lib/GCD.c: use binary GCD algorithm instead of Euclidean

The binary GCD algorithm is based on the following facts:
	1. If a and b are all evens, then gcd(a,b) = 2 * gcd(a/2, b/2)
	2. If a is even and b is odd, then gcd(a,b) = gcd(a/2, b)
	3. If a and b are all odds, then gcd(a,b) = gcd((a-b)/2, b) = gcd((a+b)/2, b)

Even on x86 machines with reasonable division hardware, the binary
algorithm runs about 25% faster (80% the execution time) than the
division-based Euclidian algorithm.

On platforms like Alpha and ARMv6 where division is a function call to
emulation code, it's even more significant.

There are two variants of the code here, depending on whether a fast
__ffs (find least significant set bit) instruction is available.  This
allows the unpredictable branches in the bit-at-a-time shifting loop to
be eliminated.

If fast __ffs is not available, the "even/odd" GCD variant is used.

I use the following code to benchmark:

	#include <stdio.h>
	#include <stdlib.h>
	#include <stdint.h>
	#include <string.h>
	#include <time.h>
	#include <unistd.h>

	#define swap(a, b) \
		do { \
			a ^= b; \
			b ^= a; \
			a ^= b; \
		} while (0)

	unsigned long gcd0(unsigned long a, unsigned long b)
	{
		unsigned long r;

		if (a < b) {
			swap(a, b);
		}

		if (b == 0)
			return a;

		while ((r = a % b) != 0) {
			a = b;
			b = r;
		}

		return b;
	}

	unsigned long gcd1(unsigned long a, unsigned long b)
	{
		unsigned long r = a | b;

		if (!a || !b)
			return r;

		b >>= __builtin_ctzl(b);

		for (;;) {
			a >>= __builtin_ctzl(a);
			if (a == b)
				return a << __builtin_ctzl(r);

			if (a < b)
				swap(a, b);
			a -= b;
		}
	}

	unsigned long gcd2(unsigned long a, unsigned long b)
	{
		unsigned long r = a | b;

		if (!a || !b)
			return r;

		r &= -r;

		while (!(b & r))
			b >>= 1;

		for (;;) {
			while (!(a & r))
				a >>= 1;
			if (a == b)
				return a;

			if (a < b)
				swap(a, b);
			a -= b;
			a >>= 1;
			if (a & r)
				a += b;
			a >>= 1;
		}
	}

	unsigned long gcd3(unsigned long a, unsigned long b)
	{
		unsigned long r = a | b;

		if (!a || !b)
			return r;

		b >>= __builtin_ctzl(b);
		if (b == 1)
			return r & -r;

		for (;;) {
			a >>= __builtin_ctzl(a);
			if (a == 1)
				return r & -r;
			if (a == b)
				return a << __builtin_ctzl(r);

			if (a < b)
				swap(a, b);
			a -= b;
		}
	}

	unsigned long gcd4(unsigned long a, unsigned long b)
	{
		unsigned long r = a | b;

		if (!a || !b)
			return r;

		r &= -r;

		while (!(b & r))
			b >>= 1;
		if (b == r)
			return r;

		for (;;) {
			while (!(a & r))
				a >>= 1;
			if (a == r)
				return r;
			if (a == b)
				return a;

			if (a < b)
				swap(a, b);
			a -= b;
			a >>= 1;
			if (a & r)
				a += b;
			a >>= 1;
		}
	}

	static unsigned long (*gcd_func[])(unsigned long a, unsigned long b) = {
		gcd0, gcd1, gcd2, gcd3, gcd4,
	};

	#define TEST_ENTRIES (sizeof(gcd_func) / sizeof(gcd_func[0]))

	#if defined(__x86_64__)

	#define rdtscll(val) do { \
		unsigned long __a,__d; \
		__asm__ __volatile__("rdtsc" : "=a" (__a), "=d" (__d)); \
		(val) = ((unsigned long long)__a) | (((unsigned long long)__d)<<32); \
	} while(0)

	static unsigned long long benchmark_gcd_func(unsigned long (*gcd)(unsigned long, unsigned long),
								unsigned long a, unsigned long b, unsigned long *res)
	{
		unsigned long long start, end;
		unsigned long long ret;
		unsigned long gcd_res;

		rdtscll(start);
		gcd_res = gcd(a, b);
		rdtscll(end);

		if (end >= start)
			ret = end - start;
		else
			ret = ~0ULL - start + 1 + end;

		*res = gcd_res;
		return ret;
	}

	#else

	static inline struct timespec read_time(void)
	{
		struct timespec time;
		clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time);
		return time;
	}

	static inline unsigned long long diff_time(struct timespec start, struct timespec end)
	{
		struct timespec temp;

		if ((end.tv_nsec - start.tv_nsec) < 0) {
			temp.tv_sec = end.tv_sec - start.tv_sec - 1;
			temp.tv_nsec = 1000000000ULL + end.tv_nsec - start.tv_nsec;
		} else {
			temp.tv_sec = end.tv_sec - start.tv_sec;
			temp.tv_nsec = end.tv_nsec - start.tv_nsec;
		}

		return temp.tv_sec * 1000000000ULL + temp.tv_nsec;
	}

	static unsigned long long benchmark_gcd_func(unsigned long (*gcd)(unsigned long, unsigned long),
								unsigned long a, unsigned long b, unsigned long *res)
	{
		struct timespec start, end;
		unsigned long gcd_res;

		start = read_time();
		gcd_res = gcd(a, b);
		end = read_time();

		*res = gcd_res;
		return diff_time(start, end);
	}

	#endif

	static inline unsigned long get_rand()
	{
		if (sizeof(long) == 8)
			return (unsigned long)rand() << 32 | rand();
		else
			return rand();
	}

	int main(int argc, char **argv)
	{
		unsigned int seed = time(0);
		int loops = 100;
		int repeats = 1000;
		unsigned long (*res)[TEST_ENTRIES];
		unsigned long long elapsed[TEST_ENTRIES];
		int i, j, k;

		for (;;) {
			int opt = getopt(argc, argv, "n:r:s:");
			/* End condition always first */
			if (opt == -1)
				break;

			switch (opt) {
			case 'n':
				loops = atoi(optarg);
				break;
			case 'r':
				repeats = atoi(optarg);
				break;
			case 's':
				seed = strtoul(optarg, NULL, 10);
				break;
			default:
				/* You won't actually get here. */
				break;
			}
		}

		res = malloc(sizeof(unsigned long) * TEST_ENTRIES * loops);
		memset(elapsed, 0, sizeof(elapsed));

		srand(seed);
		for (j = 0; j < loops; j++) {
			unsigned long a = get_rand();
			/* Do we have args? */
			unsigned long b = argc > optind ? strtoul(argv[optind], NULL, 10) : get_rand();
			unsigned long long min_elapsed[TEST_ENTRIES];
			for (k = 0; k < repeats; k++) {
				for (i = 0; i < TEST_ENTRIES; i++) {
					unsigned long long tmp = benchmark_gcd_func(gcd_func[i], a, b, &res[j][i]);
					if (k == 0 || min_elapsed[i] > tmp)
						min_elapsed[i] = tmp;
				}
			}
			for (i = 0; i < TEST_ENTRIES; i++)
				elapsed[i] += min_elapsed[i];
		}

		for (i = 0; i < TEST_ENTRIES; i++)
			printf("gcd%d: elapsed %llu\n", i, elapsed[i]);

		k = 0;
		srand(seed);
		for (j = 0; j < loops; j++) {
			unsigned long a = get_rand();
			unsigned long b = argc > optind ? strtoul(argv[optind], NULL, 10) : get_rand();
			for (i = 1; i < TEST_ENTRIES; i++) {
				if (res[j][i] != res[j][0])
					break;
			}
			if (i < TEST_ENTRIES) {
				if (k == 0) {
					k = 1;
					fprintf(stderr, "Error:\n");
				}
				fprintf(stderr, "gcd(%lu, %lu): ", a, b);
				for (i = 0; i < TEST_ENTRIES; i++)
					fprintf(stderr, "%ld%s", res[j][i], i < TEST_ENTRIES - 1 ? ", " : "\n");
			}
		}

		if (k == 0)
			fprintf(stderr, "PASS\n");

		free(res);

		return 0;
	}

Compiled with "-O2", on "VirtualBox 4.4.0-22-generic #38-Ubuntu x86_64" got:

  zhaoxiuzeng@zhaoxiuzeng-VirtualBox:~/develop$ ./gcd -r 500000 -n 10
  gcd0: elapsed 10174
  gcd1: elapsed 2120
  gcd2: elapsed 2902
  gcd3: elapsed 2039
  gcd4: elapsed 2812
  PASS
  zhaoxiuzeng@zhaoxiuzeng-VirtualBox:~/develop$ ./gcd -r 500000 -n 10
  gcd0: elapsed 9309
  gcd1: elapsed 2280
  gcd2: elapsed 2822
  gcd3: elapsed 2217
  gcd4: elapsed 2710
  PASS
  zhaoxiuzeng@zhaoxiuzeng-VirtualBox:~/develop$ ./gcd -r 500000 -n 10
  gcd0: elapsed 9589
  gcd1: elapsed 2098
  gcd2: elapsed 2815
  gcd3: elapsed 2030
  gcd4: elapsed 2718
  PASS
  zhaoxiuzeng@zhaoxiuzeng-VirtualBox:~/develop$ ./gcd -r 500000 -n 10
  gcd0: elapsed 9914
  gcd1: elapsed 2309
  gcd2: elapsed 2779
  gcd3: elapsed 2228
  gcd4: elapsed 2709
  PASS

[akpm@linux-foundation.org: avoid #defining a CONFIG_ variable]
Signed-off-by: Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>
Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/Kconfig b/arch/Kconfig
index 8f84fd268dee6a2c4bf3792352c48ff11e2e6222..b16e74e4b5afbbcfad4091c4067c638d2e50da3c 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -647,4 +647,7 @@ config COMPAT_OLD_SIGACTION
 config ARCH_NO_COHERENT_DMA_MMAP
        bool
 
+config CPU_NO_EFFICIENT_FFS
+       def_bool n
+
 source "kernel/gcov/Kconfig"

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index fe99f894e57d647f36c18177311eb1355b6332ff..7f312d80b43b4327611295a8506f1894754b4544 100644 (file)
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -26,6 +26,7 @@ config ALPHA
        select MODULES_USE_ELF_RELA
        select ODD_RT_SIGACTION
        select OLD_SIGSUSPEND
+       select CPU_NO_EFFICIENT_FFS if !ALPHA_EV67
        help
          The Alpha is a 64-bit general-purpose processor designed and
          marketed by the Digital Equipment Corporation of blessed memory,

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 8894f7e7e3de2a6e6cc1947d93a9359be961c699..0dcbacfdea4b98ca7805624a5ffb59b9298c4992 100644 (file)
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -107,6 +107,7 @@ choice
 
 config ISA_ARCOMPACT
        bool "ARCompact ISA"
+       select CPU_NO_EFFICIENT_FFS
        help
          The original ARC ISA of ARC600/700 cores
 

diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 55347662e5edafa0900b54987b219a5fb83a6b5a..cb569b65a54ddcdaa8cce08d047799a24ba75f4b 100644 (file)
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -421,18 +421,21 @@ config CPU_32v3
        select CPU_USE_DOMAINS if MMU
        select NEED_KUSER_HELPERS
        select TLS_REG_EMUL if SMP || !MMU
+       select CPU_NO_EFFICIENT_FFS
 
 config CPU_32v4
        bool
        select CPU_USE_DOMAINS if MMU
        select NEED_KUSER_HELPERS
        select TLS_REG_EMUL if SMP || !MMU
+       select CPU_NO_EFFICIENT_FFS
 
 config CPU_32v4T
        bool
        select CPU_USE_DOMAINS if MMU
        select NEED_KUSER_HELPERS
        select TLS_REG_EMUL if SMP || !MMU
+       select CPU_NO_EFFICIENT_FFS
 
 config CPU_32v5
        bool

diff --git a/arch/h8300/Kconfig b/arch/h8300/Kconfig
index 986ea84caaed48a9fd4d16a7b1195a7b97b87eb5..aa232de2d4bc854a7c25b73a538dc7bcf7df755e 100644 (file)
--- a/arch/h8300/Kconfig
+++ b/arch/h8300/Kconfig
@@ -20,6 +20,7 @@ config H8300
        select HAVE_KERNEL_GZIP
        select HAVE_KERNEL_LZO
        select HAVE_ARCH_KGDB
+       select CPU_NO_EFFICIENT_FFS
 
 config RWSEM_GENERIC_SPINLOCK
        def_bool y

diff --git a/arch/m32r/Kconfig b/arch/m32r/Kconfig
index c82b2925399116e87128e0c187fde3d9e7aadb75..3cc8498fe0fe8c983031c73a72c3b2c168f92218 100644 (file)
--- a/arch/m32r/Kconfig
+++ b/arch/m32r/Kconfig
@@ -17,6 +17,7 @@ config M32R
        select ARCH_USES_GETTIMEOFFSET
        select MODULES_USE_ELF_RELA
        select HAVE_DEBUG_STACKOVERFLOW
+       select CPU_NO_EFFICIENT_FFS
 
 config SBUS
        bool

diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
index c1beb5ae181fa61648fab7f44040c940c8372aa1..8ace920ca24aaf1d491db0f7038a60e2bbc58237 100644 (file)
--- a/arch/m68k/Kconfig.cpu
+++ b/arch/m68k/Kconfig.cpu
@@ -40,6 +40,7 @@ config M68000
        select CPU_HAS_NO_MULDIV64
        select CPU_HAS_NO_UNALIGNED
        select GENERIC_CSUM
+       select CPU_NO_EFFICIENT_FFS
        help
          The Freescale (was Motorola) 68000 CPU is the first generation of
          the well known M68K family of processors. The CPU core as well as
@@ -51,6 +52,7 @@ config MCPU32
        bool
        select CPU_HAS_NO_BITFIELDS
        select CPU_HAS_NO_UNALIGNED
+       select CPU_NO_EFFICIENT_FFS
        help
          The Freescale (was then Motorola) CPU32 is a CPU core that is
          based on the 68020 processor. For the most part it is used in
@@ -130,6 +132,7 @@ config M5206
        depends on !MMU
        select COLDFIRE_SW_A7
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5206 processor support.
 
@@ -138,6 +141,7 @@ config M5206e
        depends on !MMU
        select COLDFIRE_SW_A7
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5206e processor support.
 
@@ -163,6 +167,7 @@ config M5249
        depends on !MMU
        select COLDFIRE_SW_A7
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5249 processor support.
 
@@ -171,6 +176,7 @@ config M525x
        depends on !MMU
        select COLDFIRE_SW_A7
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Freescale (Motorola) Coldfire 5251/5253 processor support.
 
@@ -189,6 +195,7 @@ config M5272
        depends on !MMU
        select COLDFIRE_SW_A7
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5272 processor support.
 
@@ -217,6 +224,7 @@ config M5307
        select COLDFIRE_SW_A7
        select HAVE_CACHE_CB
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5307 processor support.
 
@@ -242,6 +250,7 @@ config M5407
        select COLDFIRE_SW_A7
        select HAVE_CACHE_CB
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Motorola ColdFire 5407 processor support.
 
@@ -251,6 +260,7 @@ config M547x
        select MMU_COLDFIRE if MMU
        select HAVE_CACHE_CB
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Freescale ColdFire 5470/5471/5472/5473/5474/5475 processor support.
 
@@ -260,6 +270,7 @@ config M548x
        select M54xx
        select HAVE_CACHE_CB
        select HAVE_MBAR
+       select CPU_NO_EFFICIENT_FFS
        help
          Freescale ColdFire 5480/5481/5482/5483/5484/5485 processor support.
 

diff --git a/arch/metag/Kconfig b/arch/metag/Kconfig
index e47a08d72819651446cf937ce54333792666cf58..5b7a45d99cfbfd5397554b399a6ec2bd8e4b5582 100644 (file)
--- a/arch/metag/Kconfig
+++ b/arch/metag/Kconfig
@@ -30,6 +30,7 @@ config METAG
        select OF
        select OF_EARLY_FLATTREE
        select SPARSE_IRQ
+       select CPU_NO_EFFICIENT_FFS
 
 config STACKTRACE_SUPPORT
        def_bool y

diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index 3d793b55f60c04b3b18cdf01b20a1d7c6964e65a..f17c3a4fb697d9b40752c235a43fe2fd65349db9 100644 (file)
--- a/arch/microblaze/Kconfig
+++ b/arch/microblaze/Kconfig
@@ -32,6 +32,7 @@ config MICROBLAZE
        select OF_EARLY_FLATTREE
        select TRACING_SUPPORT
        select VIRT_TO_BUS
+       select CPU_NO_EFFICIENT_FFS
 
 config SWAP
        def_bool n

diff --git a/arch/mips/include/asm/cpu-features.h b/arch/mips/include/asm/cpu-features.h
index e6f19fc61bf254d6fa05fdbec1164f76dca11c39..e961c8a7ea6626a1b044b366a99c5201e6d5beb3 100644 (file)
--- a/arch/mips/include/asm/cpu-features.h
+++ b/arch/mips/include/asm/cpu-features.h
@@ -204,6 +204,16 @@
 #endif
 #endif
 
+/* __builtin_constant_p(cpu_has_mips_r) && cpu_has_mips_r */
+#if !((defined(cpu_has_mips32r1) && cpu_has_mips32r1) || \
+         (defined(cpu_has_mips32r2) && cpu_has_mips32r2) || \
+         (defined(cpu_has_mips32r6) && cpu_has_mips32r6) || \
+         (defined(cpu_has_mips64r1) && cpu_has_mips64r1) || \
+         (defined(cpu_has_mips64r2) && cpu_has_mips64r2) || \
+         (defined(cpu_has_mips64r6) && cpu_has_mips64r6))
+#define CPU_NO_EFFICIENT_FFS 1
+#endif
+
 #ifndef cpu_has_mips_1
 # define cpu_has_mips_1                (!cpu_has_mips_r6)
 #endif

diff --git a/arch/nios2/Kconfig b/arch/nios2/Kconfig
index 87ca653eb5f3452558ea89c53d641f78dd370a62..51a56c8b04b451ce140a5708b4d5ca57bc7a6404 100644 (file)
--- a/arch/nios2/Kconfig
+++ b/arch/nios2/Kconfig
@@ -15,6 +15,7 @@ config NIOS2
        select SOC_BUS
        select SPARSE_IRQ
        select USB_ARCH_HAS_HCD if USB_SUPPORT
+       select CPU_NO_EFFICIENT_FFS
 
 config GENERIC_CSUM
        def_bool y

diff --git a/arch/openrisc/Kconfig b/arch/openrisc/Kconfig
index e118c02cc79a085eb59e10513bbf15c0272725a3..142cb057c41bd3218cb4a3634335452e161f172c 100644 (file)
--- a/arch/openrisc/Kconfig
+++ b/arch/openrisc/Kconfig
@@ -25,6 +25,7 @@ config OPENRISC
        select MODULES_USE_ELF_RELA
        select HAVE_DEBUG_STACKOVERFLOW
        select OR1K_PIC
+       select CPU_NO_EFFICIENT_FFS if !OPENRISC_HAVE_INST_FF1
 
 config MMU
        def_bool y

diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 88cfaa8af78ea2af3192472920100a4ac1de8207..3d498a676551d0fa778f0bafe0cbced3fe44765d 100644 (file)
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -32,6 +32,7 @@ config PARISC
        select HAVE_ARCH_AUDITSYSCALL
        select HAVE_ARCH_SECCOMP_FILTER
        select ARCH_NO_COHERENT_DMA_MMAP
+       select CPU_NO_EFFICIENT_FFS
 
        help
          The PA-RISC microprocessor is designed by Hewlett-Packard and used

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 1c3c43d9d1b54f5d828c45f272e7dc0be882024f..a8c259059adf0e7b6af3587d59d2d370e0b0cd5d 100644 (file)
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -123,6 +123,7 @@ config S390
        select HAVE_ARCH_AUDITSYSCALL
        select HAVE_ARCH_EARLY_PFN_TO_NID
        select HAVE_ARCH_JUMP_LABEL
+       select CPU_NO_EFFICIENT_FFS if !HAVE_MARCH_Z9_109_FEATURES
        select HAVE_ARCH_SECCOMP_FILTER
        select HAVE_ARCH_SOFT_DIRTY
        select HAVE_ARCH_TRACEHOOK

diff --git a/arch/score/Kconfig b/arch/score/Kconfig
index 366e1b599a7b7849b74f17f19308def9028cc971..507d6318138941aa5dec6cc767e992ca598e009f 100644 (file)
--- a/arch/score/Kconfig
+++ b/arch/score/Kconfig
@@ -14,6 +14,7 @@ config SCORE
        select VIRT_TO_BUS
        select MODULES_USE_ELF_REL
        select CLONE_BACKWARDS
+       select CPU_NO_EFFICIENT_FFS
 
 choice
        prompt "System type"

diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index f6254341c065da7ac4606b02a342c13b70b8e243..e803a836cb7ca81e9e1546649cc15e2e350dd88c 100644 (file)
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -20,6 +20,7 @@ config SUPERH
        select PERF_USE_VMALLOC
        select HAVE_DEBUG_KMEMLEAK
        select HAVE_KERNEL_GZIP
+       select CPU_NO_EFFICIENT_FFS
        select HAVE_KERNEL_BZIP2
        select HAVE_KERNEL_LZMA
        select HAVE_KERNEL_XZ

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 1012f7ffcdf5f5486ffe67ec185c356509d6f6b6..546293d9e6c52906f2c1f72d1a9545749595f994 100644 (file)
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -42,6 +42,7 @@ config SPARC
        select ODD_RT_SIGACTION
        select OLD_SIGSUSPEND
        select ARCH_HAS_SG_CHAIN
+       select CPU_NO_EFFICIENT_FFS
 
 config SPARC32
        def_bool !64BIT

diff --git a/lib/gcd.c b/lib/gcd.c
index 3657f129d7b8c5db0bee11a94a97d5d4bf251b52..135ee6407a5e4f0a6f7fc8da85f38a74ea37ed38 100644 (file)
--- a/lib/gcd.c
+++ b/lib/gcd.c
@@ -2,20 +2,77 @@
 #include <linux/gcd.h>
 #include <linux/export.h>
 
-/* Greatest common divisor */
+/*
+ * This implements the binary GCD algorithm. (Often attributed to Stein,
+ * but as Knuth has noted, appears in a first-century Chinese math text.)
+ *
+ * This is faster than the division-based algorithm even on x86, which
+ * has decent hardware division.
+ */
+
+#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) && !defined(CPU_NO_EFFICIENT_FFS)
+
+/* If __ffs is available, the even/odd algorithm benchmarks slower. */
 unsigned long gcd(unsigned long a, unsigned long b)
 {
-       unsigned long r;
+       unsigned long r = a | b;
+
+       if (!a || !b)
+               return r;
 
-       if (a < b)
-               swap(a, b);
+       b >>= __ffs(b);
+       if (b == 1)
+               return r & -r;
 
-       if (!b)
-               return a;
-       while ((r = a % b) != 0) {
-               a = b;
-               b = r;
+       for (;;) {
+               a >>= __ffs(a);
+               if (a == 1)
+                       return r & -r;
+               if (a == b)
+                       return a << __ffs(r);
+
+               if (a < b)
+                       swap(a, b);
+               a -= b;
        }
-       return b;
 }
+
+#else
+
+/* If normalization is done by loops, the even/odd algorithm is a win. */
+unsigned long gcd(unsigned long a, unsigned long b)
+{
+       unsigned long r = a | b;
+
+       if (!a || !b)
+               return r;
+
+       /* Isolate lsbit of r */
+       r &= -r;
+
+       while (!(b & r))
+               b >>= 1;
+       if (b == r)
+               return r;
+
+       for (;;) {
+               while (!(a & r))
+                       a >>= 1;
+               if (a == r)
+                       return r;
+               if (a == b)
+                       return a;
+
+               if (a < b)
+                       swap(a, b);
+               a -= b;
+               a >>= 1;
+               if (a & r)
+                       a += b;
+               a >>= 1;
+       }
+}
+
+#endif
+
 EXPORT_SYMBOL_GPL(gcd);