linux-kernel - Re: [PATCH] __div64_const32(): improve the generic C version

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Message-ID: <nycvar.YSQ.7.76.1908292256290.3091@knanqh.ubzr>
Date:   Thu, 29 Aug 2019 22:59:16 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Arnd Bergmann <arnd@...db.de>
cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] __div64_const32(): improve the generic C version


Ping.

On Tue, 20 Aug 2019, Nicolas Pitre wrote:

> Let's rework that code to avoid large immediate values and convert some
> 64-bit variables to 32-bit ones when possible. This allows gcc to
> produce smaller and better code. This even produces optimal code on
> RISC-V.
> 
> Signed-off-by: Nicolas Pitre <nico@...xnic.net>
> 
> diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
> index dc9726fdac..33358245b4 100644
> --- a/include/asm-generic/div64.h
> +++ b/include/asm-generic/div64.h
> @@ -178,7 +178,8 @@ static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
>  	uint32_t m_hi = m >> 32;
>  	uint32_t n_lo = n;
>  	uint32_t n_hi = n >> 32;
> -	uint64_t res, tmp;
> +	uint64_t res;
> +	uint32_t res_lo, res_hi, tmp;
>  
>  	if (!bias) {
>  		res = ((uint64_t)m_lo * n_lo) >> 32;
> @@ -187,8 +188,9 @@ static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
>  		res = (m + (uint64_t)m_lo * n_lo) >> 32;
>  	} else {
>  		res = m + (uint64_t)m_lo * n_lo;
> -		tmp = (res < m) ? (1ULL << 32) : 0;
> -		res = (res >> 32) + tmp;
> +		res_lo = res >> 32;
> +		res_hi = (res_lo < m_hi);
> +		res = res_lo | ((uint64_t)res_hi << 32);
>  	}
>  
>  	if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
> @@ -197,10 +199,12 @@ static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
>  		res += (uint64_t)m_hi * n_lo;
>  		res >>= 32;
>  	} else {
> -		tmp = res += (uint64_t)m_lo * n_hi;
> +		res += (uint64_t)m_lo * n_hi;
> +		tmp = res >> 32;
>  		res += (uint64_t)m_hi * n_lo;
> -		tmp = (res < tmp) ? (1ULL << 32) : 0;
> -		res = (res >> 32) + tmp;
> +		res_lo = res >> 32;
> +		res_hi = (res_lo < tmp);
> +		res = res_lo | ((uint64_t)res_hi << 32);
>  	}
>  
>  	res += (uint64_t)m_hi * n_hi;
>