linux-kernel - Re: [PATCH v4 11/34] vsprintf: Improve number()

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Message-ID: <82fb021d-31bb-4545-cbcc-a7622dbd2e0a@rasmusvillemoes.dk>
Date:   Tue, 21 Jun 2022 10:33:37 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, pmladek@...e.com
Cc:     rostedt@...dmis.org, enozhatsky@...omium.org, willy@...radead.org
Subject: Re: [PATCH v4 11/34] vsprintf: Improve number()

On 20/06/2022 02.42, Kent Overstreet wrote:

>  	/* generate full string in tmp[], in reverse order */
> -	i = 0;
> -	if (num < spec.base)
> -		tmp[i++] = hex_asc_upper[num] | locase;

Please don't remove that optimization in a patch titled "improve number()".

> -	else if (spec.base != 10) { /* 8 or 16 */
> +	if (spec.base == 10) {
> +		nr_digits = put_dec(tmp, num) - tmp;
> +	} else { /* 8 or 16 */
>  		int mask = spec.base - 1;
> -		int shift = 3;
> +		int shift = ilog2((unsigned) spec.base);
>  
> -		if (spec.base == 16)
> -			shift = 4;

So avoiding a branch here may be a good idea, but I'm not sure ilog2()
is very efficient on all arches. Since we know that base is either 8 or
16, we could do "shift = (spec.base/8) + 2".

Rasmus