linux-kernel - Re: [PATCH v4 next 3/9] lib: mul_u64_u64_div_u64() simplify check for a 64bit product

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Message-ID: <26p1nq66-8pq5-3655-r7n5-102o989391s2@onlyvoer.pbz>
Date: Wed, 29 Oct 2025 14:11:08 -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>, Borislav Petkov <bp@...en8.de>, 
    Dave Hansen <dave.hansen@...ux.intel.com>, 
    "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, 
    Thomas Gleixner <tglx@...utronix.de>, Li RongQing <lirongqing@...du.com>, 
    Yu Kuai <yukuai3@...wei.com>, Khazhismel Kumykov <khazhy@...omium.org>, 
    Jens Axboe <axboe@...nel.dk>, x86@...nel.org
Subject: Re: [PATCH v4 next 3/9] lib: mul_u64_u64_div_u64() simplify check
 for a 64bit product

On Wed, 29 Oct 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 mainstream.
> 
> 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>

Comment below.


> ---
> 
> Split from patch 3 for v2, unchanged since.
> 
>  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 1092f41e878e..7158d141b6e9 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -186,9 +186,6 @@ EXPORT_SYMBOL(iter_div_u64_rem);
>  #ifndef mul_u64_u64_div_u64
>  u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  {
> -	if (ilog2(a) + ilog2(b) <= 62)
> -		return div64_u64(a * b, d);
> -
>  #if defined(__SIZEOF_INT128__)
>  
>  	/* native 64x64=128 bits multiplication */
> @@ -224,6 +221,9 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>  		return ~0ULL;
>  	}
>  
> +	if (!n_hi)
> +		return div64_u64(n_lo, d);

I'd move this before the overflow test. If this is to be taken then 
you'll save one test. same cost otherwise.