linux/arch/arm/include/asm/div64.h

#ifndef __ASM_ARM_DIV64
#define __ASM_ARM_DIV64

#include <linux/types.h>
#include <asm/compiler.h>

/*
 * The semantics of __div64_32() are:
 *
 * uint32_t __div64_32(uint64_t *n, uint32_t base)
 * {
 * 	uint32_t remainder = *n % base;
 * 	*n = *n / base;
 * 	return remainder;
 * }
 *
 * In other words, a 64-bit dividend with a 32-bit divisor producing
 * a 64-bit result and a 32-bit remainder.  To accomplish this optimally
 * we override the generic version in lib/div64.c to call our __do_div64
 * assembly implementation with completely non standard calling convention
 * for arguments and results (beware).
 */

#ifdef __ARMEB__
#define __xh "r0"
#define __xl "r1"
#else
#define __xl "r0"
#define __xh "r1"
#endif

static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
{
	register unsigned int __base      asm("r4") = base;
	register unsigned long long __n   asm("r0") = *n;
	register unsigned long long __res asm("r2");
	register unsigned int __rem       asm(__xh);
	asm(	__asmeq("%0", __xh)
		__asmeq("%1", "r2")
		__asmeq("%2", "r0")
		__asmeq("%3", "r4")
		"bl	__do_div64"
		: "=r" (__rem), "=r" (__res)
		: "r" (__n), "r" (__base)
		: "ip", "lr", "cc");
	*n = __res;
	return __rem;
}
#define __div64_32 __div64_32

#if !defined(CONFIG_AEABI)

/*
 * In OABI configurations, some uses of the do_div function
 * cause gcc to run out of registers. To work around that,
 * we can force the use of the out-of-line version for
 * configurations that build a OABI kernel.
 */
#define do_div(n, base) __div64_32(&(n), base)

#else

/*
 * gcc versions earlier than 4.0 are simply too problematic for the
 * __div64_const32() code in asm-generic/div64.h. First there is
 * gcc PR 15089 that tend to trig on more complex constructs, spurious
 * .global __udivsi3 are inserted even if none of those symbols are
 * referenced in the generated code, and those gcc versions are not able
 * to do constant propagation on long long values anyway.
 */

#define __div64_const32_is_OK (__GNUC__ >= 4)

static inline uint64_t __arch_xprod_64(uint64_t m, uint64_t n, bool bias)
{
	unsigned long long res;
	register unsigned int tmp asm("ip") = 0;

	if (!bias) {
		asm (	"umull	%Q0, %R0, %Q1, %Q2\n\t"
			"mov	%Q0, #0"
			: "=&r" (res)
			: "r" (m), "r" (n)
			: "cc");
	} else if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
		res = m;
		asm (	"umlal	%Q0, %R0, %Q1, %Q2\n\t"
			"mov	%Q0, #0"
			: "+&r" (res)
			: "r" (m), "r" (n)
			: "cc");
	} else {
		asm (	"umull	%Q0, %R0, %Q2, %Q3\n\t"
			"cmn	%Q0, %Q2\n\t"
			"adcs	%R0, %R0, %R2\n\t"
			"adc	%Q0, %1, #0"
			: "=&r" (res), "+&r" (tmp)
			: "r" (m), "r" (n)
			: "cc");
	}

	if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
		asm (	"umlal	%R0, %Q0, %R1, %Q2\n\t"
			"umlal	%R0, %Q0, %Q1, %R2\n\t"
			"mov	%R0, #0\n\t"
			"umlal	%Q0, %R0, %R1, %R2"
			: "+&r" (res)
			: "r" (m), "r" (n)
			: "cc");
	} else {
		asm (	"umlal	%R0, %Q0, %R2, %Q3\n\t"
			"umlal	%R0, %1, %Q2, %R3\n\t"
			"mov	%R0, #0\n\t"
			"adds	%Q0, %1, %Q0\n\t"
			"adc	%R0, %R0, #0\n\t"
			"umlal	%Q0, %R0, %R2, %R3"
			: "+&r" (res), "+&r" (tmp)
			: "r" (m), "r" (n)
			: "cc");
	}

	return res;
}
#define __arch_xprod_64 __arch_xprod_64

#include <asm-generic/div64.h>

#endif

#endif