lib: add fast path for find_next_*_bit()

Similarly to bitmap functions, find_next_*_bit() users will benefit if
we'll handle a case of bitmaps that fit into a single word inline. In the
very best case, the compiler may replace a function call with a few
instructions.

This is the quite typical find_next_bit() user:

unsigned int cpumask_next(int n, const struct cpumask *srcp)
{
/* -1 is a legal arg here. */
if (n != -1)
cpumask_check(n);
return find_next_bit(cpumask_bits(srcp), nr_cpumask_bits, n + 1);
}
EXPORT_SYMBOL(cpumask_next);

Currently, on ARM64 the generated code looks like this:
0000000000000000 <cpumask_next>:
0: a9bf7bfd stp x29, x30, [sp, #-16]!
4: 11000402 add w2, w0, #0x1
8: aa0103e0 mov x0, x1
c: d2800401 mov x1, #0x40 // #64
10: 910003fd mov x29, sp
14: 93407c42 sxtw x2, w2
18: 94000000 bl 0 <find_next_bit>
1c: a8c17bfd ldp x29, x30, [sp], #16
20: d65f03c0 ret
24: d503201f nop

After applying this patch:
0000000000000140 <cpumask_next>:
140: 11000400 add w0, w0, #0x1
144: 93407c00 sxtw x0, w0
148: f100fc1f cmp x0, #0x3f
14c: 54000168 b.hi 178 <cpumask_next+0x38> // b.pmore
150: f9400023 ldr x3, [x1]
154: 92800001 mov x1, #0xffffffffffffffff // #-1
158: 9ac02020 lsl x0, x1, x0
15c: 52800802 mov w2, #0x40 // #64
160: 8a030001 and x1, x0, x3
164: dac00020 rbit x0, x1
168: f100003f cmp x1, #0x0
16c: dac01000 clz x0, x0
170: 1a800040 csel w0, w2, w0, eq // eq = none
174: d65f03c0 ret
178: 52800800 mov w0, #0x40 // #64
17c: d65f03c0 ret

find_next_bit() call is replaced with 6 instructions. find_next_bit()
itself is 41 instructions plus function call overhead.

Despite inlining, the scripts/bloat-o-meter report smaller .text size
after applying the series:
add/remove: 11/9 grow/shrink: 233/176 up/down: 5780/-6768 (-988)

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Yury Norov <[e-mail]>
Acked-by: Rasmus Villemoes <[e-mail]>
Acked-by: Andy Shevchenko <[e-mail]>
Cc: Alexey Klimov <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: David Sterba <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Jianpeng Ma <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: John Paul Adrian Glaubitz <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Rich Felker <[e-mail]>
Cc: Stefano Brivio <[e-mail]>
Cc: Wei Yang <[e-mail]>
Cc: Wolfram Sang <[e-mail]>
Cc: Yoshinori Sato <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/asm-generic/bitops/find.h

···
 unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
 			    unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr & GENMASK(size - 1, offset);
+		return val ? __ffs(val) : size;
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, 0UL, 0);
 }
 #endif
···
 		const unsigned long *addr2, unsigned long size,
 		unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr1 & *addr2 & GENMASK(size - 1, offset);
+		return val ? __ffs(val) : size;
+	}
+
 	return _find_next_bit(addr1, addr2, size, offset, 0UL, 0);
 }
 #endif
···
 unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
 				 unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr | ~GENMASK(size - 1, offset);
+		return val == ~0UL ? size : ffz(val);
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, ~0UL, 0);
 }
 #endif

include/asm-generic/bitops/le.h

···
 #include <asm-generic/bitops/find.h>
 #include <asm/types.h>
 #include <asm/byteorder.h>
+#include <linux/swab.h>
 
 #if defined(__LITTLE_ENDIAN)
 
···
 unsigned long find_next_zero_bit_le(const void *addr, unsigned
 		long size, unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val = *(const unsigned long *)addr;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = swab(val) | ~GENMASK(size - 1, offset);
+		return val == ~0UL ? size : ffz(val);
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, ~0UL, 1);
 }
 #endif
···
 unsigned long find_next_bit_le(const void *addr, unsigned
 		long size, unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val = *(const unsigned long *)addr;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = swab(val) & GENMASK(size - 1, offset);
+		return val ? __ffs(val) : size;
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, 0UL, 1);
 }
 #endif