Re: [PATCH v4 next 4/9] lib: Add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()

[David Laight <david.laight.linux@gmail.com>]

On Wed, 29 Oct 2025 17:38:23 +0000
David Laight <david.laight.linux@gmail.com> wrote:

There is a slight bug ...

> 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@gmail.com>
> ---
> 
> 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.
> 
> Changes for v4:
> - For x86-64 split the multiply, add and divide into three asm blocks.
>   (gcc makes a pigs breakfast of (u128)a * b + c)
> - Change the kerndoc since divide by zero will (probably) fault.
> 
>  arch/x86/include/asm/div64.h | 20 +++++++++------
>  include/linux/math64.h       | 48 +++++++++++++++++++++++++++++++++++-
>  lib/math/div64.c             | 14 ++++++-----
>  3 files changed, 67 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
> index 9931e4c7d73f..cabdc2d5a68f 100644
> --- a/arch/x86/include/asm/div64.h
> +++ b/arch/x86/include/asm/div64.h
> @@ -84,21 +84,25 @@ 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 rax, u64 mul, u64 add, u64 div)
>  {
> -	u64 q;
> +	u64 rdx;
>  
> -	asm ("mulq %2; divq %3" : "=a" (q)
> -				: "a" (a), "rm" (mul), "rm" (div)
> -				: "rdx");
> +	asm ("mulq %[mul]" : "+a" (rax), "=d" (rdx) : [mul] "rm" (mul));
>  
> -	return q;
> +	if (statically_true(add))

This needs to be:
	if (!statically_true(!add))

Do you need me to resend the full series?

	David

> +		asm ("addq %[add], %[lo]; adcq $0, %[hi]" :
> +			[lo] "+r" (rax), [hi] "+r" (rdx) : [add] "irm" (add));
> +
> +	asm ("divq %[div]" : "+a" (rax), "+d" (rdx) : [div] "rm" (div));
> +
> +	return rax;
>  }
> -#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..e889d850b7f1 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.
> + * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
> + * 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.
> + * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
> + * 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.
> + * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
> + * 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 7158d141b6e9..25295daebde9 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -183,13 +183,13 @@ 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 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
> @@ -198,8 +198,10 @@ 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);
> @@ -265,5 +267,5 @@ 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