Re: [PATCH v3 next 04/10] lib: Add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()

[Nicolas Pitre]

On Sat, 14 Jun 2025, David Laight wrote:

> The existing mul_u64_u64_div_u64() rounds down, a 'rounding up'
> variant needs 'divisor - 1' adding in between the multiply and
> divide so cannot easily be done by a caller.
> 
> Add mul_u64_add_u64_div_u64(a, b, c, d) that calculates (a * b + c)/d
> and implement the 'round down' and 'round up' using it.
> 
> Update the x86-64 asm to optimise for 'c' being a constant zero.
> 
> Add kerndoc definitions for all three functions.
> 
> Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>

Reviewed-by: Nicolas Pitre <npitre@xxxxxxxxxxxx>


> 
> Changes for v2 (formally patch 1/3):
> - Reinstate the early call to div64_u64() on 32bit when 'c' is zero.
>   Although I'm not convinced the path is common enough to be worth
>   the two ilog2() calls.
> 
> Changes for v3 (formally patch 3/4):
> - The early call to div64_u64() has been removed by patch 3.
>   Pretty much guaranteed to be a pessimisation.

Might get rid of the above in the log. Justification is in the previous 
patch.

> Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>

Double signoff.

> ---
>  arch/x86/include/asm/div64.h | 19 ++++++++++-----
>  include/linux/math64.h       | 45 +++++++++++++++++++++++++++++++++++-
>  lib/math/div64.c             | 22 ++++++++++--------
>  3 files changed, 69 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
> index 9931e4c7d73f..7a0a916a2d7d 100644
> --- a/arch/x86/include/asm/div64.h
> +++ b/arch/x86/include/asm/div64.h
> @@ -84,21 +84,28 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
>   * Will generate an #DE when the result doesn't fit u64, could fix with an
>   * __ex_table[] entry when it becomes an issue.
>   */
> -static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> +static inline u64 mul_u64_add_u64_div_u64(u64 a, u64 mul, u64 add, u64 div)
>  {
>  	u64 q;
>  
> -	asm ("mulq %2; divq %3" : "=a" (q)
> -				: "a" (a), "rm" (mul), "rm" (div)
> -				: "rdx");
> +	if (statically_true(!add)) {
> +		asm ("mulq %2; divq %3" : "=a" (q)
> +					: "a" (a), "rm" (mul), "rm" (div)
> +					: "rdx");
> +	} else {
> +		asm ("mulq %2; addq %3, %%rax; adcq $0, %%rdx; divq %4"
> +			: "=a" (q)
> +			: "a" (a), "rm" (mul), "rm" (add), "rm" (div)
> +			: "rdx");
> +	}
>  
>  	return q;
>  }
> -#define mul_u64_u64_div_u64 mul_u64_u64_div_u64
> +#define mul_u64_add_u64_div_u64 mul_u64_add_u64_div_u64
>  
>  static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 div)
>  {
> -	return mul_u64_u64_div_u64(a, mul, div);
> +	return mul_u64_add_u64_div_u64(a, mul, 0, div);
>  }
>  #define mul_u64_u32_div	mul_u64_u32_div
>  
> diff --git a/include/linux/math64.h b/include/linux/math64.h
> index 6aaccc1626ab..e1c2e3642cec 100644
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -282,7 +282,53 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
>  }
>  #endif /* mul_u64_u32_div */
>  
> -u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div);
> +/**
> + * mul_u64_add_u64_div_u64 - unsigned 64bit multiply, add, and divide
> + * @a: first unsigned 64bit multiplicand
> + * @b: second unsigned 64bit multiplicand
> + * @c: unsigned 64bit addend
> + * @d: unsigned 64bit divisor
> + *
> + * Multiply two 64bit values together to generate a 128bit product
> + * add a third value and then divide by a fourth.
> + * Generic code returns 0 if @d is zero and ~0 if the quotient exceeds 64 bits.
> + * Architecture specific code may trap on zero or overflow.
> + *
> + * Return: (@a * @b + @c) / @d
> + */
> +u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d);
> +
> +/**
> + * mul_u64_u64_div_u64 - unsigned 64bit multiply and divide
> + * @a: first unsigned 64bit multiplicand
> + * @b: second unsigned 64bit multiplicand
> + * @d: unsigned 64bit divisor
> + *
> + * Multiply two 64bit values together to generate a 128bit product
> + * and then divide by a third value.
> + * Generic code returns 0 if @d is zero and ~0 if the quotient exceeds 64 bits.
> + * Architecture specific code may trap on zero or overflow.
> + *
> + * Return: @a * @b / @d
> + */
> +#define mul_u64_u64_div_u64(a, b, d) mul_u64_add_u64_div_u64(a, b, 0, d)
> +
> +/**
> + * mul_u64_u64_div_u64_roundup - unsigned 64bit multiply and divide rounded up
> + * @a: first unsigned 64bit multiplicand
> + * @b: second unsigned 64bit multiplicand
> + * @d: unsigned 64bit divisor
> + *
> + * Multiply two 64bit values together to generate a 128bit product
> + * and then divide and round up.
> + * Generic code returns 0 if @d is zero and ~0 if the quotient exceeds 64 bits.
> + * Architecture specific code may trap on zero or overflow.
> + *
> + * Return: (@a * @b + @d - 1) / @d
> + */
> +#define mul_u64_u64_div_u64_roundup(a, b, d) \
> +	({ u64 _tmp = (d); mul_u64_add_u64_div_u64(a, b, _tmp - 1, _tmp); })
> +
>  
>  /**
>   * DIV64_U64_ROUND_UP - unsigned 64bit divide with 64bit divisor rounded up
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index ed9475b9e1ef..7850cc0a7596 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -184,11 +184,11 @@ u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
>  }
>  EXPORT_SYMBOL(iter_div_u64_rem);
>  
> -#ifndef mul_u64_u64_div_u64
> -u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> +#ifndef mul_u64_add_u64_div_u64
> +u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
>  {
> -	if (WARN_ONCE(!d, "%s: division of (%#llx * %#llx) by zero, returning 0",
> -		      __func__, a, b)) {
> +	if (WARN_ONCE(!d, "%s: division of (%#llx * %#llx + %#llx) by zero, returning 0",
> +		      __func__, a, b, c)) {
>  		/*
>  		 * Return 0 (rather than ~(u64)0) because it is less likely to
>  		 * have unexpected side effects.
> @@ -199,7 +199,7 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  #if defined(__SIZEOF_INT128__)
>  
>  	/* native 64x64=128 bits multiplication */
> -	u128 prod = (u128)a * b;
> +	u128 prod = (u128)a * b + c;
>  	u64 n_lo = prod, n_hi = prod >> 64;
>  
>  #else
> @@ -208,8 +208,10 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  	u32 a_lo = a, a_hi = a >> 32, b_lo = b, b_hi = b >> 32;
>  	u64 x, y, z;
>  
> -	x = (u64)a_lo * b_lo;
> -	y = (u64)a_lo * b_hi + (u32)(x >> 32);
> +	/* Since (x-1)(x-1) + 2(x-1) == x.x - 1 two u32 can be added to a u64 */
> +	x = (u64)a_lo * b_lo + (u32)c;
> +	y = (u64)a_lo * b_hi + (u32)(c >> 32);
> +	y += (u32)(x >> 32);
>  	z = (u64)a_hi * b_hi + (u32)(y >> 32);
>  	y = (u64)a_hi * b_lo + (u32)y;
>  	z += (u32)(y >> 32);
> @@ -223,8 +225,8 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  		return div64_u64(n_lo, d);
>  
>  	if (WARN_ONCE(n_hi >= d,
> -		      "%s: division of (%#llx * %#llx = %#llx%016llx) by %#llx overflows, returning ~0",
> -		      __func__, a, b, n_hi, n_lo, d))
> +		      "%s: division of (%#llx * %#llx + %#llx = %#llx%016llx) by %#llx overflows, returning ~0",
> +		      __func__, a, b, c, n_hi, n_lo, d))
>  		return ~(u64)0;
>  
>  	int shift = __builtin_ctzll(d);
> @@ -268,5 +270,5 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  
>  	return res;
>  }
> -EXPORT_SYMBOL(mul_u64_u64_div_u64);
> +EXPORT_SYMBOL(mul_u64_add_u64_div_u64);
>  #endif
> -- 
> 2.39.5
> 
>