linux-kernel - Re: [PATCH v2 next 1/4] lib: mul_u64_u64_div

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Message-ID: <39r936n1-0p22-4438-23q7-q8on77583r8r@onlyvoer.pbz>
Date: Mon, 19 May 2025 22:11:30 -0400 (EDT)
From: Nicolas Pitre <npitre@...libre.com>
To: David Laight <david.laight.linux@...il.com>
cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
    u.kleine-koenig@...libre.com, Oleg Nesterov <oleg@...hat.com>, 
    Peter Zijlstra <peterz@...radead.org>, 
    Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH v2 next 1/4] lib: mul_u64_u64_div_u64() rename parameter
 'c' to 'd'

On Sun, 18 May 2025, David Laight wrote:

> Change to prototype from mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
> to mul_u64_u64_div_u64(u64 a, u64 b, u64 d).
> Using 'd' for 'divisor' makes more sense.
> 
> Am upcoming change adds a 'c' parameter to calculate (a * b + c)/d.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

Reviewed-by: Nicolas Pitre <npitre@...libre.com>

> ---
> 
> Note v1 of the patchset had mul_u64_add_u64_div_u64(a, b, add, c).
> 
>  lib/math/div64.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 5faa29208bdb..a5c966a36836 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -184,10 +184,10 @@ 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 c)
> +u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  {
>  	if (ilog2(a) + ilog2(b) <= 62)
> -		return div64_u64(a * b, c);
> +		return div64_u64(a * b, d);
>  
>  #if defined(__SIZEOF_INT128__)
>  
> @@ -212,36 +212,36 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
>  
>  #endif
>  
> -	/* make sure c is not zero, trigger exception otherwise */
> +	/* make sure d is not zero, trigger exception otherwise */
>  #pragma GCC diagnostic push
>  #pragma GCC diagnostic ignored "-Wdiv-by-zero"
> -	if (unlikely(c == 0))
> +	if (unlikely(d == 0))
>  		return 1/0;
>  #pragma GCC diagnostic pop
>  
> -	int shift = __builtin_ctzll(c);
> +	int shift = __builtin_ctzll(d);
>  
>  	/* try reducing the fraction in case the dividend becomes <= 64 bits */
>  	if ((n_hi >> shift) == 0) {
>  		u64 n = shift ? (n_lo >> shift) | (n_hi << (64 - shift)) : n_lo;
>  
> -		return div64_u64(n, c >> shift);
> +		return div64_u64(n, d >> shift);
>  		/*
>  		 * The remainder value if needed would be:
> -		 *   res = div64_u64_rem(n, c >> shift, &rem);
> +		 *   res = div64_u64_rem(n, d >> shift, &rem);
>  		 *   rem = (rem << shift) + (n_lo - (n << shift));
>  		 */
>  	}
>  
> -	if (n_hi >= c) {
> +	if (n_hi >= d) {
>  		/* overflow: result is unrepresentable in a u64 */
>  		return -1;
>  	}
>  
>  	/* Do the full 128 by 64 bits division */
>  
> -	shift = __builtin_clzll(c);
> -	c <<= shift;
> +	shift = __builtin_clzll(d);
> +	d <<= shift;
>  
>  	int p = 64 + shift;
>  	u64 res = 0;
> @@ -256,8 +256,8 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
>  		n_hi <<= shift;
>  		n_hi |= n_lo >> (64 - shift);
>  		n_lo <<= shift;
> -		if (carry || (n_hi >= c)) {
> -			n_hi -= c;
> +		if (carry || (n_hi >= d)) {
> +			n_hi -= d;
>  			res |= 1ULL << p;
>  		}
>  	} while (n_hi);
> -- 
> 2.39.5
> 
>