long);
+			}
+			if (len >= sizeof(long)) {
+				len -= sizeof(long);
+				*(unsigned long *)out = val;
+				out += sizeof(long);
+			}
+			if (len) {
+_part:
+#if BITS_PER_LONG == 64
+				if (len & 4) {
+					*(u32 *)out = LSFETCH(val, 32);
+					out += 4;
+					val = LSPULL(val, 32);
+				}
+#endif
+				if (len & 2) {
+					*(u16 *)out = LSFETCH(val, 16);
+					out += 2;
+					val = LSPULL(val, 16);
+				}
+				if (len & 1)
+					*out++ = LSFETCH(val, 8);
+			}
+		}
+	} else {
+_last:
+		/* align out addr */
+		if ((uintptr_t)out & 1) {
+			*out = *(out - dist);
+			out++;
+			len--;
+		}
+		if (len >= 2) {
+#if BITS_PER_LONG == 32
+			if (dist > 1)
+				*(u16 *)out = get_unaligned((u16 *)(out - dist));
+			else
+				*(u16 *)out = out[-1] | out[-1] << 8;
+			out += 2;
+			len -= 2;
+#else
+			u16 val;
+
+			if (dist > 2) {
+				do {
+					val = get_unaligned((u16 *)(out - dist));
+					*(u16 *)out = val;
+					out += 2;
+					len -= 2;
+				} while (len >= 2);
+			} else {
+				val = dist == 2 ? *(u16 *)(out - 2)
+						: (out[-1] | out[-1] << 8);
+				do {
+					*(u16 *)out = val;
+					out += 2;
+					len -= 2;
+				} while (len >= 2);
+			}
+#endif
+		}
+		if (len) {
+			*out = *(out - dist);
+			out++;
+		}
+	}
+
+	return out;
+}
+
+#undef COPY_WORD_SHIFT
+
+#undef LSPULL
+#undef LSPUSH
+#undef LSMASK
+#undef LSFETCH
+
+#endif /* CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
+
+#undef COPY_WORD
+
+#endif /* __COPY_FROM_BACK_H */
-- 
2.28.0

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