linux-kernel - Re: [PATCH v3 next 07/10] lib: mul_u64_u64_div

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Message-ID: <rs908qr0-747n-9q5o-4soo-rr09o26o9qr3@onlyvoer.pbz>
Date: Sat, 14 Jun 2025 11:31:52 -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 07/10] lib: mul_u64_u64_div_u64() optimise multiply
 on 32bit x86

On Sat, 14 Jun 2025, David Laight wrote:

> gcc generates horrid code for both ((u64)u32_a * u32_b) and (u64_a + u32_b).
> As well as the extra instructions it can generate a lot of spills to stack
> (including spills of constant zeros and even multiplies by constant zero).
> 
> mul_u32_u32() already exists to optimise the multiply.
> Add a similar add_u64_32() for the addition.
> Disable both for clang - it generates better code without them.
> 
> Use mul_u32_u32() and add_u64_u32() in the 64x64 => 128 multiply
> in mul_u64_add_u64_div_u64().
> 
> Tested by forcing the amd64 build of test_mul_u64_u64_div_u64.ko
> to use the 32bit asm code.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
> 
> New patch for v3.
> 
>  arch/x86/include/asm/div64.h | 14 ++++++++++++++
>  include/linux/math64.h       | 11 +++++++++++
>  lib/math/div64.c             | 18 ++++++++++++------
>  3 files changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
> index 7a0a916a2d7d..4a4c29e8602d 100644
> --- a/arch/x86/include/asm/div64.h
> +++ b/arch/x86/include/asm/div64.h
> @@ -60,6 +60,7 @@ static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
>  }
>  #define div_u64_rem	div_u64_rem
>  
> +#ifndef __clang__

Might be worth adding a comment here justifying why clang is excluded.

>  static inline u64 mul_u32_u32(u32 a, u32 b)
>  {
>  	u32 high, low;
> @@ -71,6 +72,19 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
>  }
>  #define mul_u32_u32 mul_u32_u32
>  
> +static inline u64 add_u64_u32(u64 a, u32 b)
> +{
> +	u32 high = a >> 32, low = a;
> +
> +	asm ("addl %[b], %[low]; adcl $0, %[high]"
> +		: [low] "+r" (low), [high] "+r" (high)
> +		: [b] "rm" (b) );
> +
> +	return low | (u64)high << 32;
> +}
> +#define add_u64_u32 add_u64_u32
> +#endif
> +
>  /*
>   * __div64_32() is never called on x86, so prevent the
>   * generic definition from getting built.
> diff --git a/include/linux/math64.h b/include/linux/math64.h
> index e1c2e3642cec..5e497836e975 100644
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -158,6 +158,17 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
>  }
>  #endif
>  
> +#ifndef add_u64_u32
> +/*
> + * Many a GCC version also messes this up.
> + * Zero extending b and then spilling everything to stack.
> + */
> +static inline u64 add_u64_u32(u64 a, u32 b)
> +{
> +	return a + b;
> +}
> +#endif
> +
>  #if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
>  
>  #ifndef mul_u64_u32_shr
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 22433e5565c4..2ac7e25039a1 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -187,6 +187,12 @@ EXPORT_SYMBOL(iter_div_u64_rem);
>  #endif
>  
>  #if !defined(mul_u64_add_u64_div_u64) || defined(test_mul_u64_add_u64_div_u64)
> +
> +static u64 mul_add(u32 a, u32 b, u32 c)
> +{
> +	return add_u64_u32(mul_u32_u32(a, b), c);
> +}
> +
>  u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
>  {
>  	if (WARN_ONCE(!d, "%s: division of (%#llx * %#llx + %#llx) by zero, returning 0",
> @@ -211,12 +217,12 @@ u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
>  	u64 x, y, z;
>  
>  	/* 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);
> +	x = mul_add(a_lo, b_lo, c);
> +	y = mul_add(a_lo, b_hi, c >> 32);
> +	y = add_u64_u32(y, x >> 32);
> +	z = mul_add(a_hi, b_hi, y >> 32);
> +	y = mul_add(a_hi, b_lo, y);
> +	z = add_u64_u32(z, y >> 32);
>  	x = (y << 32) + (u32)x;
>  
>  	u64 n_lo = x, n_hi = z;
> -- 
> 2.39.5
> 
>