linux-kernel - Re: [PATCH v3 next 03/10] lib: mul_u64_u64_div_u64() simplify check for a 64bit product

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Message-ID: <327ss83n-nq57-n674-02ss-369638np8s21@onlyvoer.pbz>
Date: Sat, 14 Jun 2025 10:01:15 -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 v3 next 03/10] lib: mul_u64_u64_div_u64() simplify check
 for a 64bit product

On Sat, 14 Jun 2025, David Laight wrote:

> If the product is only 64bits div64_u64() can be used for the divide.
> Replace the pre-multiply check (ilog2(a) + ilog2(b) <= 62) with a
> simple post-multiply check that the high 64bits are zero.
> 
> This has the advantage of being simpler, more accurate and less code.
> It will always be faster when the product is larger than 64bits.
> 
> Most 64bit cpu have a native 64x64=128 bit multiply, this is needed
> (for the low 64bits) even when div64_u64() is called - so the early
> check gains nothing and is just extra code.
> 
> 32bit cpu will need a compare (etc) to generate the 64bit ilog2()
> from two 32bit bit scans - so that is non-trivial.
> (Never mind the mess of x86's 'bsr' and any oddball cpu without
> fast bit-scan instructions.)
> Whereas the additional instructions for the 128bit multiply result
> are pretty much one multiply and two adds (typically the 'adc $0,%reg'
> can be run in parallel with the instruction that follows).
> 
> The only outliers are 64bit systems without 128bit mutiply and
> simple in order 32bit ones with fast bit scan but needing extra
> instructions to get the high bits of the multiply result.
> I doubt it makes much difference to either, the latter is definitely
> not mainsteam.

mainstream*

> Split from patch 3 of v2 of this series.
> 
> If anyone is worried about the analysis they can look at the
> generated code for x86 (especially when cmov isn't used).
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

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

> ---
>  lib/math/div64.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 397578dc9a0b..ed9475b9e1ef 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -196,9 +196,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  		return 0;
>  	}
>  
> -	if (ilog2(a) + ilog2(b) <= 62)
> -		return div64_u64(a * b, d);
> -
>  #if defined(__SIZEOF_INT128__)
>  
>  	/* native 64x64=128 bits multiplication */
> @@ -222,6 +219,9 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  
>  #endif
>  
> +	if (!n_hi)
> +		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))
> -- 
> 2.39.5
> 
>