linux-kernel - Re: [RFC PATCH v1 08/50] fs/ext4/ialloc.c: Replace % with reciprocal

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Date:   Sun, 29 Mar 2020 04:00:12 +0000
From:   George Spelvin <lkml@....ORG>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>,
        linux-ext4 <linux-ext4@...r.kernel.org>, lkml@....org
Subject: Re: [RFC PATCH v1 08/50] fs/ext4/ialloc.c: Replace % with
 reciprocal_scale() TO BE VERIFIED

On Sat, Mar 28, 2020 at 06:10:11PM -0600, Andreas Dilger wrote:
> On Mar 28, 2020, at 5:15 PM, George Spelvin <lkml@....ORG> wrote:
>> Also, we could, if desired, eliminate the i variable entirely
>> using the fact that we have a copy of the starting position cached
>> in parent_group.  I.e.
>> 
>> 		g = parent_group = reciprocal_scale(grp, ngroups);
>> -		for (i = 0; i < ngroups; i++, ++g == ngroups && (g = 0)) {
>> +		do {
>> 			...
>> -		}
>> +			if (++g == ngroups)
>> +				g = 0;
>> +		} while (g != parent_group);

> I was looking at whether we could use a for-loop without "i"?  Something like:
> 
> 	for (g = parent_group + 1; g != parent_group; ++g >= ngroups && (g = 0))
> 
> The initial group is parent_group + 1, to avoid special-casing when the
> initial parent_group = 0 (which would prevent the loop from terminating).

That's the first option I presented, above.  Since a for() loop
tests before each iteration, if the counter is strictly modulo
ngroups, there's no way to execute the loop body more than ngroups-1
times.

That's why I changed to do{}while(), which has a minimum of 1 (it can't
handle ngroups == 0), but can mimic the current loop's execution
perfectly (no initial +1 offset).